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Prospects for diffractive and forward physics at the LHC

Prospects for diffractive and forward physics at the LHC. Monika Grothe (Turin/Wisconsin) on behalf of the CMS and TOTEM diffractive and forward physics working group. CMS + TOTEM: Unprecedented Coverage in . CMS IP

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Prospects for diffractive and forward physics at the LHC

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  1. Prospects for diffractive and forward physics at the LHC Monika Grothe (Turin/Wisconsin) on behalf of the CMS and TOTEM diffractive and forward physics working group Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  2. CMS + TOTEM: Unprecedented Coverage in  CMS IP T1/T2, Castor, BSC ZDC RPs@150m RPs@220m possibly detectors@420m T1 (CSC) 3.1 ≤ || ≤ 4.7 HF 3 ≤|| ≤ 5 T2 (GEM): 5.3 ≤ || ≤ 6.6 Castor5.3 ≤ || ≤ 6.6 Possible addition FP420: Near-beam detectors at 420 m in cold region of LHC Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  3. CMS + TOTEM: Coverage in  Depends on value of * At nominal LHC optics,*=0.5m TOTEM Points are ZEUS data det 420 det 420 xL=P’/Pbeam= 1-x ξ: fractional momentum loss of proton t: 4-momentum transfer squared at proton vertex diffractive peak Note: Totem RP’s optimized for special optics runs at high * β* is measure for transverse beam size at vertex Consider 3 different optics, at *=0.5m, 90m and 1540m Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  4. Contents of document The document is not an exhaustive and in-depth exposé on the fwd and diffractive physics program possible with CMS/TOTEM. It’s not a physics TDR. Rather, the document, for the first time at the LHC, addresses central experimental issues in measuring fwd and diffractive physics by way of a number of exemplary processes. → Acceptance and resolution of the near-beam detectors at 220 & 420m → Reconstruction of kin. variables → Background from fake diff events with protons from pile-up events → Triggering on the relatively low ET diffractive processes Ch 1: Introduction Ch 2: Experimental set-up Ch 3: Measurement of forward protons Ch 4: Machine-induced background Ch 5: Diffraction at low and medium luminosity Ch 6: Triggering with fwd detectors at high lumi Ch 7: Hard diffraction at high luminosity Ch 8: Photon-photon and photon-proton physics Ch 9: Low-x QCD physics Ch 10: Validation of hadronic shower models used in cosmic ray physics Prospects for diffractive and forward physics at the LHC Near-beam detectors at 220/420 m T1/T2 + CASTOR + ZDC forward detectors

  5. Part I: “Diffractive” partDetecting leading protons in near-beam detectors at 220/420m: Acceptance determined by tracking protons through the LHC accelerator lattice with the program MAD-X smearing of both transverse vertex position and scattering angle at the IP according to transverse beam size and beam momentum divergence assume that near-beam detectors are 100% efficient, i.e. assume all protons that reach 220/420m location outside of cutout for beam (1.3mm @220m, 4mm @420m) are detected @220m @420m , *=0.5m @220m, *=0.5m Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  6. Part I: “Diffractive” partDetecting leading protons in near-beam detectors for *=0.5m : Resolution At nominal LHC optics,*=0.5m • Measure positions and angle in two sets • of Si detectors per station. • For nominal LHC optics (*=0.5m), where • dispersion is mainly horizontal: • depends only on x-position in Si detectors take uncertainty of position measurement into account by smearing x position by 10 m per detector set take uncertainty of beam position at detector location into account by smearing x position by an additional 50 m Note: Resolutions also available for *=90m and 1540m optics Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  7. Part I: “Diffractive” partDetecting leading protons in near-beam detectors for *=0.5m : Mass reconstruction For central exclusive production pp -> pXp Note: Also available for*=90m and 1540m optics Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  8. Part I: “Diffractive” partThe processes we are concerned with Single diffraction (SD): X X Double Pomeron exchange (DPE): Near-beam detectors central CMS apparatus IP IP central CMS apparatus rap gap IP Near-beam detectors Near-beam detectors o) If X = anything: Measure fundamental quantities of soft QCD Contributes significantly to pile-up Inclusive SD  15 mb, inclusive DPE  1 mb, where 1 mb = 100 events/s @ 10 29 cm-2 s-1 o) If X includes jets, W’s, Z’s, Higgs (!): Hard processes, calculable in perturbative QCD. Measure proton structure, QCD at high parton densities, discovery physics Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  9. Part I: “Diffractive” partThe processes we are concerned with (II) The accessible physics is a function of the instantaneous and integrated luminosity “Low”: Low enough that pile-up is negligible, i.e <1032 cm-2s-1, and integrated lumi a few 100pb-1 to <1 fb-1 → Measure inclusive SD and DPE cross sections and their Mx dependence → In addition to running at nominal LHC optics: Option of running with*=90m @ 1031 cm-2s-1 Would be trade-off between improved  coverage and running for only a few days Program envisaged as part of the routine data taking at*=0.5m: “Intermediate”: Lumi < 1033 cm-2s-1, pile-up starts becoming an issue, integrated lumi 1 to a few fb-1 → Measure SD and DPE in presence of hard scale (dijets, vector bosons, heavy quarks) → Basically follow Tevatron program “High”: Lumi > 1033 cm-2s-1, pile-up substantial and integrated lumi several tens of fb-1 → Discovery physics comes into reach in central exclusive production Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  10. Part I: “Diffractive” part - Soft diffractionCoverage in M with the detectors at 220m Differential cross section of inclusive DPE taking into account the acceptance of the detectors at 220m Wide coverage in mass possible when requiring that protons be seen on both sides in the 220m detectors Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  11. Part I: “Diffractive” part - Soft diffractionCoverage in t with the detectors at 220m For inclusive DPE events Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  12. Part I: “Diffractive” partReconstruction of variables in DPE Bias in  reconstruction caused by limits in the  coverage DPE dijets with ET>20GeV inclusive DPE After application of constant correction factor Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  13. Part I: “Diffractive” part - Hard diffractionExperimental issues in selecting diffractive events at high lumi at the LHC Background from non-diffractive events that are overlaid with protons from pile-up events 2. Trigger is a major limiting factor for selecting diffractive events Results on diffractive trigger stream already approved, see CMS note 054/2006 Quick recap: → Standard CMS trigger thresholds too high for diffractive events → Use information of forward detectors to lower CMS muon and jet thresholds → The CMS trigger menus now foresee a dedicated diffractive trigger stream with 1% of the overall bandwidth on L1 and HLT Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  14. Part I: “Diffractive” part - High lumiPile-up background Number of PU events with protons within acceptance of near-beam detectors on either side: ~2 % with p @ 420m , ~6 % with p @ 220m Translates into a probability of obtaining a fake DPE signature caused by protons from PU: Eg at 1x 1034 60% of any signal event one wants to select have a fake DPE signature This is independent of the type of signal. Can be reduced by: Requiring correlation between ξ/M measured in the central detector and ξ/Mmeasured by the near-beam detectors Fast timing detectors that can determine whether the protons seen in the near-beam detector came from the same vertex as the hard scatter Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  15. Part I: “Diffractive” part - High lumiImpact of pile-up background Central exclusive production pp pHp with H (120GeV) -> bb In non-diffractive production hopeless, signal swamped with QCD dijet background Selection rule in CEP (central system is JPC = 0++ to good approx) improves S/B for SM Higgs dramatically In certain MSSM scenarios the signal cross section is three order of magnitude higher than for the SM case Two examples on which impact of PU background studied: CEP 120GeV Higgs - needs 220 & 420m detetcors inclusive DPE ttbar production - needs 220m detectors Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  16. Part I: “Diffractive” part - High lumiImpact of pile-up background Effect of cuts on the S/B for the background caused by PU for 120GeV Higgs 0.6/10 0.9/30 0.6/20 0.9/100 0.6/180 0.9/320 0.6/900 0.9/1900 Beware:Uncertainty of the leading proton spectrum normalization because of uncertainty in rap gap suppression factor and its energy dependence Beware: Further handles not yet employed against PU are track multiplicity cuts and more stringent vertex cuts which may lead to an additional reduction of PU by a factor 10-100 Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  17. Part 2: “Forward physics” partPhoton-mediated processes:Exclusive μμ production known Calibration process both for luminosity and energy scales of near-beam detectors Striking signature: acoplanarity angle between leptons Allows reconstruction of proton  values with resolution of 10-4, i.e. smaller than beam dispersion Expect ~300 events per 100 pb-1after CMS muon trigger Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  18. Part 2: “Forward physics” partLow-x QCD: Forward jets • MC-level proof-of-principle only (no det. response !) • Missing important corrections: underlying-evt. (PYTHIA CMS-Tune), hadronization (cluster vs. Lund) • Large expected yields (~107 at ~20 GeV) ! • Inclusive forward “low-ET” jet (ET ~20-100 GeV) production: • Inclusive fwd. jet reconstruction: p + p → jet1 + jet2 + X , x1 ~ 10-1 Sensitive to gluons with: x2 ~ 10-4 1 pb-1 PYTHIA ~ NLO Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  19. Part 2: “Forward physics” partLow-x QCD: Forward Drell-Yan Gives access to low-xBJ partons in proton in case of large imbalance of fractional momenta x1,2 of leptons, which are then boosted to large rapidities CASTOR range 5.3 ≤ || ≤ 6.6 gives access to xBJ~10-7 DY pairs suppressed in saturated PDF Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  20. Part 2: “Forward physics” partValidation of hadronic shower models in cosmic ray physics → Models for showers caused by primary cosmic rays (PeV = 1015 eV range) differ substantially → Fixed target collision in air with 100 PeV center-of-mass E corresponds to pp interaction at LHC → Hence can tune shower models by comparing to measurements with T1/T2, CASTOR, ZDC Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  21. Conclusions Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  22. BACKUP Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  23. Acceptance at * = 0.5m @420m @220m Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  24. Part I: “Diffractive” partDetecting leading protons in near-beam detectors for *=90m : Mass reconstruction For central exclusive production pp -> pXp Different from *=0.5m optics, the dependence of the measured position in the near-beam detectors at 220m on the horizontal vertex position can no longer be neglected Substantial improvement possible when using reconstructed vertex from CMS detector Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  25. Machine Induced Background (RP detector at 220m) scenario (b*) 1540 90 0.55 background (10-4/bx) beam halo 0.8 0. 7 0.5 beam gas 4 3 2 p-p 0.4-2 3.8-20 200-1000 ~4% probability of having a “fake” proton

  26. Major issues in selecting diffractive events: Protons from pile-up events TOTEM FP420 xL=P’/Pbeam= 1-x Average number of pile-up events overlaid to any hard scatter 7 @ 2x1033 cm-2s-1 35 @ 1x1034 cm-2s-1 Average number of protons per PU event on either side of the IP Seen within acceptance of near-beam detectors 0.012 @420m, i.e. 1.2 % of PU events with p at 420m 0.055 @220m, i.e. 5.5% of PU events with p at 220m Example: Selection of a DPE process for case with 7 PU events on average → Probability that 2 PU p look like a DPE event with protons at 420m: 7*6*0.6%*0.6% = 0.001 → Effective cross section after central detector selection cuts: sigma_centralDet_(non)Diff Signal_diff/Bckgrd_nonDiff = 1000 * sigma_centralDet_Diff / sigma_centralDet_nonDiff Can be reduced by: Requiring correlation between ξ measured in the central detector and ξmeasured by the near-beam detectors Fast timing detectors that can determine whether the protons seen in the near-beam detector came from the same vertex as the hard scatter Planned as part of FP420 with vertex resolution of better than 3mm Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  27. Major issues in selecting diffractive events: Trigger → Standard CMS trigger thresholds too high for diffractive events → Use information of forward detectors to lower CMS muon and jet thresholds → The CMS trigger menus now foresee a dedicated diffractive trigger stream with 1% of the overall bandwidth on L1 and HLT CMS note 2006/054 and TOTEM note 2006/01 “Triggering on fwd physics” Achievable total reduction: 10 x 2 (HT cond) x 2 (topological cond) = 40 Can win additional factor ~2 in reduction when requiring that the 2 jets are in the same  hemisphere as the RP detectors that see the proton Jet isolation criterion For dijet trigger adding L1 conditions on the near-beam detectors provides a rate reduction sufficient to lower the dijet threshold to 40GeV per jet while still meeting the CMS L1 bandwidth limits for luminosities up to 2x 1033 cm-1 s-1 Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  28. Part I: “Diffractive” partCEP of a 120 GeV Higgs: Trigger • Level-1: • 2-jets (ET>40GeV) & single-sided 220m • condition results in efficiency ~12% (L1) • Can add another ~10% efficiency by introducing • a 1 jet & 1 (40GeV, 3GeV) trigger condition • HLT: Efficiency ~7% • To stay within 1Hz output rate, needs to either prescale or add 420 m detectors in trigger H(120 GeV) → b bbar Trigger is a major limiting factor ! Without special diffractive trigger stream where jet thresholds can be lowered to ~40GeV no signal events at all would survive CMS dijet trigger Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  29. Part I: “Diffractive” partInclusive DPE production of t tbar Detect ttbar in semileptonic decay channel: pp  p+X+(tt)+X+ptt  bbqqEvent yield after cuts in 10 fb-1:Depending on model, between a O(1) and O(100) events Dominating background from non-diffractive inclusive t tbar production with protons from pile-up:O(100) events Selection does not assume fast timing detectors ξ matching only of limited use because of neutrino in event Signal S: gap suppression factor Background caused By PU protons Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

  30. Case-study II: Mueller-Navelet dijets in HF suppressed ratio saturation/BFKL • Mueller-Navelet dijets separated by large Δy: very sensitive to non-DGLAP evolution pp √s = 14 TeV jet1 increasing rapidity y~10 jet2 C.Marquet, Royon, hep-ph/0510266 (MC-level study only !) Expected dijet rates ET~20-40 GeV: ~104-105 Enough stats. for detailed studies of Dy-evolution 1 pb-1 1 pb-1 h1= -h2= [3.-3.5] h1= -h2= [4.5-5.] Monika Grothe, Prospects for diff and fwd physics at the LHC, CMS physics meeting Dec 2006

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