Forward Physics at the HL-LHC

Lucian Harland-Lang, University of Oxford Oxford blue Visual identity guidelines LHC Working Group on Forward Physics and Diffraction CERN, 19 December 2018

1 Outline/Context • QCD and SM Working Group for HL-LHC and HE-LHC Yellow

Report released recently: CERN-LPCC-2018-03

1 CERN-LPCC-2018-03 December 12, 2018

2 Standard Model Physics

3 at the HL-LHC and HE-LHC

4 Report from Working Group 1 on the Physics of the HL-LHC, and Perspectives at the HE-LHC 6

5 200 6.6.17 Editors: Top quark charge asymmetries at LHCb ...... 143 1 2 3 4 5 8 P. Azzi , S. Farry , P. Nason , A. Tricoli , D. Zeppenfeld 201 6.6.2 A method to determine V at the weak scale in top quark decays at the LHC . . . . . 144 | cb| 202 6.79 Contributors: Flavour changing neutral current ...... 146 6 7 8 9 9 10 203 10 R. Abdul Khalek , J. Alimena , N. Andari , L. Aperio Bella , A.J. Armbruster , J. Baglio , S. 7 Effective11 coupling12 interpretations13 for top14 quark cross8 sections and13 properties15 ...... 149 11 Bailey , E. Bakos , A. Bakshi , C. Baldenegro , F. Balli , A. Barker , W. Barter , J. de 204 7.116 , The1 top quark17 couplings18 to the W boson19 ...... 8 . . . . .20 ...... 21 . . . . . 149 • As part of this-12 Blas chapter, F. Blekman ,on D. Bloch forward, A. Bodek , M. Boonekamp physics., E. Boos , J.D. Bossio Sola , L. 22 9 23 24 25 4 205 137.2Cadamuro t¯t ...... , S. Camarda , F. Campanario , M. Campanelli , Q.-H. Cao , V. Cavaliere , A. 151 26 27 28 29 30 3 31 14 Cerri ¯, B. Chargeishvili , C. Charlot , S.-L. Chen , T. Chen , L. Cieri , M. Ciuchini , G. 206 7.3 ttZ...... 32 33 34 1 35 36 151 15 Corcella , J.M. Cruz-Martinez , M. Czakon , A. Dainese , N.P. Dang , L. Darmé , S. 207 8 Forward4 physics .37 ...... 9 . . . . .8 ...... 38 ...... 39 ...... 40 ...... 153 16 Dawson , H. De la Torre , M. Deile , F. Deliot , S. Demers , A. Denner , F. Derue , L. Di 41 42 43 20 27 208 178.1Ciaccio Photon-induced, W.K. Di Clemente collisions, D. Dominguez at the HL–LHC Damiani . ., . L. . Dudko . . . ., . A. . Durglishvili ...... , . M...... 153 9 44 45 3 99 30 18 Dünser , J. Ebadi , R. B. Ferreira De Faria , G. Ferrera , A. Ferroglia , T. M. Figy , K.D. 209 8.1.146 Anomalous quartic45 gauge couplings47 with48 proton tagging49 ,50 at the HL-LHC45 . . .51 . . . . . 153 19 Finelli , M.C.N. Fiolhais , E. Franco , R. Frederix , B. Fuks , B. Galhardo , J. Gao ,J. 210 9 52 53 54 55 56 208.2R. Gaunt Central, T. Gehrmann exclusive production:, A. Gehrmann-De QCD Ridder prospects, J. Giljanovic ...... , . F. . Giuli . . . ., E.W.N. . . . . Glover . . . ., . . . 155 49 45 41 52 9 211 218.3M. D. Goodsell Tagged proton, E. Gouveia at the HL–LHC:, C. Goy experimental, M. Grazzini prospects, J.F. Grosse-Oetringhaus ...... ,P...... 157 57 11 11 58 59 9 Mario’s talk 22 Gunnellini , C. Gwenlan , L. A. Harland-Lang , P.F. Harrison , G. Heinrich , C. Helsens , C. 212 8.4 Low-mass9 central60 exclusive production61 .62 ...... 34 ...... 63 ., .64 . . . . .9 . . . . 162 23 Helsens , M. Herndon , O. Hindrichs , A. Hoang , K. Hoepfner , J.M. Hogan , A. Huss , 52 65 66 9 13 43 67 68 24 G. Isidori , S. Jahn , Sa. Jain , S. P. Jones , A.W. Jung , H. Jung , S. Kallweit , D. Kar , 52 52 8 69 70 25 A. Karlberg , M. Kerner , M.K. Khandoga , Hamzeh Khanpour , S. Khatibi , A. 71 9 56 2 42 72 43 26 Khukhunaushvili , J. Kieseler , F. Krauss , J. Kretzschmar , J. Kroll , E. Kryshen , V.S. Lang 73 4 74 47 75 14 37 43 • Here - summarise27 , L. Lechner physics, C.A. Lee , M. Leigh prospects, D. Lelas , R. Les discussed, I. M. Lewis , B. Li , Y.here Li ,J. + beyond. 43 76 56 77 78 60 17 28 Lidrych , Z. Ligeti , J.M. Lindert , Y. Liu , K. Lohwasser , K. Long , D. Lontkovskyi , G. 66 17 79 9 17 80 29 Majumder , M. Mancini , P. Mandrik , M.L. Mangano , I. Marchesini , C. Mayer , K. 66 9 45 43 30 Mazumdar , J.A. McFayden , P. M. Mendes Amaral2 Torres Lagarelhos , A.B. Meyer , S. 81 82 83 44 9 31 Mikhalcov , S. Mishima , A. Mitov , M. Mohammadi Najafabadi , M. Moreno Llácer , M. 9 63 47 84 85 76 48 32 Mulders , M. Narain , A. Nisati , T. Nitta , A. Onofre , S. Pagan Griso , D. Pagani , A. 83 86 83 20 9 9 87 33 Papanastasiou , K. Pedro , M. Pellen , M. Perﬁlov , B.A. Petersen , M. Pierini , J. Pires , 4 62 43 52 56 5 9 ,88 34 M.-A. Pleier , S. Plätzer , K. Potamianos , S. Pozzorini , A. C. Price , M. Rauch , E. Re , 89 43 90 14 60 66 86 35 L. Reina , J. Reuter , J. Rojo , C. Royon , A. Savin , S. Sawant , B. Schneider , R. 73 9 5 9 2 47 ,9 36 Schoefbeck , M. Schoenherr , H. Schäfer-Siebert , M. Seidel , T. Shears , L. Silvestrini , M. 91 17 73 92 93 86 19 37 Sjodahl , K. Skovpen , D. Spitzbart , P. Starovoitov , C. J. E. Suster , P. Tan , R. Taus , D. 60 94 95 66 45 52 58 38 Teague , K. Terashi , J. Terron , S. Uplap , F. Veloso , M. Verzetti , M. A. Vesterinen , V.E. 58 20 20 12 12 9 39 Vladimirov , P. Volkov , G. Vorotnikov , M. Vranjes Milosavljevic , N. Vranjes , E. Vryonidou 56 9 96 8 74 40 11 40 , D. Walker , M. Wiesemann ,Y.Wu , T. Xu , S. Yacoob , J. Zahreddine , G. Zanderighi , M. 90 43 97 98 63 96 96 41 Zaro , O. Zenaiev , G. Zevi Della Porta , C. Zhang , W. Zhang , H. Zhu , H.L. Zhu , R. 43 9 42 Zlebcik , F. N. Zubair

6 1Introduction

The use of diffractive processes to study the Standard Model (SM) and New Physics at the LHC has only been fully appreciated within the last few years; see, for example [1, 2, 3, 4], or the recent reviews [5, 6, 7], and references therein. By detecting protons that have lost only about 1-3% of their longitudinal momentum [8, 9], a rich QCD, electroweak, Higgs and BSM programme becomes accessible experimentally, with the potential to study phenomena which are unique to the LHC, and difficult even at a future linear collider. Particularly interesting are the so-called central exclusive production (CEP) processes which provide an extremely favourable environment to search for, and identify the nature of, new particles at the LHC. The first that comes to mind are the Higgs bosons, but there is also a potentially rich, more exotic, physics menu including (light) gluino and squark production, searches for extra dimensions, gluinonia, radions, and indeed any new object which has 0++ (or 2++)quantumnumbersand couples strongly to gluons, see for instance [2, 10, 11]. By “central exclusive” we mean a process of the type pp → p + X + p, where the + signs denote the absence of hadronic activity (that is, the presence of rapidity gaps) between the outgoing protons and the decay products of the centrally produced system X. The basic mechanism driving the process is shown in Fig. 1. There are several reasons why CEP is especially attractive for searches for new heavy objects. Central exclusive reactionsFirst, if processes the outgoing protons remain intact and scatter through small angles then, to a very good approximation, the primary active di-gluon system obeys a Jz =0,C-even,P-even, selection rule [12]. Here Jz is the projection of the total angular momentum along the proton beam axis. This selection rule readily permits a clean determination of the quantum numbers of the observed new (for example, Higgs-like) resonance, when the dominant production is a scalar state. Secondly, because the process is exclusive, the energy loss of the outgoing protons Central Exclusiveis directly related Production to the mass of the central at system,HL-LHC allowing a potentially excellent mass resolution, irrespective of the decay mode of the centrally produced system. Thirdly, in many Central exclusive reactionstopical cases,pp in particular,p + X for+ Higgsp bosoncan production, be studied a signal-t byo-background measuring ratio ofX orderin a central 1 (or even better)! is achievable [3, 11], [13]-[18]. In particular, due to Jz = 0 selection, leading- 2 detector and the intactorder protons QCD b¯b productionpp with is suppressed forward by a factor proton (mb/E detectors.T ) ,whereET is the transverse energy ¯ < • Two productionof the b, mechanismsb jets. Therefore, of forinterest a low mass*: Higgs, MH ∼ 150 GeV, there is a possibility to observe p p

+ −

Fig. 5.31: Di-photon exclusive Standard Model productionPhoton-inducedFigure via QCD 1: (left) The and basic photon mechanism induced (right) forQCD-induced the exclusive process pp → p + X + p.ThesystemX is processes at the lowest order of pertubation theory. produced by the fusion of two active gluons, with a screening gluon exchanged to neutralize Higher mass the colour. ‘Lower’ mass p whereas the photon induced ones (QED processes) dominate at higher diphoton masses [176]. It is p very important to notice that the W loop• contributionSensitive dominates to atrather high diphoton different masses [174, physics/areas 175, 177] of phase space. Play whereas this contribution is omitted in most studies. This is the ﬁrst time that we put all terms inside a MC generator, FPMC [179]. different roles @ HL-LHC. 2

6.1.2 StandardMeasure Model WW and theZZ prduction proton fractional momentum3 loss *In⇠ addition-= p photoproduction/p with forward proton In the Standard Model (SM) of particle physics, the couplings of fermions and gauge bosons are con- i strained by thespectrometers. gauge symmetries of the Lagrangian. The measurement of W and Z boson pair pro- ductions via the exchange of two photons allows to provide directly stringent tests of one of the most important and least understood mechanism in particle physics, namely the electroweak symmetry break- ing. p 1 The processIsolate that we study signal is the W pair by production comparing induced by the exchangem of twowith photonsm [178].pp = ⇠1⇠2s,andy with ypp = 2 ln(⇠1/⇠2) It is a pure QED process in which the decay products of the W bosons are measured in the central detector and the scattered(exclusive protons leave intact processes). in the beam pipe at very small angles and are detected in AFP or CT-PPS. All these processes as well as theb different diffractive backgrounds were implemented in the FPMC Monte Carlo [179]. After simple cuts to select exclusive W pairs decaying into leptons, such as a cut on the proton momentum loss of the proton (0.0015 < x < 0.15) — we assume the protons to be tagged in AFP or CT-PPS at 210 and 420 m — on the transverse momentum of the leading and second leading leptons at miss 25 and 10 GeV respectively, on ET > 20 GeV, Df > 2.7 between leading leptons, and 160 < W < 500 GeV, the diffractive mass reconstructed using the forward detectors, the background is found to be less 1 than 1.7 event for 30 fb for a SM signal of 51 events [178].

6.2 Triple anomalous gauge couplings 1/8 In Ref. [180], we also studied the sensitivity to triple gauge anomalous couplings at the LHC. The Lagrangian including anomalous triple gauge couplings l g and Dkg is the following

L (W † W µ An W W †µ An ) ⇠ µn µn g g † µn l † µ nr +(1 + Dk )WµWn A + 2 WrµW n A ). (5.27) MW The strategy is the same as for the SM coupling studies: we ﬁrst implement this lagrangian in FPMC [179] and we select the signal events when the Z and W bosons decay into leptons. The difference is that the signal appears at high mass for l g and Dkg only modiﬁes the normalization and the low mass events

81 The exclusive channel is particularly relevant in light of the forward proton detectors approved for installation at ATLAS (AFP [16]) and already installed at CMS (CT-PPS [18]): such exclusive events can be selected by tagging the outgoing intact protons in association with a measurement of the resonance R in the central detector. The background from over- lapping non–exclusive pile–up interactions may be controlled by ensuring that the ‘missing mass’ and rapidity information reconstructed from the outgoing protons is consistent with the measurement in the central detector, as well as through the use of ‘fast timing’ detectors to check if the photon and proton scattering points are the same, see [19,43]. 1Introduction

By selecting exclusive events we naturally enhance the relative contribution from the The use of diffractive processes to study the Standard Model (SM) and New Physics at the –initiated subprocess, see [20]. In particular, for the gg–initiated case, which can occur LHC has only been fully appreciated within the last few years; see, for example [1, 2, 3, 4], or the recent reviews [5, 6, 7], and references therein. By detecting protons that have lost only exclusively through the ‘Durham’ mechanism described in [44], there is a strong Sudakov about 1-3% of their longitudinal momentum [8, 9], a rich QCD, electroweak, Higgs and BSM c programme becomes accessible experimentally, with the potential to study phenomena which suppression (given by (18) without the theta–function and with a much lower k = Q0 = ? are unique to the LHC, and difficult even at a future linear collider. Particularly interesting O(GeV)) associated with the requirement of no additional parton emission from the hard are the so-called central exclusive production (CEP) processes which provide an extremely process. As a result, the exclusive gg luminosity in the relevant kinematic regions is 3 favourable environment to search for, and identify the nature of, new particles at the LHC. The ⇠ first that comes to mind are the Higgs bosons, but there is also a potentially rich, more exotic, orders of magnitude smaller than in the inclusive case. In addition, for the final state to be physics menu including (light) gluino and squark production, searches for extra dimensions, gluinonia, radions, and indeed any new object which has 0++ (or 2++)quantumnumbersand exclusive there must be no underlying event activity associated with the hard process. The couples strongly to gluons, see for instance [2, 10, 11]. By “central exclusive” we mean a process probability for this to occur is known as the ‘survival factor’: see Appendix A for further of the type pp → p + X + p, where the + signs denote the absence of hadronic activity (that discussion. For gg–induced production this suppresses the cross section by a further 2 is, the presence of rapidity gaps) between the outgoing protons and the decay products of the ⇠ centrally produced system X. The basic mechanism driving the process is shown in Fig. 1. orders of magnitude, so that the exclusive cross section is suppressed in total by a very large There are several reasons why CEP is especially attractive for searches for new heavy objects. factor of 105. The exclusive channel is particularly relevant in lightFirst of the, if the forward outgoing protons proton remain detectors intact and scatter through small angles then, to a very ⇠ approved for installation at ATLAS (AFP [16]) and alreadygood installed approximation, at CMS the (CT-PPS primary active [18]): di-gluon system obeys a Jz =0,C-even,P-even, In the –initiated process there is also some suppression from the fact that, while the selection rule [12]. Here Jz is the projection of the total angular momentum along the proton such exclusive events can be selected by tagging the outgoingbeam intact axis. This protons selection rule in association readily permits a clean determination of the quantum numbers dominant component of the input PDF, (x, Q0), is due to coherentwith a measurement emission from of the the resonance proton,R in the central detector.of the observed The background new (for example, from Higgs-like) over- resonance, when the dominant production is a any further DGLAP evolution cannot occur, as this will producelapping secondary non–exclusive particles pile–up and interactions spoil may be controlledscalar by state. ensuringSecondly, that because the the ‘missing process is exclusive, the energy loss of the outgoing protons mass’ and rapidity information reconstructed from the outgoingisProduction directly protons related to is the consistentMechanisms mass of the with central system, allowing a potentially excellent mass the exclusivity of the final state. More precisely, we calculate the exclusive luminosity in resolution,12 irrespective of the decay mode of the centrally produced system. Thirdly, in many the measurement in the central detector, as well as through the use of ‘fast timing’ detectorsLHCb the usual equivalent photon approximation (EPA) [45]. The quasi–real photons are emitted Exclusive final statetopical can cases, be10 produced in particular, via for three Higgs different boson production, mechanisms, a signal-t o-background ratio of order to check if the photon and proton scatteringdepending points are on thekinematics1 same, (or even and see better)8 [19,43].quantum is achievable numbers [3, 11], of [13]-[18]. state: In particular, due to Jz = 0 selection, leading- 6 ¯ 2 by the incoming proton i =1, 2withanumberdensitygivenbyBy selecting exclusive events we naturally enhance theorder relative QCDEvents per 500 MeV bb production contribution is suppressed from by the a factor (mb/ET ) ,whereET is the transverse energy ¯ 4 C-even, couples to< gluons 2 of theQCD-b, b jets.induced Therefore, for a low mass Higgs, MH ∼ 150 GeV, there is a possibility to observe 2 –initiated2 subprocess, see [20]. In particular, for the gg–initiated2 case, which can occur 1 ↵ d qi qi 2 xi 2 p p ? ? exclusively through the ‘Durham’ mechanism described in [44], there0 is a strong Sudakov n(xi)= (1 xi)FE(Qi )+ FM (Qi ) , (24) C-even, Couples to photons6000 8000 10000 12000 2 2 2 2 2 2 2 c Mass (MeV) xi ⇡ qi + xi mp qi + xi mp suppression2 (given by (18) without the theta–function and with a much lower+ − k = Q0 = Z ? ✓ ? ◆ Fig. 5.10: Invariant mass of the J?/yJ/y system in (left) exclusive and (right) inclusive events. The O(GeV)) associated with the requirement of no additional partonshaded area emission is the theoretical from prediction the of Ref. hard [26] X = H, jj... where xi and qi are the longitudinal momentumPhoton-induced fraction and transverse momentum production of the ? process. As a result, the exclusive gg luminosity in the relevant kinematic regions is 3 2 p p ⇠ photon i, respectively, and Qi is the modulus of the photon virtuality.orders of magnitude The functions smaller thanFE and in the inclusive case. In addition,3 Future for measurement the final at low/medium state to luminosity: be motivation Fig. 88: Schematic diagram of the production of a system X in (left) two–photon (right) QCD–initiated FM are the usual proton electric and magnetic form factors exclusive there must be no underlying event activity associated3.1 with Photon–induced the hard processes process. The centralFigure exclusive3.1.1 1: Theproduction. Diffractive basic photoproduction mechanism forg p theVp exclusive process pp → p + X + p.ThesystemX is Exclusive photon-initiated production of particular interest for BSM. C-odd, couples! to photons + gluons • probability for this to occur is known as the ‘survival factor’:produced see by Appendix the fusion of twoA for active further gluons, with a screening gluon exchanged to neutralize 2 2 2 2 2 2 Photon-induced V (z,k ) Fig. 5.31: Di-photon exclusive Standard Model production via QCD (left) and photon induced (right) ~ Q (z, k ) ? 4mpGE(Qi )+Qi GM (Qi ) ? 2 2 2 2 discussion. For ggprocesses–induced at the lowest production order of pertubation theory.this suppresses3909 the8 theForward cross colour. physics section by a further 2 F (Q )=G (Q ) F (Q )= , (25) Q¯ VM = J/ , 0, ⌥, ⌥0,... M i M i E i (1 z, ~k ) ⇠ ? 2orders2 of magnitude, so that the exclusive cross section3910 is8.1 suppressed Photon-induced in collisions total at the by HL–LHC a very large ★ Theoretical framework4mp + Qwelli understood. Cross section give in terms of ~ ~ 5 3911 PhotoproductionCentral exclusive production (CEP)W corresponds2 to the production of central system X, and nothing else, factor of 10 . 3912 with two outgoing intact protons: whereas the photon induced ones (QED processes) dominate at higher diphoton masses [176]. It is with ⇠ pp p + X + p. 2 (57) well known equivalent photonIn the approximation:–initiatedvery important process to notice that there the W loop is contribution also some dominates suppression at high diphoton masses from [174, 175, the 177] fact that,! while the 2 2 p p whereas this contribution is omitted in most studies. This is the first time3913 thatSuch we a processput all terms may be inside mediated a by photon exchange, with the elastic photon emission vertex leaving 2 2 GM (Qi ) 1 (x, )=@G(x, )/@ log¯2 G (Q )= = dominant, componentMC generator, of the FPMC input [179]. (26) PDF, (x, Q Each0), is one due offers3914 tothe coherent protonsdifferent intact, emission see possibilities… Fig. 88 (left). from A range the of SM proton, (e.g. X F= , Z, ZZ, ``) and BSM states (e.g. E i 4 3915 X = axion–like particles,Fig. monopoles, 5.11: Diagrams SUSY representing particles) may the exclusive be produced diffractive in this way.g p TheseVpamplitude. have the 7.78 2 anypp2 furtherpXp DGLAP evolutionEPA cannot occur, as this will produce secondary particles and spoil ! 3916 benefit of: 1+Qi /0.71GeVd ! 6.1.2d Standard Model WW and ZZ prduction 6 the exclusivity ofIn the theL Standard final Model state.ˆ (SM)( of More particleX physics, precisely,) the couplings we of calculate fermions and gauge the bosons exclusive are con- luminosity in in the dipole approximation, where G and G are the ‘Sachs’2 form factors.2 The ‘EPA’ 3917 – The theoreticalTwo framework largely equivalent to model approaches the underlying to exclusive production diffractive mechanism, production based on of thea vector equiva- meson of mass E M dM dyX ⇠ dstrainedM byd theyX gauge symmetries! of the Lagrangian. The measurement3918 of W andlent photonZ boson approximation pair pro- [728], is very well understood. Moreover,2 due2 to the peripheral nature theX usual equivalentX photon approximation (EPA) [45]. The quasi–realMV at g p cms energy photonsW, applicable are at small emitted values of x = MV /W , are the color-dipole approach and the ductions via the exchange of two photons allows to provide directly3919 stringent testsof the of interaction one of the the most possibility for additional inelastic proton–proton interactions (in other words kT -factorization. luminosity is given by by the incomingimportant proton andi least=1 understood, 2withanumberdensitygivenby mechanism in particle physics, namely3920 the electroweakof multiple–particle symmetry break- interactions) is very low. Within the color-dipole framework, the forward diffractive amplitude shown in Fig. 6.8 takes the EPA ing. d 1 2 2 3921 – As theform mass of the2 central system increases, the relative size of any contribution from QCD– 1 The↵ process thatd weq studyi is the W pair productionqi induced by the3922 exchangeinitiated of two photons production,x [178]. see Section 8.2, becomes increasingly small [729], due to the strong Sudakov 2 i 2 1 L = n(x1) n(x2) . n(x )=It is a pure QED process in which?(27) the decay products? of the W bosons(1 are3923x measured)F suppression( inQ the)+ central in vetoingdetectorF on( additionalQ2 ) QCD, radiation.(24) 2 2 with i 2 2 2 2 i E i ¡mAM(g⇤(Q )pi Vp;W,t = 0)= dz d r yV (z,r)yg (z,r,Q )s(x,r), (5.3) 2 2 2 2 ⇤ x ⇡ q + x m q + x m 2 ! 0 dMX dyX s and thei scattered protonsi leavei intactp in thei beam pipei at veryp small angles and are detected in AFP or Z Z ? ✓ ? 3924 ◆ CT-PPS. AllZ these processes as well as theb different diffractive backgrounds wereCEP implemented therefore offers in2 the a unique2 opportunity at the LHC to observe the purely photon–initiated pro- where x = M /W , yV and yg are the light-cone wave functions for the quark-antiquark Fock states of 3925 duction of electromagneticallyV charged objects at the LHC in a clean and well understood environment; where xi and qi FPMCare Monte the Carlo longitudinal [179]. momentum fraction and transversethe vector meson and momentum photon respectively. The ofq theq¯ separation r is conserved during the interaction (and so ? 3926 we in effect use the LHC as a photon–photon collider. The cross sections for such processes can be After simple cuts2 to select exclusive W pairs decaying into leptons, such as a cutare onthe the longitudinal proton momentum fractions z,1 z carried by q andq ¯). Color dipoles of size r are diagonal photon i, respectively, and Q is the modulus of the photon3927 relatively virtuality. small, in particular The at higher functions mass, and thereforeFE toand select such events it is essential to run dur- 14 momentum loss of thei proton (0.0015 < x < 0.15) — we assume the protons to be taggedstates of in the AFPS-matrix or and interact with the proton with the cross section 3928 ing nominal LHC running with tagged protons. The increased statistics available during the HL–LHC CT-PPS at 210 and 420 m — on the transverse momentum of the leading and second leading leptons at FM are the usual proton electric and magnetic form factors3929 stage will allow these studies to push to higher masses and2 lower cross2 sections, increasing the discov- • Photon flux depends on well measured25 and 10 GeV respectively, EM on Emiss > 20form GeV, Df > 2factors..7 between leading leptons, and 160 < W < 500 4p d k ∂xg(x,k ) T 3930 ery potential. A detailed study for thes( examplex,r)= caseaS of anomalous quartic1 gaugeexp(i couplingskr) , is discussed (5.4) GeV, the diffractive mass reconstructed using the forward detectors,2 the2 background2 is found2 to2 be less 2 3 k4 ∂ log(k2) 4m G3931 (below.Q )+Q G (Q ) Z 2 2 1 2 2 p E i i M i h i Fthan( 1.7Q event)= forG 30 fb(Qfor a) SM signalF of 51(Q events)= [178]. which in turn is related, to the transverse-momentum(25) dependent (or unintegrated) gluon distribution (see M i M i Ep i 2 2 3932 8.1.14mp Anomalous+Ref.Qi [35] quartic and references gauge couplings therein). with Let proton us try tagging to understand at the HL-LHC the behaviour of the amplitude A salient Impact of QCD, via additional6.2 Triple anomalous gauge couplings • 3933 We discuss the discovery potential of anomalous quartic gauge interactions at the LHC via the mea- with In Ref. [180], we also studied the sensitivity to triple gauge anomalous3934 surement couplings of at central the LHC. exclusive The production. The central system X is reconstructed in the central detector 2 2 g g 58 hadron-hadron interactions veryLagrangian including2 anomalous2 G tripleM gauge(Qi couplings) l and Dk is3935 the1 following(CMS, ATLAS) while the outgoing protons, which remain intact due to the coherent photon exchange, G (Q )= = 3936 can be reconstructed, with dedicated tracking detectors located(26) in the very forward region at about E i † µ n †µ n 2 2 4 ± L (W7µn.W78A WµnW A ) 2 3937 210 m (220 m) with respect to the interaction point of the CMS (ATLAS) experiment. The fractional ⇠ b 1+SQi /0.71GeV0.7 0.9 g soft3938 ⇠ = p/p ⊥ g † µn l † µ nrmomentum loss of the outgoing protons is reconstructed offline.Central exclusive production low. +(1 + Dk )W Wn A + W W n A ). (5.27)fwd fwd 1 ⇠1 µ 2 rµ 3939 ⇠processes satisfymX = mX = ⇠1⇠2s and yX = yX = log , where mX and yX are the mass in the dipole approximation, where GE and GM areMW the ‘Sachs’ form factors. The ‘EPA’ 2 ⇠2 p luminosity is givenThe strategy by is the same as for the SM coupling studies: we first implement this lagrangian in FPMC [179] 153 and we select the signal events whenEPA the Z and W bosons decay into leptons. The difference is that the signal appears at high massp fordl g andDkg only modifies1 the normalization and the low mass events L2 = n(x1) n(x2) . (27) dMX dyX s 4 81 14 ★ As mass of central system M X increases, QCD-initiated production cross section suppressed by no radiation probability BG often low*. d ) dM [fb/GeV] ps = 14 TeV 101 X gg 100 ! SuperChic 3 1 ! 10 , W loop ! 10 2 , fermion loop ! 3 • Example of 10 4 10 production: 5 10 6 10 7 10 8 10 50 100 150 200 250 300 350 400 450 500 MX [GeV] • CEP: unique possibility to observe photon-initiated production of states with EM coupling in clean/well understood environment. • However typically considering high mass region (RPs) and relatively low cross sections (EM couplings). Statistics limited. Increased statistics from HL-LHC running offer clear advantage here, ! in particular in terms of pushing to higher mass.

5 *Precise level depends on particular process. Example 1 - Anomalous Couplings

• DiscussedSensitivity in improvement detail in Cristian’s at the HL-LHC talk:

1 1 Left: Comparison between 300 fb and 3000 fb . Right: (Zoomed-in; change of scale in X-Y axis) comparison between the use of timing t =2, 5, 10 ps. Z hadrons + beneﬁts the ! most from the use of timing. • ExpectedSensitivity improvements on couplings at 95% CL couldfrom improve HL-LHC roughly one impressive order of magnitude ~ down an to,order of 2 2 2 14 4 ⇣1 + ⇣2 ⇣1⇣2 1.6 10 GeV (5) magnitude (~ 5 ordersr of magnitude 3  better· than current best inclusive limits). 8/8

6 Example 2 - Compressed SUSY

• Searches for compressed SUSY scenarios - potential to increase reach in regions of parameter space (EW couplings, low mass difference) where inclusive searches struggle. Valery, Jesse + Lydia’s talks • Signal cross section low - gain from HL-LHC. • In addition - detector upgrades can help reduce BGs:

‣ Increased tracker coverage. Event yields / µ h iPU = 300 fb 1 0 10 50 L ‣ Timing in forward detectors at IP. Excl. sleptons 0.6—2.9 0.5—2.4 0.3—1.4 + Excl. l l 1.9 1.6 0.9 ‣ + Radiation-hard ZDC with timing? Excl. K K 0 0 0 + ⇠ ⇠ ⇠ Excl. W W 0.7 0.6 0.3 • However higher pile will clearly lead Excl. cc¯ 0 0 0 ⇠ ⇠ ⇠ Excl. gg 0 0 0 ⇠ ⇠ ⇠ to more challenging environment - Incl. ND jets 0/ 0 0.1/0.1 1.8/2.4 ⇠ ⇠ further studies needed. 1 Table 8: Final event yields corresponding to an integrated luminosity of 300 fb as a function of amount of pile–up events per bunch crossing for the slepton signal and all considered background processes. All • Note: here and elsewherenumbers ~420m correspond RPs to the could di-lepton also mass greatly range 2 5 GeV and a tracker coverage of ⌘ < 2.5. The ranges in the signal event yields illustrate the spread obtained from | | things, increasing acceptancethe entire studiedtowards slepton lower mass range: mass the lower region. value comes from the M˜l = 300 GeV, the higher from the M˜l = 120 GeV scenario. 7

Event yields / µ h iPU = 300 fb 1 0 10 50 L Excl. sleptons 0.6—3.0 0.5—2.6 0.3—1.5 + Excl. l l 1.4 1.2 0.7 + Excl. K K 0 0 0 + ⇠ ⇠ ⇠ Excl. W W 0.6 0.5 0.3 Excl. cc¯ 0 0 0 ⇠ ⇠ ⇠ Excl. gg 0 0 0 ⇠ ⇠ ⇠ Incl. ND jets 0/ 0 0.03/0.05 0.6/0.7 ⇠ ⇠ Table 9: The same as in Table 8 but for the enlarged tracker coverage ⌘ < 4.0. | |

452 The signal yield can be doubled by taking all di-lepton masses into account. This would, 453 however, not only increase the background but also the average di-lepton mass itself and hence 454 limit the possibility of estimating the unknown mass of the DM particle by measuring the central 455 system mass via the FPDs. Another way to increase the signal yield would be to increase the 456 lepton reconstruction eciencies. In the ATLAS study [15] they start at 70% for muons and at 457 50% for electrons and slowly rise with increasing pT , nevertheless since we deal with two leptons, 458 any increase in the single-lepton eciency could have a reasonable impact. 459 The background contamination, in turn, could be lowered by considering the following points. 460 Since a big part of background leptons comes from decays of heavier particles, they originate 461 from a vertex displaced from the primary one. It would thus be natural to consider restricting 462 z0 and d0 distances to some small values for all found leptons in background samples (as is done 463 for example when determining reconstruction eciencies of primary vertices, see [74]). Actual 464 LHC b–physics analyses rather use a pseudo-proper lifetime as a more appropriate observable 465 to separate primary from secondary vertices. Furthermore, as discussed above, both ATLAS 466 and CMS are upgrading their trackers to cover the additional region 2.4 < ⌘ < 4. Both | | 467 are also considering adding timing detectors in these forward areas with resolution of about 468 30 ps [74, 75]. The time information for tracks in this forward area will improve the track-to- 469 vertex association, leading to a performance similar to that in the central region for both jet 470 and lepton reconstruction, as well as the tagging of heavy-ﬂavour jets. The timing in the central

14 Further Possibilities

What if?

What if New Physics ...

Doesn’t couple to quarks? •

Is invisible to detector? •

Decays to a cascade of particles? •

Produces so decay products? •

Is a broad resonance? •

Long-lived particles? •

Photon collider for new physics searches | Lydia Beresford & Jesse Liu | Dec

• See also - t t , ALPs, dileptons, VB pairs - all clearly beneﬁt from increased statistics. • General motivation: accessing regions that are challenging/impossible for conventional HL-LHC searches.

8 1Introduction

good approximation, the primary active di-gluon system obeys a Jz =0,C-even,P-even, selection rule [12]. Here Jz is the projection of the total angular momentum along the proton beam axis.QCD-initiated This selection rule readilyProduction permits a clean determination of the quantum numbers of the observed new (for example, Higgs-like) resonance, when the dominant production is a scalar state. Secondly, because the process is exclusive, the energy loss of the outgoing protons • QCD-initiatedis directly related production to the more mass challenging of the central at HL-LHC, system, but: allowing a potentially excellent mass ★resolution, Possibility to irrespective extend to ~ of420m the RP decay positions mode (revive of the FP420) centrally produc push toed system. Thirdly, in many ) lowertopical mass, cases, and open in particular, up possibility for Higgsof e.g. exclusive boson production, Higgs observation. a signal-to-background ratio of order

★1 Dedicated (or even better)ALICE isstudies achievable in their [3, low 11], pile-up [13]-[18]. environment In particular, planned, due to Jz = 0 selection, leading- ¯ 2 makingorder QCDuse of blargerb production future dataset. is suppressed by a factor (mb/ET ) ,whereET is the transverse energy ¯ < of the b, b jets. Therefore, for a low mass Higgs, MH ∼ 150 GeV, there is a possibility to observe

Figure 1: The basic mechanism for the exclusive process pp → p + X + p.ThesystemX is produced by the fusion of two9 active gluons, with a screening gluon exchanged to neutralize the colour.

2 4

I. INTRODUCTION (a) jet jet p Exclusive dijet production inpp ¯ collisions is a process p IP in which both the antiproton and proton escape the in- p teraction point intact and a two-jet system is centrally produced: (b) jet ′ ′ jet p p¯ + p p¯ +(jet1 + jet2)+p . (1) → p p IP This process is a particular case of dijet production in IP double Pomeron exchange (DPE), a diffractive process in _ p ηp 0 η ηp which the antiproton and proton suffer a small fractional momentum loss, and a system X containing the jets is produced, FIG. 1: Illustration of event topologies in pseudorapidity, ′ ′ ′ ′ η,andassociatedPomeronexchangediagramsfordijetpro- p¯ + p [¯p + IP p¯]+[p + IP p] p¯ + X + p , (2) duction in (a) single diffraction and (b) double Pomeron ex- → → change. The shaded areas on the left side represent “underly- where IP designates a Pomeron, defined as an exchange ing event” particles not associated with the jets [from Ref. [2]]. consistingPhysics of a colorless with combination tagged of gluons protons and/or - QCD quarks carrying the quantum numbers of the vacuum. • QCD-inducedIn a particle-like baseline Pomeron high picture mass (e.g. channels- see [1]), theexclusive jets and Higgs. system X may be thought of as being produced by the p p p p collision★ Exclusive of two Pomerons,jets: IP p¯ and IP p, g Jet g IP p¯ + IP p X YIP / p¯ +(jet1 + jet2)+YIP / p , (3) (a) g (b) g H ‣ Exclusive→ ⇒ jets: CEP theory: dominantly gg colour Jet where in addition to the jets the final state generally con- g g tains Pomeronsinglet dijets. remnants Novel designated features by (radiationYIP / p¯ and patterns/YIP / p . p p p p Dijet productionzeros) in trijets in DPE - isa agluon sub-process factory! to dijet production in single diffraction (SD) dissociation, where only the FIG. 2: Leading order diagrams for (a) exclusive dijet and antiproton‣ New (proton) QCD survivesregime - while data thefrom proton Tevatron (antipro- but not(b) exclusive Higgs boson production inpp ¯ collisions. ton) dissociates. Schematic diagrams for SD and DPE dijet productionfrom LHC are (yet). shown in Fig. 1 along with event topologies in pseudorapidity space (from Ref. [2]). In ‣ Challenging measurement, but cross section high. SD, the escapingp ¯ is adjacent to a rapidity gap, defined as a region of pseudorapidity devoid of particles [3]. A Mjj [GeV] (gg)[pb] Exclusive production may also occur through a t- rapidity gap arises because the Pomeron exchanged in a channel color-singlet two gluon exchange at leading order j 100 35 diffractivep > process25 GeV is a colorless object of effective spin (LO) in perturbative quantum chromo-dynamics (QCD), J 1andcarriesthequantumnumbersofthevacuum.? 200 1 ≥ ⌘ < 3 as shown schematically in Fig. 2 (a), where one of the two In DPE,| j two| such rapidity gaps are300 present. 0.14 gluons takes part in the hard scattering that produces the Dijetq production= u, d, s, c in DPE may occur400 as an exclusive0.024 jets, while the other neutralizes the color flow [6]. A simi- process [4] with only the jets in the500 final state0.0053 and no lar diagram, Fig. 2 (b), is used in [6] to calculate exclusive Pomeron remnants, either due to a fluctuation of the 600 0.0014 Higgs boson production. Pomeron remnants down to zero or with10 a much higher Exclusive dijet production has never previously been cross section in models in which the Pomeron is treated observed in hadronic collisions. In addition to providing as a parton and the dijet system is produced in a 2 2 information on QCD aspects of vacuum quantum num- process analogous to γγ jet + jet [5]. → → ber exchange, there is currently intense interest in using In a special case exclusive dijets may be produced measured exclusive dijet production cross sections to cal- through an intermediate state of a Higgs boson decay- ¯ ibrate theoretical predictions for exclusive Higgs boson ing into bb: production at the Large Hadron Collider (LHC). Such 0 IP p¯ + IP p H (¯b jet1)+(b jet2). (4) predictions are generally hampered by large uncertain- → → → → ties due to non-perturbative suppression effects associated with the rapidity gap survival probability. As these effects are common to exclusive dijet and Higgs boson burgh EH9 3JZ, United Kingdom, kUniversity of Heidelberg, D- production mechanisms, dijet production potentially pro- 69120 Heidelberg, Germany, lUniversidad Iberoamericana, Mexico vides a “standard candle” process against which to cali- D.F., Mexico, mUniversity of Manchester, Manchester M13 9PL, brate the theoretical models [6, 7]. n England, Nagasaki Institute of Applied Science, Nagasaki, Japan, In Run I (1992-96) of the Fermilab Tevatronpp ¯ col- oUniversity de Oviedo, E-33007 Oviedo, Spain, pQueen Mary, Uni- versity of London, London, E1 4NS, England, qTexas Tech Univer- lider operating at 1.8 TeV, the Collider Detector at Fer- sity, Lubbock, TX 79409, rIFIC(CSIC-Universitat de Valencia), milab (CDF) collaboration made the first observation of 46071 Valencia, Spain, dijet production by DPE) [2] using an inclusive sample →+poster+by+J.+Chwastowski,+R.+Staszewski,+M.+Trzebiński++ PhysicsRoman+Pots+@+High=Luminosity+LHC+ with tagged protons - QCD ★ Exclusive Higgs: Possible+physics+cases+for+HL=LHC+‣ Completely novel (and so far unseen) production • Exclusive+Higgs+channel. Complementarity with inclusive modes. ‣ H b b : QCD gg b b background dynamically++ +++ – quantum+numbers+are+constrained+to+0! ! V.A. Khoze et al.+or+2 Phys. Lett. B suppressed. – produced+by+gluon+fusion,+constraints+on+401 330 (1997) Hbb+ ‣ Combined with proton tagging: handle on CP /spin- • New+physics+searches+parity analyser. – two=photon+processes+are+sensi've+to+new+physics+ ‣ Cross section O (fb) clear benefit from higher lumi. – clean+exclusive+events,+pp) +→+pγγp+ ‣ Other BSM Higgs scenarios? • Diffrac'on+ • Comprehensive Higgs measurement programme major factor of HL- – * high+LHCβ :+pile=up+prevents+measurements+of+so`+ programme. This would add completely new ingredient. diffrac'on+ – low+• Lowerβ*:+study+hard+diffrac'on+ mass acceptance highly desirable for jets and essential for Higgs detectors at & 300m needed. • HI+collisions:+) 11 – provides+centrality+based+on+spectators + +and+not+on+par'cipants+ – veto+for+γγ+(and+γ=nucleus)+processes+ + 10/31/2017+ C.+Mayer+ 4+ Lower luminosity running - ALICE Benchmark analyses Benchmark analyses:1 • ALICE - luminosity estimate of 200 pb• precision in scalar pp and by tensor end meson of spectroscopy run 4,including potential strangeonia and charmonia states (ππ,KK,4π,2π2K etc), partial wave analysis to increase in run 5. • gluonic jets (EPJC 76, 9 (2016)), two particle correlations and femtoscopy in CEP • glueball searches (including exotics like 0-- and 0+- at high masses) • All low pile-up broad programme• ofmagnetic lower monopole massand monopolium QCDsearches studies (arXiv:1707.04170) and spectroscopy anticipated.) Benefits from excellent PID, low thresholds and larger rapidity coverage of ALICE detector. • Benchmark analyses -

★ Precision scalar and tensor meson spectroscopy:

‣ Many decay channels ( ⇡⇡ , KK, pp... ), partial wave analyses. 3

‣ Probe of soft QCD dynamics.

‣ Expand knowledge of still poorly understood sector of QCD.

12 Glueball searches

• f0(1710) and f2(2300) are most promising 0++, 2++ glueball candidates • 0++, 2++ glueballs mix with qqbar mesons with similar quantum numbers – precise determination of branching ratios needed to determine gluonic content • Pseudoscalar glueball (0-+) expected ~2.6 GeV: – BR(G→KKπ)=49% according to PRD95 (2017), 014028

– KsK+π- and KsK-π+ might be promising in ALICE: Axε(|y|<1) is about 10% • Lower luminosityKsK+π- and KsK-π+ also promisingrunning for 0-- oddball -Glueball searches ALICE (2.8 GeVsearches in JHEP 1510 (2015) 137)

• Predictedf0(1710) and glueball f2(2300) arespectra most promising 0++, 2++ glueball candidates • 0++, 2++ glueballs mix with qqbar mesons with similar quantum numbers – precise determination of branching ratios needed to determine gluonic content • Pseudoscalar glueball (0-+) expected ~2.6 GeV: ★ Glueball searches: – BR(G→KKπ)=49% according to PRD95 (2017), 014028 – KsK+π- and KsK-π+ might be promising in ALICE: Axε(|y|<1) is about 10% • K K+π- and K K-π+ also promising for 0-- oddball searches (2.8 GeV in JHEP 1510 (2015) 137) ‣ s s As yet no clear evidence of states Predicted glueball spectra that QCD tells us should be there.

‣ Gluon rich CEP environmentLattice Holography ideal channel to look.

‣ Data on decays clear indication of Lattice Holography 5 gluball nature Charm5 sector → gluon PDFs σ(5.5 TeV) Y (5.5 TeV) Y(5.5 TeV) Y (14 TeV) Y(14 TeV) Channel BR A x ε σ(14 TeV) pb ★ Exclusive charmonia: probe of gluon PDFs, gg c modelling. Many pb 1 /pb 6 /pb 1 /pb 200 /pb ! χc0 → ππ 0.83% 25.2% 97933 205 1229 118120 247 49412 decay channels analysed. Realistic possibilities for c (0 , 2) . χc0 → KK 0.59% 20.5% 97933 118 711 118120 143 28573 χc0 → 4π 2.24% 9.40% 97933 206 1237 118120 249 49743

χc0 → 2π2K 1.75% 1.70% 97933 29 175 118120 35 7028 χ → ππ <0.10% 25.2% 968 0 <1 1009 0.3 <51 ★ c1 Other benchmarks: exclusive jets, magnetic monopole searches. χc1 → KK <0.10% 20.5% 968 0 <1 1009 0.2 <41

χc1 → 4π 0.76% 9.40% 968 1 4 1009 0.7 144 ★ LHCb? High luminosity + moderate pile-up + good hadron PID - χc1 → 2π2K 0.45% 1.70% 968 0 0 1009 0.1 15 χc2 → ππ 0.23% 25.2% 5779 4 22 7634 4.4 885 studies of e.g. quarkonia. Not clear will be possible. χc2 → KK 0.11% 20.5% 5779 1 9 7634 1.7 344 χc2 → 4π 1.07% 9.40% 5779 6 38 7634 7.7 1536

13 χc2 → 2π2K 0.89% 1.70% 5779 1 6 7634 1.2 231 • σ estimated with SuperChic2 generator • using lowest prediction with NNPDF3.0 and model 1 for the gap survival probability • caveat: Superchic2 predicts at least x3 higher cross section wrt LHCb preliminary • χc1 and χc2 states are suppressed due to Landau-Yang theorem and helicity-zero selection rule respectively

• Χc2 might be feasible with ~200 /pb • continuum background need to be estimated with available double gap data 6 Heavy Ion Collisions • Future possibilities not just limited to pp collisions - heavy ions can also play a role. Runs approved for Runs 3 + 4, possibilities being considered beyond that. • Variety of applications to BSM physics focus of recent (ﬁrst) workshop + ESPP document. Motivation for HIs beyond Run 4.

New physics searches with heavy-ion collisions at the LHC

Roderik Bruce1, David d’Enterria2, Albert de Roeck2, Marco Drewes3, Glennys R. Farrar4, Andrea Giammanco3, Oliver Gould5, Jan Hajer3, Lucian Harland-Lang6, Jan Heisig3, John M. Jowett1, Sonia Kabana7, Georgios K. Krintiras3, Michele Lucente3, Guilherme Milhano11,12, Swagata Mukherjee13, Jeremi Niedziela2, Vitalii A. Okorokov14, Arttu Rajantie15, and Michaela Schaumann1

1CERN, BE Department, CH-1211 Geneva, Switzerland 2CERN, EP Department, CH-1211 Geneva, Switzerland 3Centre for Cosmology, Particle Physics and Phenomenology, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium 4Centre for Cosmology and Particle Physics, New York University, NY, NY 10003, USA 5Helsinki Institute of Physics, University of Helsinki, FI-00014, Finland 6Rudolf Peierls Centre, Beecroft Building, Parks Road, Oxford, OX1 3PU, UK 7Department of Physics, University of Nantes, and SUBATECH, 4 rue Alfred Kastler, 44307 Nantes, France 11LIP, Av. Prof. Gama Pinto, 2, P-1649-003 Lisboa, Portugal 12Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001, Lisboa, Portugal 13III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany 14National Research Nuclear University MEPhI, Moscow, Russia 15Department of Physics, Imperial College London, London SW7 2AZ, UK

December 18, 2018

Abstract This document summarizes proposed searches for new physics accessible in the heavy-ion mode at the LHC, both through hadronic and ultraperipheral ““ interactions, and that have a competitive or, even, unique discovery potential compared to standard proton-proton collision studies. Illustrative examples include searches of new particles, such as axion-like pseudoscalars, radions, monopoles, dark photons, new long-lived particles, and sexaquarks as dark matter candidates; as well as new interactions, such as non-linear or non-commutative QED extensions. We argue that such interesting possibilities constitute a well-justiﬁed scientiﬁc motivation, complementing standard quark-gluon- plasma physics studies, to continue running with ions at the LHC after the Run-4, i.e. beyond 2030, including light and intermediate-mass species, accumulating nucleon-nucleon integrated luminosities 1 • Here exclusive (ultraperipheral) photon-initiatedin the accessible fb≠ range per month. production plays key role… 14 2 David d’Enterria for the CMS Collaboration / Nuclear Physics A 00 (2018) 1–4 Heavy Ion Collisions • Photon-initiated CEP equally possible in heavy ion

collisions. Indeed in some cases has significant advantages: Fig. 1. Diagrams of light-by-light scattering ( , left), QED dielectron ( e+e , center), and central exclusive diphoton ! ! (gg , right) production in ultraperipheral PbPb collisions (with potential electromagnetic excitation ( ) of the outgoing Pb ions). ! ⇤ ‣ 4 Significant Z enhancement in rate. After accounting forundetected differing at very low angles (Fig. 1, left). The dominant backgrounds are the QED production of an exclusive electron-positron pair ( e+e , Fig. 1 center) where the e are misidentified as photons, and ⇠ ! ± gluon-induced central exclusive production (CEP) [6] of a pair of photons (Fig. 1, right). Simulations of luminosities, still ~ 2 relative to pp , but with no pile-up. the light-by-light signal are generated with madgraph v.5 [7] Monte Carlo (MC) generator, modified [1, 8] + to include the nuclear fluxes and the elementary LbL scattering cross section [9]. Background QED e e events are generated with starlight v2.76 [10]. The CEP process, gg , is simulated with superchic ! ‣ QCD-initiated production essentially absent - clear interpretation.2.0 [11], where the computed pp cross section [6] is conservatively scaled to the PbPb case by multiplying it by A2R4, where A = 208 is the lead mass number and R 0.7 is a gluon shadowing correction in the g g ⇡ relevant kinematical range [12], and where the rapidity gap survival factor is taken as 100%. Given the ‣ Low pile-up - can go to low M . large theoretical uncertainty of the CEP process for PbPb collisions, the absolute normalization of this MC contribution is directly determined from a control region in the data. ‣ Conversely, steep fall off at high mass - pp essential here ) 2. Experimental measurement The measurement is carried out using the following detectors of the CMS experiment [13]: (i) the silicon complementary. dLe↵ 2 1 1 pixel and strip tracker measures charged particles within pseudorapidities ⌘ < 2.5 inside the 3.8 T magnetic [cm s GeV ] | | dM field, (ii) the lead tungstate crystal electromagnetic calorimeter (ECAL) and a brass and scintillator hadron 34 1 10 calorimeter (HCAL) reconstruct , e , and hadrons respectively over ⌘ = 3, and (iii) the hadron forward ± | | ⇥ P bP b, L =6 1027 cm 2s 1, ps =5.52 TeV calorimeters (HF) measure particle production up to ⌘ = 5.2. Exclusive diphoton candidates are selected | | 33 ⇥ with a dedicated level-1 trigger that requires at least two electromagnetic (e.m.) clusters with E above 1 10 pp, L =2 1034 cm 2s 1, ps = 14 TeV T ⇥ ⇥ 2 GeV and at least one HF detector with total energy below the noise threshold. O✏ine, photons and 1 1032 pp, RP 220 m electrons are reconstructed with the particle flow algorithm [14]. In the case of photons, to keep to a ⇥ minimum the e± contamination, we require them to be fully unconverted. Additional particle identification ⇡0 31 pp, RP 220 + 420 m (ID) criteria are applied to remove from high-pT decays, based on a shower shape analysis. Electron 1 10 candidates are identified by the association of a charged-particle track from the primary vertex with clusters ⇥ of energy deposits in the ECAL. Additional e± ID criteria discussed in Ref. [15] are applied. 1 1030 Charged and neutral exclusivity requirements are applied to reject events with any charged particles ⇥ with pT > 0.1 GeV over ⌘ < 2.4, and neutral particles above detector noise thresholds over ⌘ < 5.2. | | | | 29 Nonexclusive backgrounds, characterized by a final state with larger transverse momenta and larger diphoton 1 10 acoplanarities, A = (1 /⇡), than the back-to-back exclusive events, are eliminated by requiring ⇥ the transverse momentum of the diphoton system to be pT < 1 GeV, and the acoplanarity of the pair to be 28 + 1 10 A < 0.01. The same analysis carried out for the LbL events is done first on exclusive e e candidates, ⇥ with the exception that exactly two opposite-sign electrons, instead of exactly two photons, are exclusively 27 + 1 10 reconstructed. Figure 2 (top left) shows the acoplanarity distribution measured in exclusive QED e e ⇥ events passing all selection criteria (circles) compared to the starlight MC expectation (histogram). A good 1 1026 ⇥ 1 10 100 1000 15 M [GeV] Figure 1: Eective ““ luminosity as a function of photon-fusion mass in PbPb and pp collisions at the LHC, for their nominal ÔsNN and instantaneous luminosity values. In the pp case, the (actually "useable") ““ luminosity is also shown with proton tagging in RP detectors at 220 m (currently installed) and 420 m (proposed).

which to observe the photon-initiated production of BSM states with QED couplings. The cross section for the ““ production of any particle X can be calculated within the equivalent photon approximation [16]

d e ‡N N N XN = dx1dx2 n(x1)n(x2)‡““ X = dm““ L ‡““ X , (1) 1 2æ 1 2 æ dm æ ⁄ ⁄ ““ ‚ ‚ where xi is the longitudinal momentum fraction of the photon emitted by ion i. This factorizes the result in terms of a ““ X subprocess cross section ‡ of a (BSM) system X, and fluxes æ n(xi) of photons emitted by the ions. These latter objects are precisely determined in terms of the ion EM form factors, and are in particular enhanced by‚ Z2 for each ion, leading to an Ã overall Z4 enhancement in ion-ion collisions. The experimental signal for UPC processes is ≥ very clean, with the system X and nothing else produced in the central detector. Moreover, the virtuality of the emitted photons is restricted to be very small Q2 1/R2 ,whereR is the ion ≥ A A radius, and any produced object is almost at rest. In terms of the impact parameter b of the ‹ AA collision, ultraperipheral interactions with b 2RA lying beyond the range of additional ‹ ∫ strong interactions, are significantly larger than in the pp case, and the associated gap survival probability is also significantly bigger than for proton interactions. This eect can be accounted for precisely and enters at the (10 %) level in terms of corrections to ““ interactions, with O rather small uncertainties [17]. In addition, the background from QCD-initiated production is essentially completely removed by the requirement that the system X and nothing else is seen in the central detector [17]. A wide program of photon-photon measurements and theoretical work is ongoing in the context of pp collisions at the LHC [18], with dedicated proton taggers (Roman Pots, RP) installed inside the LHC beam-line about 220 meters away from the ATLAS [19] and CMS [20] interaction points. In comparison to the pp mode, HI collisions oer the distinct advantage of studying such photon-fusion processes in an environment where pileup is absent, forward tagging is unnecessary, and considerably lower masses can be probed. Indeed, the eective ““ luminosity in the HI case, due to the Z4 enhancement, is larger than that in pp collisions [21] ≥ at lower masses. In addition, two-photon collisions in pp collisions at high luminosity can only

4 Processes of interest • Many processes of interest in the lower mass region. Main channel of interest - ﬁnal-state.

★ Light-by-light SM signal, but also e.g. Born-Infeld extensions. ★ Axion-like particle production. ★ Magnetic monopoles. ★ Other possibilities? Gravitons, radions, unparticles, SUSY?

• A principle drawback of heavy ions for these studies is the low luminosity beneﬁt from increased datasets can be signiﬁcant. ) • Input from wider forward physics community could be crucial in 2 David d’Enterria for the CMS Collaboration / Nuclear Physics A 00 (2018) 1–4 supporting this.

Production mode BSM particle/interaction Remarks Axion-like particles ““ a, m 0.5–100 GeV æ a ¥ Radion ““ „, m 0.5–100 GeV Ultraperipheral æ „ ¥ Born-Infeld QED via ““ ““ anomalies æ Non-commutative interactions via ““ ““ anomalies æ Schwinger process Magnetic monopole Only viable in HI collisions Dark photon m 1 GeV, advanced particle ID Hard scattering AÕ . Long-lived particles (heavy ‹) mLLP . 10 GeV,improvedvertexing Thermal QCD Sexaquarks DM candidate

Table 1: Examples of new-physics particles and interactions accessible in searches with HI collisions at the LHC, listed by production mechanism. Indicative competitive mass ranges and/or the associated measurement advantages compared to the pp running mode are given. 16 a novel mode of operation with pPb collisions that was not initially foreseen. The excellent performance was made possible through many improvements in the LHC and the injector chain. In particular, the average colliding bunch intensity in 2018 was up to about 2.3 108 Pb/bunch, which is more than 3 times · higher than the LHC design value. For the next PbPb run in 2021, it is planned to further increase the + total LHC intensity through a decrease of bunch spacing to 50 ns, in order to fit 1 232 bunches in the Fig. 1. Diagrams of light-by-light scattering ( , left), QED dielectron ( e e, center), and central exclusive diphoton LHC. A further increase of the injected intensity seems dicult without additional hardware [1]. In the ! ! (gg , right) production in ultraperipheral PbPb collisions (with potential electromagnetic excitation ( ) of the outgoing Pb ions). LHC, any increase of ion luminosity is ultimately limited by the risk of quenching magnets, either by ! ⇤ secondary beams with the wrong magnetic rigidity created in the collisions [2–6] or by leakage from the halo cleaning by the collimators [7–9]. As a mitigation, it is planned to install additional collimators in the next long shutdown of the LHC (2019–2020) to allow higher luminosity and intensity [10–12]. Using the predicted beam and machine configuration, the future luminosity performance has been estimated for PbPb and pPb [13]. During a 1-month run, assuming that the instantaneous luminosity is leveled at the 27 2 1 1 current values around 6 10 cm≠ s≠ , the integrated luminosity per experiment is estimated to 3.1 nb≠ undetected at very low angles (Fig. 1, left). The dominant backgrounds are the QED production of an ex- 1 · 1 for PbPb and 700 nb≠ for pPb (without levelling), equivalent to NN luminosities of 0.15 fb≠ . LNN ¥ + In the presently approved CERN planning, it is foreseen to perform another four and a half PbPb runs clusive electron-positron pair ( e e, Fig. 1 center) where the e± are misidentified as photons, and 1 before the end of LHC Run-4 in 2029, accumulating 13 nb≠ in total. Furthermore, one short pPb run is ≥ ! planned, as well as one reference pp run. No further HI runs have so far been planned after Run-4. These gluon-induced central exclusive production (CEP) [6] of a pair of photons (Fig. 1, right). Simulations of plans are detrimental for the full exploitation of the BSM possibilities opened in HI collisions, that require as large integrated luminosities as possible. A revised proposal for Runs-3 and 4 and plans to extend the light-by-light signal are generated with madgraph v.5 [7] Monte Carlo (MC) generator, modified [1, 8] the LHC nuclear programme beyond Run-4 have been formulated [13]. The BSM physics possibilities + summarized here, complement and reinforce that scientific case. These studies involve more time spent to include the nuclear fluxes and the elementary LbL scattering cross section [9]. Background QED e e on pPb runs and also collisions of lighter nuclei, e.g. Ar, O, or Kr [13, 14]. As shown in Table 2, the latter starlight superchic have the potential to reach (2–15) higher NN luminosities, which would benefit any BSM search based events are generated with v2.76 [10]. The CEP process, gg , is simulated with ◊ ! on hard-scattering processes (Section 5), although the corresponding ““ luminosities (Section 3) would 2.0 [11], where the computed pp cross section [6] is conservatively scaled to the PbPb case by multiplying be reduced by a (Z /Z )4 factor. The estimated parameters for a range of lighter ions rely on the PbPb AA 2 4 assumption that the achievable bunch intensity Nb for a nucleus with charge number Z and mass number it by A R , where A = 208 is the lead mass number and Rg 0.7 is a gluon shadowing correction in the A can be scaled from the Pb bunch intensity as N (Z, A)=N (82, 208) (Z/82)p,wherethepower g b b ◊ ⇡ p =1.5 is estimated from previous experience of nuclear beams for the CERN fixed-target experiments relevant kinematical range [12], and where the rapidity gap survival factor is taken as 100%. Given the and the short run with Xe in the LHC in 2017 [15]. It should be noted that these estimates carry a significant uncertainty, since no attempts have been made experimentally at optimizing these beams large theoretical uncertainty of the CEP process for PbPb collisions, the absolute normalization of this MC for the LHC. Furthermore, the integrated luminosity per month in Table 2 has been calculated using 1 a simplified model, which gives slightly more optimistic values for Pb than the 3.1 nb≠ stated above, contribution is directly determined from a control region in the data. which was simulated with a more detailed and accurate model. Total integrated luminosities in the range 1 0.2–3 fb≠ are expected depending on the ion-ion colliding system. We stress that, if BSM or other physics cases eventually justify it, one can consider running a full “pp year” with ions at the LHC, leading to roughly factors of 10 larger integrated luminosities than those listed in Table 2. ◊ 2. Experimental measurement

2 The measurement is carried out using the following detectors of the CMS experiment [13]: (i) the silicon pixel and strip tracker measures charged particles within pseudorapidities ⌘ < 2.5 inside the 3.8 T magnetic | | field, (ii) the lead tungstate crystal electromagnetic calorimeter (ECAL) and a brass and scintillator hadron calorimeter (HCAL) reconstruct , e , and hadrons respectively over ⌘ = 3, and (iii) the hadron forward ± | | calorimeters (HF) measure particle production up to ⌘ = 5.2. Exclusive diphoton candidates are selected | | with a dedicated level-1 trigger that requires at least two electromagnetic (e.m.) clusters with ET above 2 GeV and at least one HF detector with total energy below the noise threshold. O✏ine, photons and electrons are reconstructed with the particle flow algorithm [14]. In the case of photons, to keep to a minimum the e± contamination, we require them to be fully unconverted. Additional particle identification 0 (ID) criteria are applied to remove from high-pT ⇡ decays, based on a shower shape analysis. Electron candidates are identified by the association of a charged-particle track from the primary vertex with clusters of energy deposits in the ECAL. Additional e± ID criteria discussed in Ref. [15] are applied. Charged and neutral exclusivity requirements are applied to reject events with any charged particles with p > 0.1 GeV over ⌘ < 2.4, and neutral particles above detector noise thresholds over ⌘ < 5.2. T | | | | Nonexclusive backgrounds, characterized by a final state with larger transverse momenta and larger diphoton acoplanarities, A = (1 /⇡), than the back-to-back exclusive events, are eliminated by requiring the transverse momentum of the diphoton system to be pT < 1 GeV, and the acoplanarity of the pair to be + A < 0.01. The same analysis carried out for the LbL events is done first on exclusive e e candidates, with the exception that exactly two opposite-sign electrons, instead of exactly two photons, are exclusively + reconstructed. Figure 2 (top left) shows the acoplanarity distribution measured in exclusive QED e e events passing all selection criteria (circles) compared to the starlight MC expectation (histogram). A good Tools for future studies - SuperChic MC

‣ QCD-initiated CEP. • A MC event generator for CEP processes. Common platform for: ‣ Photoproduction. ‣ Photon-initiated CEP.

• Previously generated pp collisions only, but recently updated to include pA and AA collisions. LHL, V. A. Khoze, M. G. Ryskin, arXiv:1810.06567 IPPP/18/90

Exclusive LHC physics with heavy ions: SuperChic 3

L.A. Harland–Lang1, V.A. Khoze2,3,M.G.Ryskin3

1Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom. 2Institute for Particle Physics Phenomenology, University of Durham, Durham, DH1 3LE 3Petersburg Nuclear Physics Institute, NRC Kurchatov Institute, Gatchina, St. Petersburg, 188300, Russia

Abstract We present results of the updated SuperChic 3 Monte Carlo event generator for central exclusive production. This extends the previous treatment of proton–proton collisions to include heavy ion (pA and AA) beams, for both photon and QCD–initiated production, the first time such a unified treatment of exclusive processes has been presented in a single generator. To achieve this we have developed a theory of the gap survival factor in heavy ion collisions, which allows us to derive some straightforward results about the A scaling of the corresponding cross sections. We compare against the recent ATLAS and CMS measurements of light–by–light scattering at the LHC, in lead–lead collisions. We find that the background from QCD–initiated production is expected to be very small, in contrast to some earlier estimates. We also present results from new photon–initiated processes that can now be generated, namely the production of axion–like particles, monopole pairs and monopolium, https://superchic.hepforge.orgtop quark pair production, and the inclusion of W loops in light–by–light scattering.

1 Introduction

17 Central Exclusive Production (CEP) is the reaction

hh h + X + h (1) ! where ‘+’ signs are used to denote the presence of large rapidity gaps, separating the system X from the intact outgoing hadrons h. This simple signal is associated with a broad and varied phenomenology, from low energy QCD to high energy BSM physics, see [1–6] for reviews. arXiv:1810.06567v1 [hep-ph] 15 Oct 2018 Consequently an extensive experimental programme is planned and ongoing at the LHC, with dedicated proton tagging detectors installed and collecting data in association with both ATLAS and CMS [7,8], while multiple measurements using rapidity gap vetoes have been made by LHCb and ALICE. CEP may proceed via either QCD or photon–induced interactions, see Fig. 1, as well as through a combination of both, namely via photoproduction. Although producing the same basic exclusive signal, each mechanism is distinct in terms of the theoretical framework underpinning it and the phenomenology resulting from it. The QCD–initiated mechanism beneﬁts from a ‘J PC =0++’ selection rule, permitting the production of a range of strongly interacting states in a precisely deﬁned gluon–rich environment, while also providing a non–trivial test of QCD 1Introduction

The use of diffractive processes to study the Standard Model (SM) and New Physics at the LHC has only been fully appreciated within the last few years; see, for example [1, 2, 3, 4], or the recent reviews [5, 6, 7], and references therein. By detecting protons that have lost only about 1-3% of their longitudinal momentum [8, 9], a rich QCD, electroweak, Higgs and BSM programme becomes accessible experimentally, with the potential to study phenomena which are unique to the LHC, and difficult even at a future linear collider. Particularly interesting are the so-called central exclusive production (CEP) processes which provide an extremely favourable environment to search for, and identify the nature of, new particles at the LHC. The first that comes to mind are the Higgs bosons, but there is also a potentially rich, more exotic, physics menu including (light) gluino and squark production, searches for extra dimensions, gluinonia, radions, and indeed any new object which has 0++ (or 2++)quantumnumbersand couples strongly to gluons, see for instance [2, 10, 11]. By “central exclusive” we mean a process of the type pp → p + X + p, where the + signs denote the absence of hadronic activity (that is, the presence of rapidity gaps) between the outgoing protons and the decay products of the centrally produced system X. The basic mechanism driving the process is shown in Fig. 1. There are several reasons why CEP is especially attractive for searches for new heavy objects. First, if the outgoing protons remain intact and scatter through small angles then, to a very good approximation, the primary active di-gluon system obeys a J =0,C-even,P-even, Exclusive production: theory z selection rule [12]. Here Jz is theSuperChic projection of the total angular MC momentum - processes along the proton generated beam axis. This selection rule readily permits a clean determination of the quantum numbers Recall formula for exclusive -initiated production in terms of EPA • of the observed new (for example, Higgs-like) resonance, when the dominant production is a photon fluxscalar state. Secondly, because the process is exclusive, the energy loss of the outgoing protons is directly related to the mass of the central system, allowing a potentially excellent mass resolution, irrespective of the decay mode of the centrally produced system.+ Thirdly+, in many pp pXp d EPA ★ W W , l l , LbyL, SM Higgs, dtopical ! cases, in particular, for Higgs boson production,RX a signal-to-background ratio of order 2 L2 ˆ( X) dMX dyX ⇠ dMX dyX ! 1 (or even better) is achievable [3, 11], [13]-[18]. In particular, dueALPs, to Jz = 0monopoles, selection, leading- monopolium. 2 order QCD b¯b production is suppressed by a factor (mb/ET ) ,whereET is the transverse energy ¯ < of the b, b jets. Therefore, for a low mass Higgs, MH ∼ 150 GeV, there is a possibility to observe • Why is this not an exact equality? Because we are asking for final state with intact protons, object X and nothing else- colliding protons may ★ SM Higgs, dijets, trijets, light interact independently: ‘Survival factor’. meson pairs, heavy quarkonia 16 (single and double), .

Figure 1: The basic mechanism for the exclusive process pp → p + X + p.ThesystemX is produced by the fusion of two active gluons, with a screening gluon exchanged to neutralize the colour. ★ Light to heavy vector meson V V production.k

(a) bare (b) screened

Figure 2: Schematic• diagramsIn all for cases the exclusive in pp photoproduction , pA and pr AA ocess ppcollisions.pV p with (a) → and without (b) screening corrections included. 18

3.2 Soft survival effects For photon–mediated processes, survival effects can be included exactly as described in Sec- tion 2.2, however some additional care is needed. From (10) we can see that it is the amplitude for the production process that is the relevant object when including these effects. On the other hand, (16) and (17) and the flux (12) are defined at the cross section level, with the squared amplitude for the photon–initiated subprocesses summed over the (transverse) photon polarisations. To translate these expressions to the appropriate amplitude level, it is important to include the photon transverse momentum q dependence in the appropriate way, corresponding ⊥ to a correct treatment of the photon polarisation, see [48]. To demonstrate this, we will only consider the FE term in (12) in what follows, but will comment on the contribution of the magnetic form factor at the end. Schematic diagrams for the bare and screened photoproduction amplitudes are shown in Fig. 2, with the relevant momenta indicated; for the bare

amplitude we have qi⊥ = pi⊥ , while for the screened we have qi⊥ = pi′⊥ , see Section 2.2. Using the same decomposition− that leads to (4), the photoproduction amplitude− corresponding to the ﬁgure behaves as

x + y + T (q ) q (A A−)+iq (A + A−) , (20) 1⊥ ∼ 1⊥ − 1⊥

where A± is the γp Vpamplitude for a photon of helicity, and if the photon is emitted → ± from the other proton we simply interchange 1 2. In the bare case (q1 = p1 )we ⊥ ⊥ simply square this, and after performing the azimuthal↔ angular integration, the cross− terms x y p1⊥ p1⊥ vanish and we have ∼ 2 2 T (p1⊥ ) p1⊥ σγp Vp , (21) | | ∼ → where σγp Vp is the subprocess cross section summed over the incoming photon transverse → polarisations. This is consistent with the FE term in (12) and with (16), and indeed a full

9 SuperChic MC - Recent Developments LHL, V. A. Khoze, M. G. Ryskin, arXiv:1810.06567 2 2 ps [TeV] 0 [mb] a [GeV ] b [GeV ] c 5.02 •146 Extension0.180 to heavy20.8 ion collisions-0.414 ﬁrst ever treatment of QCD-initiated 8.16 production.159 0.190 26.3 0.402 39 228 Requires0.144 detailed 23.3treatment0.397 of survival factor of no additional particle 63 •245 0.150 28.0 0.390 production in ion-ion QCD interactions. Table 1: The parameters of the one Relatively channel eikonal staightforward description of proton–protonin photon-initiated amplitude, case (~ no overlap), but described in the text. • −3 −4 great care needed in QCD-initiated case.where λC (A)istheComptonwavelengthoftheion.xmax is 4 10 ,3 10 , 1.4 10−4 for O, Sn, Pb ions, respectively. Here and also throughout× th×erest ⌦A A (b ) e 1 2 ? × 1 of the paper we use natural units, i.e.,h ¯ = c =1. 0.9 Pb-Pb 0.8 0.7 Z 0.6 0.5 b>R +R 12 0.4 0.3 Z 0.2 0.1 0 0 5 10 15 20 25 30 0 5 102R 15 20 25 30 b [fm] ? Fig. 1. A fast moving nucleus with charge Ze is surrounded by a strong electro- QED magnetic ﬁeld. This can be viewed as a cloud of virtual photons. These photons can Figure 3: Ion–ion opacity (left) and probability for no inelastic scattering (right) foroften lead–lead be considered as real. They are called “equivalent”or“quasireal photons”. In QCD the collision of two ions these quasireal photons can collidewitheachotherandwith collisions, as a function of the lead impact parameter b . 19 the other nucleus. For very peripheral collisions with impact parameters b>2R, ? this is useful for photon-photon as well as photon-nucleus collisions.

The collisions of e+ and e− has been the traditional way to study γγ collisions. For lower values of b . 2R ( Rp,n), where the colliding ions are overlappingSimilarly in impact photon-photon collisions can also be observed in hadron-hadron col- ? 2 parameter space, we can see that⇠ the probability is close to zero, while for larger blisions,2 seeR Fig.this 1. Since the photon number scales with Z (Z being the charge ?number& of the nucleus) such effects can be particularly large.Thisfactorof approaches unity, as expected. However we can see that this transition is not discrete,> 6000 (corresponding with to Au, Z =79)isthereasonwhythename“gold the probability being small somewhat beyond 2R, due both to the non–zero skin∼flashlight” thickness [9] has been used to describe very peripheral (AuAu) collisions at RHIC. of the ion densities and range of the QCD single–Pomeron exchange interaction. This will be missed by the approach that is often taken in the literature, namely to simplySimilarly to cuto the strong↵ electromagnetic field can be used as a source of photons to induce electromagnetic reactions in the second ion, see Fig. 1. Since the the cross in impact parameter space when b < 2R. Comparing to (18), we can seeion, whichthat is this hit by these photons, is moving in the collider frame the photon- ? corresponds to taking instead hadron invariant masses can become very high. In the rest frame of one of the ⌦(b)/2 ions (sometimes called the ”target frame”) the Lorentz factor of the other ion e = ✓(b 2R) . (33) 2 is given by γion =2γlab 1, where γlab is the Lorentz factor in the collider (cm) frame. The maximum− photon energy in this frame is 500 TeV for the LHC The value at which this would turn on is indicated in Fig. 3. As discussedand above, 600 GeV the for RHIC. more realistic results above turn on smoothly above 2R, and so will correspond to somewhat This high equivalent photon flux has already found many usefulapplicationsin suppressed exclusive cross sections (i.e. without secondary particle production) innuclear comparison physics [10], nuclear astrophysics [11,12], particle physics [13] (some- to this. For ultra–peripheral photon-initiated interactions, where the dominant contributiontimes called the “Primakoff effect”), as well as, atomic physics [14]. Previous to the cross section comes from b 2R, this will have a fairly small impact, butreviews as we of the will present topic can be found in [15–18]. ? see for QCD–initiated production this is no longer the case. The theoretical tool to analyze very peripheral collisions is the equivalent photon method. This method is described in Chap. 2. The equivalent photon

7 • Application of this - possible QCD-initiated BG to LbyL scattering. • Previously no MC available for this - in ATLAS/CMS2 analysesDavid scale d’Enterria superchic for the CMS Collaboration / Nuclear Physics A 00 (2018) 1–4 prediction for pp by: PbPb = pp A2 R4 · · Scale by Nuclear nucleon pairs shadowing (0.7)4 ⇠

• Full calculation - not the case. Not all ATLAS Collab., Nature Phys. 13 + Fig. 1. Diagrams of light-by-light scattering ( , left), QED dielectron ((2017)e e ,no.9, center), 852-858 and central exclusive diphoton ! ! ( ) nucleons (cangg participate, right) production in ultraperipheralin short-range PbPb collisions (with potential electromagnetic excitation ⇤ of the outgoing Pb ions). ! QCD interaction while leaving ions intact. Find:undetected at very low angles (Fig. 1, left). The dominant backgrounds are the QED production of an exclusive electron-positron pair ( e+e , Fig. 1 center) where the e are misidentified as photons, and ! ± gluon-inducedPbPb centralpp exclusive1/3 production (CEP) [6] of a pair of photons (Fig. 1, right). Simulations of the light-by-light signal areA generated with madgraph v.5 [7] Monte Carlo (MC) generator, modified [1, 8] + to include the⇠ nuclear · fluxes and the elementary LbL scattering cross section [9]. Background QED e e events are generated with starlight v2.76 [10]. The CEP process, gg , is simulated with superchic Find that QCD-initiated BG ! 2.0 [11], where the computed pp cross section [6] is conservatively scaled to the PbPb case by multiplying expectedit by A2 Rto4, wherebe negligible.A = 208 is the lead mass number and R 0.7 is a gluon shadowing correction in the g g ⇡ ! relevant kinematical range [12], and where20 the rapidity gap survival factor is taken as 100%. Given the large theoretical uncertainty of the CEP process for PbPb collisions, the absolute normalization of this MC contribution is directly determined from a control region in the data.

2. Experimental measurement

The measurement is carried out using the following detectors of the CMS experiment [13]: (i) the silicon pixel and strip tracker measures charged particles within pseudorapidities ⌘ < 2.5 inside the 3.8 T magnetic | | field, (ii) the lead tungstate crystal electromagnetic calorimeter (ECAL) and a brass and scintillator hadron calorimeter (HCAL) reconstruct , e , and hadrons respectively over ⌘ = 3, and (iii) the hadron forward ± | | calorimeters (HF) measure particle production up to ⌘ = 5.2. Exclusive diphoton candidates are selected | | with a dedicated level-1 trigger that requires at least two electromagnetic (e.m.) clusters with ET above 2 GeV and at least one HF detector with total energy below the noise threshold. O✏ine, photons and electrons are reconstructed with the particle flow algorithm [14]. In the case of photons, to keep to a minimum the e± contamination, we require them to be fully unconverted. Additional particle identification 0 (ID) criteria are applied to remove from high-pT ⇡ decays, based on a shower shape analysis. Electron candidates are identified by the association of a charged-particle track from the primary vertex with clusters of energy deposits in the ECAL. Additional e± ID criteria discussed in Ref. [15] are applied. Charged and neutral exclusivity requirements are applied to reject events with any charged particles with p > 0.1 GeV over ⌘ < 2.4, and neutral particles above detector noise thresholds over ⌘ < 5.2. T | | | | Nonexclusive backgrounds, characterized by a final state with larger transverse momenta and larger diphoton acoplanarities, A = (1 /⇡), than the back-to-back exclusive events, are eliminated by requiring the transverse momentum of the diphoton system to be pT < 1 GeV, and the acoplanarity of the pair to be + A < 0.01. The same analysis carried out for the LbL events is done first on exclusive e e candidates, with the exception that exactly two opposite-sign electrons, instead of exactly two photons, are exclusively + reconstructed. Figure 2 (top left) shows the acoplanarity distribution measured in exclusive QED e e events passing all selection criteria (circles) compared to the starlight MC expectation (histogram). A good my code, which I can’t work out at the moment as required by the Dirac quantisation condition 2⇡ g = N , (52) e where we take N =1,andg is the monopole charge. We also allow for the so–called g coupling scenario [22], for which we simply replace g g, where is the monopole velocity. In the monopolium case we apply the cross section! of [22], with the wave function of [23], and include the decay to two photons.

3.4 tt ! We include photon–initiated top quark production. This is implemented using the same matrix elements as the lepton pair production process, with the mass, electric charge and colour factors suitably modiﬁed. We ﬁnd a total cross section of 0.25 fb in pp collisions at ps = 14 TeV, and 36 fb in Pb–Pb collisions at ps =5.02 TeV.

4 Light–by–light scattering: a closer look

Evidence for the the production of light–by–light scattering in ultra–peripheral Pb–Pb collisions has been found by ATLAS [13] and more recently by CMS [24]. In both cases, the production of a diphoton system accompanied by no additional particle production is measured, while in the ATLAS case ZDCs are in addition applied to veto on additional neutral particle production in the forward direction, which would be a signal of semi–exclusive production accompanied by ion break–up. LbyL However,- Predictions in addition to the desired photon–initiated signal, there is the possibility that QCD–initiated diphoton production may contribute as an irreducible background. We are now in a position for the first time to calculate this background, using the results of Sec- • Predictions for ATLAS/CMStion LbyL 2.4. The event results forselection: the QCD–initiated background (both coherent and incoherent), as well as the prediction for the light–by–light signal, are shown in Table 2. We consider both the ATLASLbyL and CMSQCD event (coh.) selectionQCD in (incoh.) the centralA detectors.2R4 Namely, the produced photons are required to have transverse energy E > 2(3)GeVandpseudorapidity ⌘ < 2.4in ATLAS 50 0.008 0.05? 50 | | the case of CMS (ATLAS). We show results before and after further cuts on the diphoton [nb] ATLAS (aco < 0.01, p < 2 GeV) 50 0.007 0.01 10 ? system p and acoplanarity (1 /⇡)areimposed,whicharedesignedtosuppressthe CMS ?103 0.03 0.2 180 non–exclusive background. CMS (aco < 0.01, p < 1 GeV)For the102 light–by–light0.02 signal cross sections0.03 are fully30 consistent with the ATLAS and CMS ? results: Table 2: Predicted cross sections, in nb, for diphoton final states within the ATLAS [21] and ATLAS =70 24 (stat.) 17 (syst.)nb, (53) CMS [22] event selections, in Pb–Pb collisions at ps =5.02 TeV.± That is,± the photons are required to have transverse energy E > 2 (3) GeV and pseudorapidityCMS =122 46⌘ (stat <.)2.4,29 while (syst.) in the4(th.)nb. (54) ? ± | | ± ± CMS caseQCD-initiated an additional cut BG of m negligible! > 5 GeV is imposed.1 Resultsd with and without an additional We find that the200 QCD–initiated daco.() background is expected to be very small. In particular, acoplanarity cut aco < 0.01, and cut on the combined transverse momentum p < 1 (2) GeV in ! the coherent contribution,180 which represents? the truly irreducible background to exclusive the CMS (ATLAS) case are shown. The cross sections for the light–by–light scatteringQCD (LbyL) - incoh. 160 and In QCD–initiated addition: photonsuppress pair production,further with in both the coherent and incoherent cases, areQCD given. - coh. • 140 16 The result of simply scaling the pp cross section (including the pp survival factor) by A2R4 with acoplanarity cuts. 120 R =0.7 is also shown. 100 80 60 light signal, are shown in Table 2. We consider both the40 ATLAS and CMS event selection in the central detectors. Namely, the produced photons are required20 to have transverse energy E > 2 0 ? (3) GeV and pseudorapidity ⌘ < 2.4 in the case of CMS00 (ATLAS),.01 0.02 0. while03 0.04 for 0. CMS05 0.06 an 0 addition.07 0.08 0.09 0.1 | | acoplanarity. cut of m > 5 GeV is imposed. We show results before and after further cuts on the diphoton system p < 1(2) GeV for CMS (ATLAS) and21 acoplanarity (1 /⇡ < 0.01) are imposed, which are? designed to suppressFigure the 8: non–exclusive Normalized background. di↵erential cross sections for exclusive and semi–exclusive diphoton For the light–by–light signalproduction the predicted with respect cross sections to the are diphoton fully consistent acoplanarity. with the The ATLAS QED–initiated and QCD–initiated and CMS results: (both coherent and incoherent) processes are shown.

ATLAS = 70 24 (stat.) 17 (syst.)nb, (57) ± ± CMSwhere= 120 we have46 (stat used.) that28 (systy =.) y 4(th= y.)nband.z = z = z. We(58) then have ± ± 1 ±2 1 2 2 2 2 ri Ri y +z xi y xi On the other hand, we ﬁnd that the QCD–initiated background is expected to be very2 small.R d In 2R d T (bi )= dz ⇢(r) ⇢0 dze d ⇢0 dze i e d = ⇢0 2⇡Ride i e d . particular, both the incoherent and? coherent contributions⇡ are expected⇡ to be negligible, even before imposing additional acoplanarityZ cuts. Z Z p (56) We can see that incoherentIn what background, follows, we which will we consider recall corresponds for simplicity to thea point–like case that QCD the interaction. In other words, colliding ions do not remainin intact, the case is further of exclusive suppressed production by the additional for the ion–ion acoplanarity opacity and wep have cuts; as we would expect, due to the broader p spectrum of the incoherent cross section. This? is seen more clearly in Fig. 9, which shows the (normalized)? acoplanarity2 2 distributions in the three ⌦A1A2 (b )= d b1 d b2 TA1 (b1 )TA2 (b2 )Ann(b b1 + b2 ) , (57) ? ? ? ? ? ? ? ? cases. We can see that the QED–initiated process is stronglyZ peaked at low acoplanarity (< 0.01), as is the coherent QCD–initiated process, albeit withnn a somewhat2 broader distribution due tot d b1 TA1 (b1 )TA2 (b1 b ) , (58) to the broader QCD form factor in this case. On the⇡ other hand, for? incoherent? QCD–initiated? ? production we can see that the spectrum is spread quite evenlyZ over the considered acoplanarity region. which is valid as the range of QCD amplitude Ann is much smaller than the extent of the ion It was suggested in [48]transverse that to calculate densities. the QCD–initiated In setting the background, normalization understood we have to used be (26). For the case of semi– exclusive production, we simply replace nn , see (28). As we are only interested in the dominant incoherent part, we can simply scale the corresponding pp crosstot ! sectioninel. by a factor of A2R4,whereR 0.7 accountsthe overall for nuclear scaling shadowing with A, ewe↵ects. will As for discussed simplicity in Section assume 2.4,nn this nn, and work with the latter ⇡ inel ⇠ tot A2 scaling is certainly farvariable too extreme, in what due follows; to the short–range however, nature strictly of speaking the QCD interaction this replacement should be made when ⇠ and corresponding requirementconsider that semi–exclusive only peripheral interactions production. can lead to exclusive or semi– exclusive production. In addition, we note that as the dominant contribution in this case will Following the same logic, we simply take TA0 TA for the incoherent case, see (35), and come from nucleons situatedapply close (43) to for the the ion coherent, peripherary, corresponding where the nucleon to a point–like number⇡ density QCD interaction in the unscreened is relatively low, we can expectcross section shadowing calculation. e↵ects to We be minimal, now consider and hence the proton–ion we are justiﬁed and ion–ion cases in turn. in using the standard proton PDF in the calculation of the CEP cross section. Nonetheless,

A.1 Proton–ion18 collisions (2) ~ In this case, we take TA2 (b )= (b ), so that the opacity simply becomes ? ? x x nn nn 1/2 d d ⌦pA(b )=totTA(b ) tot⇢0(2⇡Rd) e ˜e . (59) ? ? ⇡ ⌘ 19 SuperChic MC - Recent Developments

• See Valery’s talk - prospects for compressed SUSY searches in photon- initiated CEP. • Signal for this study: + + ˜l ˜l l l˜ ˜ ! ! 0 0 implemented in SuperChic, as well as more challenging case: + ˜ ˜ (hadron/leptonic decays) ! • To be included in ofﬁcial version early in new year. LHL et al., arXiv:1812.04886 • In addition, contribution from proton IPPP/18/103 dissociation included in effective way, LHC Searches for Dark Matter in Compressed Mass Scenarios: Challenges in the Forward Proton Mode interfaced to SuperChic. L.A. Harland-Lang1∗, V.A. Khoze2,3†, M.G. Ryskin3‡ and M. Tasevsky4§ • Future work: include (more) complete 1Rudolf Peierls Centre, Beecroft Building, Parks Road, Oxford, OX1 3PU, UK 2IPPP, Department of Physics, University of Durham, Durham, DH1 3LE, UK 3Petersburg Nuclear Physics Institute, NRC “Kurchatov Institute”, Gatchina, St. Petersburg, 188300, treatment of dissociation. Stay tuned! Russia 4Institute of Physics, Czech Academy of Sciences, CS-18221 Prague 8, Czech Republic

Abstract 22 We analyze in detail the LHC prospects for charged electroweakino searches, decaying to leptons, in compressed supersymmetry scenarios, via exclusive photon-initiated pair production. This provides a potentially increased sensitivity in comparison to inclusive channels, where the background is often overwhelming. We pay particular attention to the challenges that such searches would face in the hostile high pile–up environment of the LHC, giving close consideration to the backgrounds that will be present. The signal we focus on is the exclusive production of same-flavour muon and electron pairs, with missing energy in the final state, and with two outgoing intact protons registered by the dedicated forward proton detectors installed in association with ATLAS and CMS. We present results for slepton masses of 120–300 GeV and slepton–neutralino mass splitting of 10–20 GeV, and find that the relevant backgrounds can be controlled to the level of the expected signal yields. The most significant such backgrounds are due to semi–exclusive lepton pair production at lower masses, with a proton produced in the initial proton dissociation system registering in the forward detectors, and from the coincidence of forward protons produced in pile-up events with an inclusive central event that mimics the signal. We also outline a range of potential methods to further suppress these backgrounds as well as to enlarge the signal yields. arXiv:1812.04886v1 [hep-ph] 12 Dec 2018

1Introduction

One of the main goals of the physics program at the LHC and future colliders is the search for beyond the Standard Model (BSM) physics. One well–motivated, and much explored, candidate among the existing BSM scenarios is the Minimal Supersymmetric Standard Model (MSSM), see e.g. [1–4] for reviews. This in particular oﬀers a natural candidate for cold Dark Matter

∗email: [email protected] †email: [email protected] ‡email: [email protected] §email: [email protected] Summary/Points for Discussion/Wishlist

★ Clear physics case for forward physics @ HL-LHC: access regions that are highly challenging in conventional channels, open completely new Higgs observation channel with RPs@420m, sensitive to novel QCD observables…

• Experiment: Detector improvements we need for HL-LHC running: improved timing in RPs (O(few ps) ideally), timing in central detectors, radiation hard ZDCs + timing. What is feasible? • Theory: complete account of proton dissociation (in progress)… • 420m RPs- clear/new/distinct physics case vs. ~220m. Push for this? • Expanding physics case. Identify further challenging areas for conventional channels. Thank you for listening! 23