PoS(EPS-HEP2015)102 http://pos.sissa.it/ -boson, there W -tagged jet is required. We show b ]. 1 ∗ [email protected] Speaker. is currently an unconstrainedmasses region around in 100 the GeV. In stop- thisoptimized mass note for we plane, the will four-body still propose decay mode, allowing athat, in new for which monojet-like already at stop search by least for one using light theunconstrained stops, region. existing This 8 note is TeV based LHC on data ref. [ set, such a search would cover the entire The LHC searches for light compressedcase stop squarks where have the resulted stop in decays considerablestop bounds to in decays a the to neutralino and a a neutralino, charm a . bottom However, quark in and the two case where via the an off-shell ∗ Copyright owned by the author(s) under the terms of the Creative Commons c The European Physical Society Conference on22-29 High July Energy 2015 Physics Vienna, Austria Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). Christoffer Petersson Physique Théorique et Mathématique, Université LibreC.P. 231, de 1050 Bruxelles, Brussels, Belgium, International Solvay Institutes, Brussels, Belgium, Department of Fundamental Physics, Chalmers University412 of 96 Technology, Göteborg, Sweden. E-mail: Proposing a new LHC search forstop light squarks compressed PoS(EPS-HEP2015)102 . 0 1 ˜ χ + 0 , which f 1 ˜ t + f . The current 0 1 + ˜ χ b m → − Christoffer Petersson , the stop decays via 1 1 ˜ t t . ˜ t 1 m m = > m m ∆ ∆ -quark from the off-shell top, and . We distinguish three main kine- b 0 1 ˜ χ -tagging, to test the compressed region b , the two body decay is precluded and the stop t m 2 is forced to be off-shell, i.e. - and the < W W m ∆ 200 GeV, but this triangle has been essentially closed by , while the second option is the logical extension of the < 0 < 1 b ˜ 1 χ , the above channels are closed and the stop decays to the ˜ t m b m + m + c < ] – for a neutralino lighter than 250 GeV, stop masses below 600– + W 3 m , → W 2 1 m ˜ t < m plane are represented by the dashed curves in Figure ∆ ]. The current data exclude stop masses up to roughly 300 GeV in the central 0 1 ˜ 6 χ < , ]. m b 5 , 4 , 1 m ˜ t 3 m ). To cover this unconstrained region is clearly important from the point of view of the T / E production [ Lastly, for In the intermediate region In the case when the mass difference is larger than the top mass, With the discovery of a -like , the so called is The LHC searches have placed stringent limits on the masses of several , includ- The simplified model we consider involves only the lightest of the two stop squarks, ¯ t t This is perhaps the most difficult regionwith to investigate model since there dependent are branching two ratios. competing decay The channels model dependency is mainly due to the first process, decays via a three body process involving the region of this domain. the neutralino [ neutralino and either one orthe three one-loop light Standard decay Model process fermions.decays discussed The above, first where option now corresponds even to the 750 GeV are excluded. Up untilmassless recently, there neutralino and was 180 a GeV unconstrained triangularthe region for constraint an on almost the stop productionof cross section coming from the spin correlation measurement decays to the lightest supersymmetric ,matical regions, the characterized neutralino by the stop-neutralinobounds mass in difference the the two-body decay to a top quarkon and the the stop neutralino. mass This are is attained the [ region where the strongest bounds more sharply defined than ever.Higgs The mass top parameter, quark introducing gives rise aStandard to quadratic Model. the sensitivity leading to If quantum the the correction scalethe to scale of parameters the of new of physics new the beyond physics newalmost the is physics exactly significantly are cancel, above so the this electroweak fine-tunedHiggs correction mass scale, that destabilizes parameter unless the the is quadratically sum electroweak sensitive ofcorresponding scale. to to the all the mass In the scale of of , quantum the new top the corrections should physics. quark be In , around order the or to stop, below evade the the need electroweak for scale, fine-tuning, and the hence stop be mass observableing at the the stop. LHC. However,mainly there due exists to a the particularly region challenging ining final to which states compressed light that spectra are stops that produced are giveenergy in rise ( still this to region, unconstrained. soft correspond- This andhierarchy is a problem. small amount In of this missingrun, transverse note, we after propose a a new brief monojet-like review analysis, of exploiting stop squark searches after the first LHC Proposing a new LHC search for light compressed stop squarks 1. Introduction in which the stop decays via a four body decay. 2. Current bounds on light compressed stops PoS(EPS-HEP2015)102 ) 0 1 < ˜ → χ T 1 340 ˜ + t p , 0 f < -tagging + 340 b ( f , ) + ], the ATLAS 110 GeV GeV, b 9 220 , of the leading jet region where the , . 3 (left) summarizes → 1 b ˜ T t Christoffer Petersson 1 1 ˜ t p m 280 m . The very nature of T = . / E ]. Under this assumption, m ) = ( 8 ∆ T , / 7 E , 1 j T p -tagged jet with 30 GeV ] are sensitive for larger values of ( b 9 , 3 ]. 7 ], in which 100% BR is assumed. However, under 3 -tag and will not be considered further. The remaining -tag requirement on one of the (at most) three jets. 9 b 0 b 1 , ˜ 7 χ range, the ATLAS calibration algorithm for , 3 + η c → and 1 ˜ t T p 8 and an azimuthal angular separation between these jets and the ]. . 2 10 < | ], introduced in the above discussion, cover complementary kinematical η 9 | ] is basically a monojet search in which at least one hard jet is required , 7 7 , 3 5. In the above ] and LEP [ . 7 2 < ] ATLAS performed the search for pair produced compressed stop squarks that our | 4. The difference between the three regions is then based on the 7 in the event. The values for these last two cuts are . 30 GeV and η 0 | T / > E > T , assuming this decay mode to have 100% branching ratio (BR) [ φ p 0 1 After a preselection that includes a veto, the three ATLAS monojet signal regions are In ref. [ The searches [ We are now ready to discuss the targeted region of our proposal. Figure The majority of searches in the most squeezed region have targeted the decay mode More specifically, we require the presence of at least one The different signal regions are optimized for different regions of parameter space. Since we ∆ ˜ χ . This is so because these searches require the presence of one or two , respectively, in , T m + / first characterized by a common setwith of selections criteria, namely the presence of at most three jets monojet search [ three (M1, M2 and M3)four-body are decay monojet-like mode searches we that are target interested both in. the charm decay mode and the proposal is trying to improve(C1 upon. and C2) They target considered the decay a mode total of five signal regions. Two of them E c stop masses below 250 GeV have beenhas excluded. been The targeted four by body the decay ATLAS searches process [ the status of searches forinto a light four-body stops final in state.bounded the There by squeezed the is region exclusion an with curves unconstrained from the region the for assumption ATLAS one 80 of and GeV a two lepton 100% searches BR [ 3. Proposal for a monojet search with a and the these assumptions, there is still a small unconstrained region at stop massesregions. around 100 The GeV. search [ in order to have a sufficient recoil that gives rise to a large amount of Proposing a new LHC search for light compressed stop squarks which involves the masses ofstructure. all the superpartners that enter the loop as well as the squark flavor ∆ the final state that needreconstruction to of soft be leptons sufficiently using hardsearch. fast to detector Similarly, be simulation, the reconstructed. proposal it of isleptons. Due its in For to improvement general this the suffers reason difficult we from difficulties to do the in recastchosen not difficulty to this the attempt focus of to on dealing recast improving and with the improve soft former this search latter strategy search. [ Instead we have the selection criteria is suchstop that decay the products search are is soft. most On sensitive the near contrary, the the searches [ 300 GeV, are interested in targeting the low-mass regionOur we proposal only consider is the quite M1 straightforward selectionslow in and mass the can region following. be can be summarized improved by by saying adding that a the sensitivity to the and (450,450) GeV in the signal regions M1, M2 and M3 respectively. PoS(EPS-HEP2015)102 ], at 1 -tag 12 − b . The 960 64 decays 6 [ ν ± ` -tagging 3fb Z ± . b → 250 GeV and YTHIA W ] for a discussion. ≤ 1 80 GeV. As men- 1 ˜ ], P t Christoffer Petersson 720 come from SM ≤ m 301 33450 11 m ± ± ≤ ∆ 5 [ background and thus the ¯ t ≤ t ] is used throughout the sim- RAPH 18 99 565 G 17 - 767 , ± -tag in rejecting invisible ± AD b 17 -tag. ] while for the few points below 100 of 8 TeV LHC data. The background is b Dibosons Others Total 1 19 -tag". − ] are now subleading with respect to the b 7 ) error. The error in the M1 case is simply taken 337 650 7 55 ν ` ± ± σ → ( 4 W the 1 B δ 960 background events, 17400 337 come from the processes involving ]. In the fast detector simulation we used the standard ) ± ± -jets, is only mildly affected by the extra cut. The signal, 720 14100 22 144 4 and MLM matching [ b . 15 νν ± ± 0 -tag region we quote twice the relative error, see [ → 3 [ b ( = R ∆ ] to determine the main slope and fixed the absolute normalization and 14100 20 ELPHES 73 17400 57 261 [ νν ¯ t Z ± ± ] with t -tag is expected to improve the sensitivity to the signal. For 20 b → 16 780 307 Z ] and D being the central value and summarizes the expected leading backgrounds with and without the 200 GeV in steps of 10 GeV inside the region 10 GeV ]) ROSPINO 14 B 7 decays. The two leading backgrounds in [ 1 , , ≤ B 0 1 W 13 conveys the idea of the discriminating power of the ˜ δ χ algorithm [ 1 m ± Estimated numbers of background events with 20.3 fb t 3 [ -tag k B -tag efficiency we get when passing from M1 to M1+ ≤ b b ET J After imposing the M1 cuts we again found agreement within 20% with ATLAS. We used the Table Looking at the background estimations by ATLAS for the M1 signal region it is easy to see In order to reproduce as faithfully as possible the ATLAS situation we used the following Turning now to the signal, we simulated a grid of points with 70 GeV We simulated both signal and background by using M M1+ M1 (ATLAS [ Background background which, since it contains AST -tagging, we normalize the expected number of events to the ATLAS numbers and only multiply ¯ t F same normalization procedure as for the background, normalizing the number of events before the and leptonic GeV, we used P with the ATLAS values above 100 GeV. why the addition of a strategy. We generated, fully independently, a largethrough sample the of same background events cuts andcentral as passed them those values performed for by allconfidence ATLAS the in that backgrounds the M1. remaining to From backgroundthe be sample this cuts. within is analysis, 20% representative Since we of we of foundb the are the the physical only ATLAS situation interested results. after in theby the This further improvements gives in us efficiency arising from the is data-driven, thus reducing theIn systematic what follows uncertainties we coming denote from the Monte new signal Carlo region simulations. by “M1+ 8 TeV, out of a total of expected 33450 ATLAS detector specification. Thethe PDF anti- set used isulation. the CTEQ6L1, Table jets arerequirement. reconstructed using 0 GeV tioned above, we only considered stop100 pair GeV production we as used production the mode. NLO+NLL cross For sections stop of masses ATLAS above [ as will be discussed below, is found to behave in a similar way as the Table 1: Proposing a new LHC search for light compressed stop squarks given as from ATLAS. For the error in the M1+ processes involving key point is that both of these leading backgrounds are dramatically reduced by the extra overall sensitivity is significantly improved. requirement. t PoS(EPS-HEP2015)102 . 1

B

)

1

1

0

χ 1

/

χ 0

∼ 350 ˜ ∼ t

cuts S

1 + m

b ˜ + m t t

T + m

=

m W

1 /

t E →

∼

= m m 1

t

∼ 300

m

1

pp

0 χ

∼

( and

+ m b σ T

/ =

m p

1 . Notice that the .

t

∼

1 )) 250 m B ]). is our main result. δ Christoffer Petersson 21 1 96 [GeV] . 1 1 ∼ t 200 m > 200 GeV N > T 150 p ( j ] (see also ref. [ ] are broader since they take into 1 1 ˜ -tag efficiency to obtain the final re- t 7 LEP(hep-ex/0204036) M1+b-tag M1+b-tag(+20%δB) ATLAS(1506.08616) 2L ATLAS monojet(1407.0608) ATLAS(1407.0583) 1L ATLAS(1403.4853) 2L 1 b ˜ t 100 → 0 50 pp

200 150 100 300 250

(

1 χ

] [

GeV m 0 ∼ σ = 5

100 GeV. Right: The main result from our proposed search

1

1

0 χ

χ 0

∼ 350

∼

<

+

m 200 GeV

b + m t 1

˜ t

> + m

= m

W T 1 m t

p

∼

= m

m

1 ε

8 TeV data. In fact, we tested changing the t

∼ 300

m

1

1

0 χ ∼

−

+

m 200 GeV. The ratio of the LO cross sections obtained is used to cut: b

=

m T

1 3fb t >

.

∼ p

250 m T p [GeV] 1 ∼ t 200 m -tag requirement on those regions as well but the low signal efficiency makes them 150 b LEP(hep-ex/0204036) M1 ATLAS(1407.0583) 1L ATLAS(1403.4853) 2L ATLAS(1506.08616) 2L ATLAS monojet(1407.0608) Left: Existing limits in the stop-neutralino mass plane, superimposed with the M1 exclusion curve 100 -tag", (red solid curve), with the change induced by increasing the background error by 20 % (red b We set limits by excluding points for which the number of events With an eye to the Run-2 of LHC it is worth studying if a search of this type can be further 0 50

100 250 200 150 300

-tag requirement to the ATLAS numbers and using only the

1 χ

] [ GeV m analysis is essentially all driven by systematics and not by statistics. In the left panel of Figure The one-jet unmatched sample iscedure then on a used number throughout of thematched points sample. analysis. and we We have have found checked good this agreement pro- with the results from the fully within the intervals defined by M1 and M2 and we found that M1 essentially gives the best not viable, at least for the 20 optimized to cover an even larger region. Here we conclude by pointing out that it may be possible account the signal regions M2 andimposing M3 the that become more relevant for large masses. We have tested ratio within statistical fluctuations. The red curve in the right panel of Figure we present, for comparison, the(red exclusion exclusion curve). limits We we are able obtainvalidates to repeating reproduce the the their M1 exclusion analysis boundaries analysis on fairly of accurately, the which ATLAS right. Note that the limits in [ estimate the efficiency of the “M1+ dashed curve). This figure is an updated version of the figure in ref. [ b sults for the signal. The mainlarge difference number with of the points previous on background the simulation grid is andsignificant the that, fully small due matched efficiencies, to sample we the are of unable events. towe We generate thus generate a resort two statistically to exclusive samples, the one followingexactly strategy. containing For one zero each jets such point at jet the with parton level and one containing It fully covers the so far unconstrained region exposed in the left panel of Figure Figure 1: as obtained in our analysis and extended to Proposing a new LHC search for light compressed stop squarks 0 ∼ PoS(EPS-HEP2015)102 from CMS T , D 90 / E Phys. Rev. Lett. (2014) 124 Phys. Rev. , Christoffer Petersson CMS-PAS-SUS-13-009 1406 collisions with the ATLAS 8 TeV with the ATLAS = pp JHEP s √ ]. CMS Note ]. ]. 8 TeV = no.~12, (2013) 2677 Inspire s Inspire .[ √ .[ HepData C 73 collisions at ][ ]. pp of Inspire 6 1 -jet however already dramatically cuts the QCD Inspire − b .[ ]. .[ Eur. Phys. J. , “Exclusion limits on and top-squark pair production , “Search for top-squark pair production in the single-lepton , “Search for top squarks decaying to a and a Inspire 8TeV with the ATLAS detector”, , “Search for pair-produced top squarks decaying into a charm , “Search for top squark pair production in final states with one , “Search for direct top-squark pair production in final states with , “Measurement of Spin Correlation in Top-Antitop Quark Events arXiv:1412.4742 et al. et al. et al. arXiv:1407.0583 = , s = 8 TeV”, et al. et al. et al. ]. ]. et al. s √ (2014) . [ √ Inspire Inspire arXiv:1407.0608 (2014) 118 .[ .[ 4. This is necessary to reduce the QCD jet contamination and , . arXiv:1502.01721 (2015) 142001, 0 , > 1411 ]. 114 φ ∆ , JHEP T Inspire / Collaboration, G. Aad Collaboration, G. Aad Collaboration, G. Aad Collaboration, G. Aad E Collaboration, S. Chatrchyan Collaboration, S. Chatrchyan Collaboration, S. Chatrchyan CMS-PAS-SUS-14-011 (2015) 201801 quark and the lightest with 20.3 fb final state in pp collisions at isolated lepton, jets, and missing transverse momentum in two leptons in pp collisions at 114 Note (2013) . [ ATLAS in natural SUSY scenarios with inclusive razor and exclusive single-lepton searches at 8 TeV.”, arXiv:1308.1586 arXiv:1403.4853 neutralino in events with a jet and missing transverse momentum”, CMS ATLAS ATLAS CMS ATLAS CMS detector”, and Search for Top Squark PairPhys. Production Rev. in Lett. pp Collisions at s=8 TeV Using the ATLAS Detector”, detector at the LHC, ATLAS-CONF-2013-068, ATLAS-COM-CONF-2013-076”, no.~5, (2014) 052008 I would like to thank G. Ferretti, R. Franceschini and R. Torre for the collaboration on the [7] [2] [3] [4] [5] [6] [8] [1] G. Ferretti, R. Franceschini, C. Petersson and R. Torre, “Spot the stop with a b-tag”, Proposing a new LHC search for light compressed stop squarks also to move in thesearch. opposite The direction M1 and ATLAS relaxjets cut some and flow of the was the heavily constraints relying imposed on by the the isolation current requirement between the jet mismeasurements. The requirement of a project I discussed here.the This contract work 637-2013-475, is by supportedand IISN-Belgium by (conventions by 4.4511.06, the the 4.4505.86 Swedish “Communauté andd’Impulsion Research Française Scientifique" 4.4514.08) Council de of Belgique" (VR) the F.R.S.-FNRS. through under the ARC program and byReferences a “Mandat background and could be consideredthe uncertainties an involved in alternative calculating to the QCD thefrom background former with making our requirement. any simulation tools, estimates However, weencourage due of refrain the to the experimental collaborations potential to also gain consider in this sensitivity. possibility. Nevertheless, weAcknowledgments would like to PoS(EPS-HEP2015)102 , ]. , JHEP (2006) Inspire up to PoS JHEP (2006) .[ s (2008) 063 √ B 641 0605 0804 , (2012) 1986 Christoffer Petersson JHEP JHEP C 72 Phys. Lett. collisions at − ]. ]. e presented at the Fermilab (2007) 013 + e Inspire Inspire 0701 jet-finder”, .[ .[ Eur. Phys. J. t k ]. JHEP hep-ex/arXiv:1506.08616 Inspire jet clustering algorithm”, t .[ k ]. ]. myth for the 7 3 N . hep-ph/9710451 hep-ex/0204036 Inspire Inspire , , .[ .[ , “Search for scalar in ]. , “ATLAS Run 1 searches for direct pair production of et al. ]. et al. ]. ]. Inspire ]. (1998) 3–14 (2002) 5–20 .[ Inspire arXiv:1506.00604 .[ Inspire Inspire Inspire B 515 B 537 .[ .[ .[ arXiv:1307.6346 arXiv:1106.0522 , , (2015) 076, Phys. Lett. Nucl. Phys. ]. Collaboration, A. Heister Collaboration, G. Aad hep-ph/0512210 , (2011) 128 (2014) 057 hep-ph/0603175 , Webpage third-generation squarks at the Large Collider”, CORFU2014 evolution for top-quark production in hadronic collisions”, colliders”, ME/MC Tuning Workshop, October 4, 2002 hep-ph/0611129 ALEPH arXiv:0802.1189 [ arXiv:1111.6097 “DELPHES 3, A modular framework for fast simulation of a generic collider experiment”, 209-GeV”, 1106 026 57–61 1402 ATLAS [9] [21] G. Ferretti, R. Franceschini, C. Petersson and R. Torre, “Light stop squarks and b-tagging”, [18] M. L. Mangano, M. Moretti, F. Piccinini and M. Treccani, “Matching matrix elements and shower [20] W. Beenakker, M. Krämer, T. Plehn, M. Spira and P. M. Zerwas, “Stop production at hadron [10] [11] J. Alwall, M. Herquet, F. Maltoni, O. Mattelaer and T. Stelzer, “MadGraph 5: going beyond”, [17] M. L. Mangano, “The so-called mlm prescription for me/ps matching”, [19] LHC SUSY Cross Section Working Group, “SUSY Cross Sections using 8 TeV pp collisions.”. [12] T. Sjöstrand, S. Mrenna and P. Z. Skands, “PYTHIA 6.4 physics and[13] manual”, M. Cacciari, G. P. Salam and G. Soyez, “FastJet user manual”, [14] M. Cacciari and G. P. Salam, “Dispelling the [15] J. de Favereau, C. Delaere, P. Demin, A. Giammanco, V. Lemaitre, A. Mertens and M. Selvaggi, [16] M. Cacciari, G. P. Salam and G. Soyez, “The anti- Proposing a new LHC search for light compressed stop squarks