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Prospects for diffractive and forward physics at the LHC

Prospects for diffractive and forward physics at the LHC. Monika Grothe U Turin/ U Wisconsin CALTECH, March 2007. Brief introduction to diffraction and the Pomeron Its interest at the LHC Experimental situation around the CMS interaction point The CMS/TOTEM/FP420 program.

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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 U Turin/ U Wisconsin CALTECH, March 2007 Brief introduction to diffraction and the Pomeron Its interest at the LHC Experimental situation around the CMS interaction point The CMS/TOTEM/FP420 program Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  2. Diffraction ?! Pomeron ?!Esoterica ?! Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  3. Brief introduction to diffraction and the Pomeron:Diffraction Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  4. Brief introduction to diffraction and the Pomeron:Diffraction in optics o) Forward peak for q=0 (diffraction peak) o) Diffraction pattern related to size of target and wavelength of beam Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  5. Brief introduction to diffraction and the Pomeron:Diffraction in hadron scattering p p p p Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  6. Brief introduction to diffraction and the Pomeron:Diffraction in hadron scattering (II) a X LRG vacuum quantum numbers IP IP IP b b Single Dissociation a a a X b b Y b Double Dissociation Elastic Diffraction is a feature of hadron-hadron interactions (30% of stot): o) Beam particles emerge intact or dissociated into low-mass states. Energy  beam energy (within a few %) o) Final-state particles separated by large polar angle (or pseudorapidity, ln tan(q/2)):Large Rapidity Gap (LRG) o) Interaction mediated by t-channel exchange of object with vacuum quantum numbers (no colour):the Pomeron (IP) Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  7. Brief introduction to diffraction and the Pomeron:Pomeron !? - A new way to probe the proton p p IP p p • Pomeron goes back to the ‘60s: Regge trajectory, ie a moving pole in complex angular momentum plane (huh ?!) • Thanks to HERA and the Tevatron, we now understand diffraction in • terms of the theory of strong interactions, QCD, and of the constituents • of the proton – quarks and gluons (need a hard process) In diffractive events look at the proton constituents through a lens that filters out all parton combinations except those with the vacuum quantum numbers 2-gluon exchange: LO realisation of vacuum quantum numbers in QCD Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  8. Great. And why bother at the LHC ? The Pomeron as little helper in tracking down the Higgs ?! Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  9. shields color charge of other two gluons Vacuum quantum numbers “Double Pomeron exchange” Why bother with diffraction at the LHC ?Suppose you want to detect a light SM Higgs (say MH=120 GeV) at the LHC... Central exclusive production pp  pXp Suppression of gg jet jet because of selection rules forcing central system to be JPC = 0++ SM Higgs with ~120 GeV: gg  H, H  b bbarhighest BR But signal swamped by gg  jet jet Best bet with CMS: H  Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  10. Why bother with diffraction at the LHC - Detecting CEP of a light Higgs:Necessary ingredients Central exclusive production pp pXp: Discover a light (~120 GeV) Higgs • 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 orders of magnitude higher than for the SM case • CP violation in the Higgs sector can be measured via azimuthal asymmetry of the protons In addition: Rich QCD program In addition: Rich program of gamma-gamma mediated processes Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  11. beam dipole dipole x=0.015 x=0.002 p’ x=0 (beam) roman pots p’ roman pots CEP of a light Higgs:Necessary ingredients (II) Proton spectrometer using the LHC beam magnets: Detect diffractively scattered protons inside of beam pipe With nominal LHC optics: 12 s = M2 With √s=14TeV, M=120GeV on average:  0.009  1% fractional momentum loss of the proton Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  12. CEP of a light Higgs:Necessary ingredients (III) 12s = M2 With √s=14TeV, M=120GeV on average:  0.009  1% Nominal LHC beam optics Low * (0.5m): Lumi 1033-1034cm-2s-1 @220m: 0.02 <  < 0.2 @420m: 0.002 <  < 0.02 • Detectors at 420m • complement acceptance of 220m detectors • needed to extend acceptance down to low  values, i.e. low M(Higgs) • Detectors at ~220m • needed in addition to optimize acceptance (tails of  distr.) • needed because 420m too far away for detector signals to reach L1 trigger within latency Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  13. Have 220 m detectors, want 420 m ones:Current experimental situation at the CMS IP Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  14. pp interactions at 14TeV +TOTEM +FP420 Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  15. Experimental situation at the CMS IPCMS + TOTEM (+ FP420): Coverage in  CMS IP T1/T2, Castor ZDC RPs@150m RPs@220m possibly detectors@420m Castor Castor ZDC ZDC • TOTEM: • An approved experiment at LHC for measuring totandelastic, uses same IP as CMS • TOTEM’s trigger and DAQ system will be integrated with those of CMS , i.e. common data taking CMS + TOTEM possible • TOTEM aims at start-up at the same timescale as CMS (second half of 2007) Possible addition FP420 Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  16. Experimental situation at the CMS IPCMS + TOTEM (+ FP420): Coverage in  At nominal LHC optics,*=0.5m diffractive peak TOTEM Points are ZEUS data FP420 xL=P’/Pbeam= 1-x Note: Totem RP’s optimized for special optics runs at high* β* is measure for transverse beam size at vertex TOTEM coverage in improves with increasing * Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  17. Experimental situation at the CMS IPAdditional option: The FP420 R&D project The aim of FP420 is to install high precision silicon tracking and fast timing detectors close to the beams at 420m from ATLAS and / or CMS • FP420 R&D fully funded (~1000K CHF) • Proposal to ATLAS and CMS in first half of 2007 • Detector installation could take place during first long LHC break (~2009) Technological challenge: 420m is in the cryogenic region of the LHC Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  18. Scattered protons emerge here FP420 project:How to integrate detectors into the cold section of the LHC 420m from the IP is in the cold section of the LHC ! Need to modify LHC cryostat: Use Arc Termination Modules for cold-to-warm transition such that detectors can be operated at ~ room temperature Movable beam-pipe (pipelets) with detector stations attached Move detectors toward beam envelope once beam is stable Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  19. Injection of gas (~ atmospheric pressure) pump Ejection of gas Aluminium Cerenkov medium (ethane) Mirror Protons (Flat or Spherical?) ~ 15 cm ~ 5 cm Lens? (focusing) ~ 10 cm PMT FP420 project:Which technology for the detectors ? • 3D edgeless Silicon detectors: • Edgeless, i.e. distance to beam envelope can be minimized • Radiation hard, can withstand 5 years at 1035 cm-2 s-1 Time-of-Flight detectors: Time resolution of ~10ps would translate in z-vertex resolution of better than 3mm GASTOF (UC Louvain) Cherenkov medium is a gas QUARTIC (U Texas Arlington) Cherenkov medium is fused Silica Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  20. Integration of the moving beampipe and detectors ATM BPM Line X Bus Bar Cryostat Vacuum Space BPM QRL Fixed Beampipe ATM Pockets Transport side Vacuum Space Benoît Florins, Krzysztof Piotrzkowski, Guido Ryckewaert Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  21. The goal: Carry out a program of diffractive and forward physics as integral part of the routine data taking at CMS, i.e. at nominal beam optics and up to the highest available luminosities. This program spans the full lifetime of the LHC. Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  22. A step on the way ... Joint CMS/TOTEM program Central experimental issues in measuring fwd and diffractive physics at the LHC addressed for the first time - by way of a number of exemplary processes. Included already as option: Near-beam detectors @420m submitted to the LHCC in Dec ‘06 Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  23. Prospects for diffractiveand forward physics at the LHC Central experimental issues for selecting diffraction at the LHC Trigger is a major limiting factor for selecting diffractive events Background from non-diffractive events that mimic diffractive events because of protons from pile-up events Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  24. “Prospects for diffractive and forward physics at the LHC”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, March 2007

  25. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part Trigger (I) 2 x 1033 cm-2 s-1 L1jet trigger rates pp  p jj X 2-jet trigger no fwd detectors condition Efficiency Events per pb-1 single-arm 220m single-arm 420m L1 output bandwidth: 100 kHz L1 trigger threshold [GeV] Attention: Gap survival probability not taken into account; normalized to number of events with 0.001 <  < 0.2 and with jets with pT>10GeV • At 2x 1033 cm-1 s-1 without any additional condition on fwd detectors: • L1 1-jet trigger threshold O(150 GeV) • L1 2-jet trigger threshold O(100 GeV)

  26. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part Trigger (II) → CMS trigger thresholds for nominal LHC running too high for diffractive events → Use information of forward detectors to lower in particular CMS jet trigger thresholds → The CMS trigger menus now foresee a dedicated diffractive trigger stream with 1% of the total bandwidth on L1 and HLT (1 kHz and 1 Hz) single-sided 220m condition without and with cut on  Achievable total reduction: 10 (single-sided 220m) x 2 (jet iso) x 2 (2 jets same hemisphere as p) = 40 Adding L1 conditions on the near-beam detectors provides a rate reduction sufficient to lower the 2-jet threshold to 40 GeV per jet while still meeting the CMS L1 bandwidth limits for luminosities up to 2x 1033 cm-1 s-1 Much less of a problem is triggering with muons, where L1 threshold for 2-muons is 3 GeV Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  27. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part Trigger (III) Central exclusive production pp pHp with H (120GeV)  bb Trigger is a major limiting factor ! Efficiency H(120 GeV) → b bbar L1 trigger threshold [GeV] • Level-1: • 2-jets (ET>40GeV, ||<3) & single-sided 220m condition results in efficiency ~12% (L1) • HLT: • Efficiency of above 2-jet condition ~7% • To stay within 1 Hz output rate, needs to either prescale or add 420 m detectors in trigger •  Can add another ~10% efficiency by introducing a 1 jet & 1 (40GeV, 3GeV) trigger condition Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  28. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive partPile-up background TOTEM d(epeXp)/dxL [nb] det@420 xL=P’/Pbeam= 1-x 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 2x 1033 cm-2s-1 10% probability for a fake DPE signature. This is independent of the type of signal. Depends critically on the leading proton spectrum at the LHC which in turn depends on size of soft rescattering effects (rapidity gap survival factor) ! Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  29. Side remark:Rapidity gap survival probability hard scattering jet dPDF • Diffractive PDFs: • probability to find a parton of given x under • condition that proton stays intact – • sensitive to low-x partons in proton IP dPDF b jet Closely related to the underlying event at the LHC • Proton and anti-proton are large objects, unlike pointlike virtual photon • In addition to hard diffractive scattering, there may be soft interactions among spectator partons. •  Fill rapidity gap & slow down outgoing protons  Hence reduce the rate of diffractive events. • Quantified by rapidity gap survival probability. F2D without and with rescattering effects CDF data b Predictions based on HERA diffractive PDFs Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  30. “Prospects for diff and fwd physics at the LHC” - Part I: Diffractive partPile-up background (II) 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 (planned in FP420) ; 12 s = M2 incl QCD di-jets + PU CEP H(120) bb (jets) (220m) CEP of H(120 GeV) → b bbar: Possible to retain O(10%) of signal up to 2 x 1033 cm-2 s-1 in a special forward detectors trigger stream S/B in excess of unity for a SM Higgs might be in reach M(2-jets)/M(p’s) Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  31. Program on diffractive and forward physics1. Diffraction Low Luminosity (≤1032 cm-2s-1): low & high b* Measure inclusive SD and DPE cross sections and t, MX dependence Rapidity Gap selection possible High Luminosity (> 1032 cm-2s-1) : low b* Measure SD and DPE in presence of hard scale (dijets, vector bosons, heavy quarks) gg and gp phyics Highest Luminosity (> 1033 cm-2s-1) : low b* Discovery physics in central exclusive production (SM or MSSM Higgs, other exotic processes) Running with TOTEM optics: large proton acceptance No pile-up Pile-up not negligible: main source of background Need additional forward proton detector Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  32. Castor Castor ZDC ZDC Prospects for diffractive and forward physicsat the LHC Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  33. Program on diffractive and forward physics2. Forward physics 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 cosmic ray shower models at the LHC Study of the underlying event at the LHC: → Multiple parton-parton interactions and rescattering effects accompanying a hard scatter → Closely related to gap survival and factorization breaking in hard diffraction Heavy-ion and high parton density physics: Proton structure at low xBj → saturation → Color glass condensates Photon-photon and photon-proton physics: Also there protons emerge from collision intact and with very low momentum loss Multiple connection points to other areas in High-Energy-Physics ! Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  34. “Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” partPhoton-mediated processes:Exclusive μμ production (di-muons) - (true) known rms ~ 10-4 • Calibration process both for luminosity and energy scales of near-beam detectors •  Striking signature: acoplanarity angle between leptons • Allows reco of proton  values with resolution of 10-4, i.e. smaller than beam dispersion •  Expect ~300 events/100 pb-1 after CMS muon trigger Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  35. “Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” partLow-x QCD Saturation (Colour glass condensate) Qs2(x) 1/x Non perturbative region pQCD Q2 [GeV2] • When x  0 at Q2 > a few GeV2 • DGLAP predicts steep rise of • parton densities • At small enough x, this violates • unitarity • Growth is tamed by gluon fusion: • saturation of parton densities at • Q2=Qs2(x) • So far not observed in pp • interactions Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  36. “Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” partLow-x QCD: Forward jets 1 pb-1 PYTHIA ~ NLO Inclusive forward (3<|| < 5) low-ET (20-100GeV) jet production Inclusive single jet cross section would be reduced by saturation Di-jets sensitive to gluons with: x2 ~ 10-4, x1 ~ 10-1 Large expected yields (~107 at ~20 GeV) ! At the LHC, accessible xBJ(min) decreases by ~10 every 2 units of rapidity Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  37. “Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” partLow-x QCD: Forward Drell-Yan Sensitivity to saturation: saturated PDF • Gives access to low-xBJ quarks in proton • in case of large imbalance of fractional • momenta x1,2 of leptons, which are then • boosted to large rapidities • CASTOR with 5.3 ≤ || ≤ 6.6 gives access to xBJ~10-6 to 10-7 • Measure angle of electrons with T2 • Pdf’s known at large xBJ, hence can extract pdf’s at low xBJ DY pairs suppressed in saturated PDF Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  38. “Prospects for diff and fwd physics at the LHC” - 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, March 2007

  39. Summary CMS aims at carrying out a program of diffractive and forward physics as integral part of its scientific program during routine data taking at the LHC and up to the highest available luminosities The combination of CMS (CASTOR, ZDC) with TOTEM (T1, T2, near-beam detectors at 220m) and with FP420 offers ideal conditions for carrying out a rich program of diffractive and forward physics as integral part of the routine data taking at the LHC and up to the highest achievable luminosities. CMS aims at a formal agreement with TOTEM on this collaboration. Feasibility of the detectors suggested by FP420 essentially proven. Decision on building detectors at 420m, as suggested by FP420, as part of CMS to be addressed soon. Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

  40. Last words... “QCD at a hadron collider is always a ghetto, and there the worst neighborhood is diffraction.” - W.Smith Recently the diffraction and forward physics group was promoted to one of the principal physics analysis groups of CMS. We are on the way to respectability ! Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007

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