Hard diffraction at the LHC
Christophe Royon DAPNIA-SPP, CEA Saclay Seminar at Brookhaven National Laboratory, December 21 2006
Contents: Standard Higgs physics at the LHC • What is diffraction (HERA and Tevatron)? • Inclusive diffraction at the LHC • Diffractive Higgs production at the LHC: exclusive and • inclusive production Background studies: How to measure the high β gluon? • Exclusive top and stop production • Work done in collaboration with M. Boonekamp. J. Cammin, O. Kepka, A. Kupco, R. Peschanski, L. Schoeffel NB: Many results coming from the “Saclay” group, see also results from Durham group in particular (A. Martin, V. Khoze, M. Ryskin) LEP experiments limits on Higgs mass Q: ratio of the probability to observe what has been seen • if it is a Higgs signal by the probability to observe the same if it is only background Limit on Higgs mass: 114.4 GeV •
50
40 LEP
-2 ln(Q) 30
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-10 Observed Expected for background -20 Expected for signal plus background
-30 106 108 110 112 114 116 118 120 2 mH(GeV/c ) Electroweak fits and mass of Higgs boson
Use new Mtop, width of W boson from Tevatron and • LEP, and mass of W from LEP
MHiggs = 89 + 42 30 GeV (68% CL), and < 175 GeV • at 95% CL −
6 Theory uncertainty LEP1 and SLD ∆α(5) = had LEP2 and Tevatron (prel.) 5 0.02758±0.00035 80.5 0.02749±0.00012 68% CL 2
4 incl. low Q data ] 2 GeV
3 [ 80.4 ∆χ
W
2 m
1 80.3 ∆α mH [GeV] Excluded 114 300 1000 0 30100 300 150 175 200
mH [GeV] mt [GeV] Standard search for Higgs boson at the LHC Low masses: difficult region at the LHC: other ways of • finding the Higgs boson Properties of Higgs boson difficult to be analysed •
H → γ γ ttH (H → bb) H → ZZ(*) → 4 l H → WW(*) → lνlν 2 H → ZZ → llνν 10 H → WW → lνjj Total significance Signal significance
10
5 σ
ATLAS ∫ L dt = 30 fb-1 (no K-factors)
1 2 3 10 10 mH (GeV)
H → γ γ + WH, ttH (H → γ γ ) ttH (H → bb) H → ZZ(*) → 4 l H → WW(*) → lνlν 2 H → ZZ → llνν 10 H → WW → lνjj Total significance Signal significance
10
5 σ
ATLAS ∫ L dt = 100 fb-1 (no K-factors)
1 2 3 10 10 mH (GeV) SM Higgs decay
Low masses: b¯b and ττ dominate High masses: W W dominates Definition of diffraction: example of HERA Typical DIS event: part of proton remnants seens in • detectors in forward region (calorimeter, forward muon...) HERA observation: in some events, no energy in forward • region, or in other words no colour exchange between proton and jets produced in the hard interaction Leads to the first experimental method to detect • diffractive events: rapidity gap in calorimeter Second method to find diffractive events: Tag the proton • in the final state DIS and Diffractive event at HERA
Scheme of a roman pot detector Scheme of roman pot detector Forward Detectors (DØ ) Diffractive kinematical variables
Momentum fraction of the proton carried by the 2+ 2 • Q MX colourless object (pomeron): xp = ξ = Q2+W 2 Momentum fraction of the pomeron carried by the • interacting parton if we assume the colourless object to 2 Q xBj be made of quarks and gluons: β = 2+ 2 = Q MX xP 4-momentum squared transferred: t =(p p0)2 • − D Measurement of the diffractive structure function F2 Measurement of the diffractive cross section using the • rapidity gap selection over a wide kinematical domain in 2 (xP ,β,Q ) Definition of the reduced cross section: • 3 D 2 2 d σ 2παem y D 2 2 = 4 1 y + σr (xP ,Q , β) dxP dQ dβ βQ − 2 ! As an example: H1 data • D Measurement of the diffractive structure function F2
H1 Data H1 2006 DPDF Fit (extrapol. fit) 2 Q 2 β β β β β β β [GeV ] 0.05 =0.01 =0.04 =0.1 =0.2 =0.4 =0.65 =0.9
D(3) 3.5 r
σ 0 0.05 5 IP
x 0 0.05 6.5 0 0.05 8.5 0 0.05 12 0 0.05 15 0 0.05 20 0 0.05 25 0 0.05 35 0 0.05 45 0 0.05 60 0 0.05 90 0 0.05 200 0 0.05 400 0 0.05 800 0 0.05 1600 0 -4 -2 -4 -2 -4 -2 -4 -2 -4 -2 -4 -2 -4 -2 10 10 10 10 10 10 10 10 10 10 10 10 10 10 xIP Extraction of the parton densities in the pomeron (H1) Assume pomeron made of quarks and gluons: perform • QCD DGLAP fits as for the proton structure function 2 starting from xG and xq distributions at a given Q0, and evolve in Q2 (the form of the distributions is MRS like)
Bq Cq βq = Aqβ (1 β) − Cg βG = Ag(1 β) − At low β: evolution driven by g qq¯, at high β, q qg • becomes important → → Take all data for Q2 > 8.5 GeV2, β < 0.8 to be in the • perturbative QCD region and avoid the low mass region (vector meson resonances)
D dF2 αS [Pqg g + Pqq Σ] d log Q2 ∼ 2π ⊗ ⊗ Parton densities in the pomeron (H1) Extraction of gluon and quarks densities in pomeron: • gluon dominated Gluon density poorly constrained at high β (imposing • Cg = 0 leads to a good fit as well, Fit B) Good description of final states ) • ) 2 2 2 Singlet Gluon Q 2 0.2 0.5 [GeV ]
(z,Q 8.5 Σ 0.1 0.25 z z g(z,Q
0 0
0.2 0.5 20 0.1 0.25
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0.2 0.5 90 0.1 0.25
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0.2 0.5 800 0.1 0.25
0 0 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 z z H1 2006 DPDF Fit Fit B (exp. error) (exp.+theor. error) Diffraction at Tevatron/LHC
(a) (b) (c)
φ Gap Jet+Jet φ Jet Gap Jet φ Gap Jet+Jet Gap η η η
Kinematic variables t: 4-momentum transfer squared • ξ1,ξ2: proton fractional momentum loss (momentum • fraction of the proton carried by the pomeron)
β1,2 = xBj,1,2/ξ1,2: Bjorken-x of parton inside the • pomeron 2 M = sξ1ξ2: diffractive mass produced • ∆y1,2 ∆η log 1/ξ1,2: rapidity gap • ∼ ∼ “Inclusive” models “Inclusive” models: Take the hadron-hadron “usual” • cross section convoluted with the parton distributions in the pomeron Take shape of H1 measurement of gluon density • Normalisation coming from CDF run I cross section • measurement Inclusive cross sections need to be known in detail since • it is a direct background to search for exclusive events
p x 1 , v 1 P
g k x 1 1 H , QQ, gg g k x 2 2
P
p x 2 , v 2 Inclusive Higgs mass production Large cross section, but mass poorly reconstructed since part of the energy lost in pomeron remnants (M = √ξ1ξ2S Higgs + remnant mass) ∼ “Exclusive models”
p p P
g k 1 g k 1 x x , γγ 1 H , QQ , γγ 1 H , QQ g x k 2 g k 2 x 2 2
P p "Standard" p "Exclusive "
p P
g k x 1 1 H , QQ , γγ g x k 2 2
P
p
"Inclusive"
All the energy is used to produce the Higgs (or the dijets), namely xG δ (See in particular “Saclay model (R. Peschanski, M. Boonekamp,∼ J. Cammin, A. Kupco, O. Kepka, C. R.) (coupling via a pomeron), and Durham model (A. Martin, V. Khoze, M. Ryskin) (direct coupling to the proton) Advantage of exclusive Higgs production? Good Higgs mass reconstruction: fully constrained • system, Higgs mass reconstructed using both tagged protons in the final state (pp pHp) → MH = ξpξp¯S • No energyq loss in pomeron “remnants” •
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Mx DPEMC Monte Carlo DPEMC (Double Pomeron Exchange Monte Carlo): New • generator to produce events with double pomeron exchange (both “Durham” and “Saclay” models) http://boonekam.home.cern.ch/boonekam /dpemc.htm, hep-ph/0312273 Interface with Herwig: for hadronisation • Exclusive and inclusive processes included: Higgs, dijets, • diphotons, dileptons, SUSY, QED, Z, W ... DPEMC generator interfaced with a fast simulation of • LHC detector (as an example CMS, same for ATLAS), and a detailled simulation of roman pot acceptance Gap survival probability of 0.03 put for the LHC: is it • possible to check this at the Tevatron? Another available MC: “Exhume” for Durham model • “Exclusive” production at the LHC As an example: Saclay model (For Durham model, similar • results at 120 GeV with a stronger mass dependence) Higgs decaying into b¯b: study S/B • Exclusive b¯b cross section (for jets with pT > 25 GeV): • 2.1 pb Exclusive Higgs production (in fb) • MHiggs σ (fb) 120 3.9 125 3.5 130 3.1 135 2.5 140 2.0 NB: a survival probability of 0.03 was applied to all cross • sections Signal and background Signal and background for different Higgs masses for 100 fb−1
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total mass pots total mass pots Signal over background: standard model Higgs For a Higgs mass of 120 GeV and for different mass windows as a function of the Higgs mass resolution S/B
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1 0.9 0.8 0.7 0.6 0.5 0.4
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0.2 0 1 2 3 4 5 Mass resolution Diffractive SUSY Higgs production At high tan β, possibility to get a S/B over 50 (resp. 5.) for 100 (resp.10) fb−1! S/B
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0 1 2 3 4 5 Mass resolution Uncertainty on high β gluon Important to know the high β gluon since it is a • contamination to exclusive events Experimentally, quasi-exclusive events indistinguishable • from purely exclusive ones Uncertainty on gluon density at high β: multiply the • gluon density by (1 β)ν (fit: ν = 0.0 0.6) − ± zG 2
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0 -3 -2 -1 -3 -2 -1 -3 -2 -1 10 10 10 10 10 10 10 10 10 z Dijet mass fraction measurement at the LHC Dependence on high-β gluon
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dijet mass fraction dijet mass fraction Existence of exclusive events Test of the existence of exclusive events ll /dM σ / d γγ
/dM 10 σ d
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Mass fraction
Dilepton and diphoton cross section ratio as a function of • the diphoton/dilepton mass: no dilepton event for exclusive models (gg γγ ok, gg l+l− direct: impossible) → → Change of slope of ratio if exclusive events exist • Other method: ratio b-jets / all jets, • Look for exclusive events at the Tevatron Look for exclusive events (events where there is no • pomeron remnants or when the full energy available is used to produce diffractively the high mass object) Select events with two jets only, one proton tagged in • roman pot detector and a rapidity gap on the other side Comparison with POMWIG Monte Carlo using H1 gluon • density in pomeron and DPEMC for exclusive signal Will be necessary to see the effect of new H1/ZEUS • PDFs in pomeron on these results since gluon density changed by a lot; a direct Tevatron measurement would be also interesting
CDF Run II Preliminary CDF Run II Preliminary DPE data (stat. only) -1 3.6 < |η | < 5.9 CDF gap POMWIG: CDF⊕H1-fit2 10 3000 jet2 ET > 10 GeV Exclusive DPE (DPEMC) jet3 ET < 5 GeV Best Fit to Data Arbitrary ηjet1 || 2<-1.0
Arbitrary ± -2 Fexcl = 29.1 0.8 % 10 2000 (stat. only)
-3 10 DPE data (stat. only) 1000 POMWIG (CDF) Background POMWIG + Background -4 10 0 0 0.2 0.4 0.6 0.8 1 1.2 0 0.2 0.4 0.6 0.8 1 1.2 R = M / M R = M / M jj jj X jj jj X Exclusive production at the Tevatron? Look at the dijet mass fraction: exclusive events are supposed to appear around 1
700 700 B.-L. INC B.-L INC+EXC DPE data (stat.only) DPE data (stat.only) ν=0 ν=0
Events 600 ν=0.5 Events 600 ν=0.5 ν=-0.5 ν=-0.5 ν=1 ν=1 ν ν 500 =-1 500 =-1
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0 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1
Rjj=Mjj/Mx Rjj=Mjj/Mx χC exclusive production at the Tevatron?
CDF observation: Upper limit of χC exclusive production • at the Tevatron in the J/Ψγ channel σ 49 pb 18 ∼ ± ± 39 pb for y < 0.6 (result not corrected for cosmics, χ2 contamination) Exclusive prediction: 59 pb • Quasi-exclusive contamination: • mass fraction ν = 0 ν = 1 ν = 0.5 ν = +0.5 ν = +1.0 − − 0.8 5.4 119.1 27.2 0.9 0.2 ≥0.85 2.0 62.0 11.2 0.2 0.0 ≥ 0.9 0.3 19.6 2.9 0.0 0.0 ≥0.95 0.8 1.7 0.8 0.0 0.0 ≥ Contamination of quasi-exclusive events strongly • dependent on assumption on high-β gluon density in pomeron (completely unknown...), and also on precision and smearing of dijet mass distribution (a cut at 0.9 at generator level corresponds to ??? at reconstructed level), true also for jet studies... W, top and stops p p Pom
γ 1 g W W g t t g γ
Pom
p p
All the energy is used to produce the W, top (stop) pairs: W: QED process, cross section perfectly known, top: QCD diffractive process Top and W events
9 1.4 res = 3.0 GeV (GeV)
8 W 1.2 res = 2.0 GeV Events res = 1.0 GeV 7 1 6 0.8 5
4 0.6
3 0.4 2 0.2 Expected stat. uncertainty m 1 0 0 100 200 300 400 500 600 140 150 160 170 180 190 200 210 220 total pot mass (GeV) Integrated luminosity (fb-1)
Turn-on fits: fit missing mass distribution at threshold • (events appearing when M > 2MW ) W boson cross section and acceptance: σ 56 fb, about • 60% acceptance from pots, W mass resolution:∼ 400 MeV, not competitive, check the roman pot alignment∼ Top quark cross section and acceptance: σ 40 fb, pots • at 220 m, about 85% acceptance, Top mass∼ resolution: 1 GeV, competitive measurement ∼ WARNING: W W cross section certain (QED process), • whereas tt¯ cross section uncertain and model dependent (to be tested at the LHC) Top and stops
Cross section for a stop mass of 250 GeV: σtot = 8 fb, • σacc = 6 fb Possibility to distinguish between top and stop even if • they have about the same mass: using the differences in spin (as an example: mt˜ = mtop) Very fast turn-on for stops Resolution on stop mass measurement 1 GeV at high • lumi, if cross section high enough (model∼ dependent)
100
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80 4 m~t = 393 GeV (GeV) t ~ 3.5
60 3
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40 2
1.5 m~t = 210 GeV
20 1
Expected stat. uncertainty m 0.5
m~t = 174.3 GeV 0 0 300 350 400 450 500 550 600 20 40 60 80 100 120 140 160 180 200 -1 total mass pots Integrated luminosity (fb ) Concept of survival probability Survival probability: Probability that there is no soft • additional interaction, that the diffractive event is kept Important to measure the survival probability in data: • estimated to be of the order of 0.1 at the Tevatron
π + LRG LRG φ + Y 0
a) b) ∆Φ dependence of survival probabilities Survival probability strongly ∆Φ-dependent where ∆Φ is the difference in azimuthal angles between p and p¯
-1 2 corr 10 2 S t1 = t2 = -0.1GeV
2 2 t1 = -0.2GeV , t2 = -0.7GeV
10-2
10-3
2 t1 = t2 = -0.7GeV
0 20 40 60 80 100 120 140 160 180 ∆φ [°] Forward Proton Detector in DØ Forward Proton Detector (FPD) installed by DØ allowing to measure directly ∆Φ
Dipole Quadrupole Quadrupole spectrometer spectrometer spectrometer
Q-UP
Q-OUT p Dé p
D-IN Q-IN
Q-DOWN
Possibility to combine D-IN with quadrupole on the other side, or two quadrupole detectors (Q-UP and Q-UP, or Q-UP and Q-DOWN...) Possible measurement at DØ Diffractive cross section ratios in different regions of ∆Φ • at the Tevatron same side: ∆Φ < 45 degrees, opposite side: ∆Φ > 135, • middle: 45 < ∆Φ < 135 degrees; 1st measurement: asymmetric cuts on t (dipole and • quadrupole), 2nd measurement: symmetric cuts on t (quadrupole on both sides) Possible to distinguish between SCI and pomeron-based • models, and test the survival probabilities
Configuration model middle/same opp./same Quad. SCI 1.3 1.1 + Dip. Pom. 0.36 0.18 Quad. SCI 1.4 1.2 + Quad. Pom. 0.14 0.31 Roman pot projects TOTEM project accepted, close to CMS • FP420: Project of installing roman pot detectors at 420 • m both in ATLAS, CMS; collaboration being built Roman pot detectors at 220 m in ATLAS: • – Natural follow-up of the ATLAS luminosity project at 240 m to measure total cross section – Complete nicely the FP420 m project – Collaboration between Saclay. Prague, Cracow and Stony Brook (so far) being pursued – Collaboration with the FP420 m project concerning detectors, triggers, simulation... For more information, see the web pages of FP420, CMS, • TOTEM, ATLAS Scheme of roman pot detectors
Roman pots at ATLAS 4m
IP 220m
As close as possible to the beam: 10 s = 1mm
8 s
3cm Silicon Detectors
Assume roman pots located at 216 and 224 m Acceptance for 220 m pots Steps in ξ: 0.02 (left), 0.005 (right), t =0 or 0.05 GeV2 • | | Detector of 2 cm 2 cm will have an acceptance up to • ξ 0.16, down to× 0.008 at 10 σ, 0.016 at 20 σ ∼ As an example Higgs mass acceptance using 220 m pots • down to 135 GeV and upper limit due to cross section and not kinematics
ξ = 0, 0.005, 0.010, 0.015, 0.020 0 1 S = 216 m Y [mm] Y [mm]
∆ S = 216 m ∆ 2 -5 0.5 t = 0, -0.1 GeV
0 -10
-0.5 -15 S = 224 m -1 -20 -1.5 10σ: ξ~0.008 -25 σ ξ -2 20 : ~0.016 0 5 10 15 20 25 -0.5 0 0.5 1 1.5 2 2.5 ∆X [mm] ∆X [mm] Hit maps at 216 and 224 m Study difference between hit maps at 216 and 224 m: • test the idea of using displacement at the trigger level to distinguish with halo No unique shift direction between 216 and 224 m •
0 ξ = 0.02, t = -0.1 GeV2 ξ = 0.02, t = -0.4 GeV2 -0.6 Y [mm] Y [mm]
∆ S = 216 m ∆ S = 216 m -0.8 -0.5
-1 -1 -1.2
-1.4 -1.5
-1.6 S = 224 m S = 224 m -2 -1.8 1.8 2 2.2 2.4 2.6 2.8 3 1.5 2 2.5 3 3.5 ∆X [mm] ∆X [mm] Roman pots at 220 m Schematic view of 220 m pots: keep horizontal pots only from the TOTEM pots Fluxes in roman pot detectors (from Vadim Talanov) Fluxes at 220 m at high lumi (1034) (plots for 2.1029)
RATE EVOLUTION WITH CUTS [MD2005] Particle flux in [Hz]
pn%+ %– e+ e– Ȗ Pot at 220m 344 174 616 406 4630 3361 9.4x104
BEAM-GAS [LHC360] Si strip detectors specifications Size: 2cm 2cm • × Spatial resolution of the order of 10-15 µm: Si strip • detectors of 50 µm, as a first proposal: 9 layers, 3 vertical, 2 horizontal, 2 U, 2 V (45 degrees) Edgeless detectors: Between 30 to 60 µm • First prototype of detector to be sent by CANBERRA by • the end of December: test-stand (laser and radioactive source) to be installed in Saclay following the Paris 6, Prague, Cracow, CERN experience Readout: ABCD chip for test, ABCNext chip for final • detectors (needs longer latency since needs as an input L1 ATLAS trigger and distance to be covered: 2 220 = 440 m) × 2 layers used for the trigger: Strip detectors of 100-200 • µm (to be optimised given the fact that we have 1 µs to send the trigger to ATLAS) Timing from the Si strip detector: 5-10 ns required to • identify from which bunch crossing the event is coming, implies a fast shaping time of 20-30 ns Different kinds of detectors
Y X U V
Edgeless cut
5 detectors to be used in readout per pot • 2 additional detectors to be used for triggering (larger • strips) Test stand Installation of a test stand for Si strip detectors • Tests performed using a laser and radioactive source • Needs a precise table and labview software to control the • motion of the laser (benefitting from the experience of Paris 6 to test Si strip detectors at the ILC) DAQ using ABCD chip: to be obtained from CERN, • Cracow, Prague... Tests should be starting next December when the first • prototype from Canberra is coming Connection to the readout chips: Hybrids As a start, obtain hybrids from CERN for test purposes
Silicon detector Pitch adapter Readout chips Output to DAQ Trigger and timing: principle Roman pot trigger to be transmitted to L1: compatibility • of hit strips between 2 different planes and 2 different roman pots (local L1 roman pot trigger), and timing information L1 positive trigger sent back to the pots: starts the • readout of the Si strip detector FP420 and 220 m pot trigger: In ATLAS, there is a large • overlap between FP420 and 220 m pot kinematical domain (220 m pots sensitive down to MH 130 GeV, allows FP420 trigger to be given by 220 m tags∼ (offline, the 420 m information will be used) Timing with a precision of the order of 5 ps needed to • identify from which vertex the protons are coming: 1 mm resolution, up to 25 interactions by bunch crossing Collaboration in progress with the University of Chicago, • Argonne National Laboratory, Stony Brook, Saclay: test and develop new timing detectors specially for medical applications and linear collider, roman pots New 2 2 inch multi anode phototubes developped by • Burle and× Photonis Use of timing/Si strip detectors needed for future • developments: ILC, CLIC, medical applications Trigger: principle
Horizontal roman pots (a la TOTEM) Diffractive Higgs Trigger 7 plans Si strips /Roman Pot x,y - 224 m - 216 m σ position 5 microns xA xB σ time < 10 psec T T jet L L
Front end T T PA R R +730 ns SH xA xB jet Left Right Pretrigger ATLAS detector Pretrigger T xA - xB =0 xD - xC =0 Chip ABC next T 1,0 µsec LR Trigger Logic Pipeline • LP AND RP buffer +850 ns (air cable) •TR - TL < 200 psec (> 3.6 µsec) 2,0 µsec 2 Jets with L1L1 CTP CTP Pt > 20 Gev/c Max 75KHz 2,5µsec
ATLAS standard R R HLT Trigger Refined Jet Pt cut O HLT Trigger Vertice within millimeter O (ROB)(ROB) ∆ time < 5 to 10 psec DD ATLAS Standard US15 Timing detectors
Ultra fast (psec) Timing
MCP Principle
Anode pads and equal read out design Photonis Prototype Conclusion Study of inclusive events (the only events which are • existing for sure): determination of gluon at high β, search for SUSY events (or any resonance) when dijet background is known Exclusive events still to be observed in particular at the • Tevatron Exclusive Higgs: Signal over background: 1 if one gets • a very good resolution using roman pots (better∼ than 1 GeV), enhanced by a factor up to 50 for SUSY Higgs at high tanβ QED WW pair production: cross section known precisely, • allow to calibrate prescisely the roman pot detectors Diffractive top, stop pair production: possibility to • measure top and stop masses by performing a threshold scan with a precision better than 1 GeV if cross section high enough (same idea as linear collider, without ISR problem) Project of installing roman pot detectors in ATLAS well • started: collaboration between Prague, Cracow, Saclay, Stony Brook,... Collaboration about timing detectors (and medical • applications): University of Chicago, Argonne, Saclay