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Pushing the limits: Triggering and forward physics at the LHC

Pushing the limits: Triggering and forward physics at the LHC. Monika Grothe U Wisconsin/ U Turin. Boundary conditions for triggering at the LHC Trigger architecture at the LHC/CMS Level-1 trigger algorithm at the LHC/CMS High level trigger algorithms at the LHC/CMS

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Pushing the limits: Triggering and forward physics at the LHC

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  1. Pushing the limits: Triggering and forward physics at the LHC Monika Grothe U Wisconsin/ U Turin • Boundary conditions for triggering at the LHC • Trigger architecture at the LHC/CMS • Level-1 trigger algorithm at the LHC/CMS • High level trigger algorithms at the LHC/CMS • Pushing the limits: Triggering on forward physics • Diffractive and forward physics with CMS + Totem (+ FP420) • Outlook: A Level-1 track trigger for the SLHC Monika Grothe, Triggering and forward physics at the LHC, July 2007

  2. Boundary conditions for triggering at the LHC Monika Grothe, Triggering and forward physics at the LHC, July 2007

  3. Simple example of a trigger (U Bonn) some logic operation and synchronization of signals: “trigger logic” Record pulse height spectrum of cosmic rays in a 30 degree slice trigger decision data is stored depending on trigger decision An interesting event: Pulses with height above a threshold are seen in coincidence with Z12 in 30o slice around Z12 Monika Grothe, Triggering and forward physics at the LHC, July 2007

  4. Boundary conditionsFirst try: A trigger at the LHC Job of a trigger: Indicate that something of interest was seen in the detector If an interesting “event” occured, record all its data for later detailed analysis  Attention: An event not flagged as interesting by the trigger is lost forever • Simple-minded approach: At the LHC, each crossing of bunches that leads to one • (or more) pp collisions is an interesting event. Why don’t we simply trigger on • each of them and record all associated subdetector data ? • At the LHC, per year 1016 events • For each event total size of data to eventually store is about 1 MByte • 1016 x 1 MHz = 107 PetaByte Need to compromise between physics and affordability Affordable ? CMS Tier-1 centers will provide about O(1) PByte of storage in 2008 Reduction of 7 orders of magnitude !!  What does this mean for physics ? ! ? Monika Grothe, Triggering and forward physics at the LHC, July 2007

  5. Boundary conditionsEvent rates at the LHC Rates for L = 1034 cm-2 s-1: (LHC) • Inelastic pp reactions: 109 / s • bb pairs 5 106 / s • tt pairs 8 / s • W  e n 150 / s • Z  e e 15 / s • Higgs (150 GeV) 0.2 / s • Gluino, Squarks (1 TeV) 0.03 / s • For cost reasons: • First level trigger output limited to • ~100 kHz • Higher level triggers output limited to • ~100 Hz Monika Grothe, Triggering and forward physics at the LHC, July 2007

  6. Boundary conditionsThe LHC and its experiments pp ATLAS and CMS : pp, general purpose 27 km ring used for e+e- LEP machine in 1989-2000 ALICE : heavy ions LHCb : pp, B-physics + TOTEM at the CMS IP + LHCf at the ATLAS IP s = 14 TeV search for new massive particles up to m ~ 5 TeV 7 x higher than Tevatron Ldesign = 1034 cm-2 s-1 search for rare processes with small s (N = Ls ) 102 x higher than Tevatron Monika Grothe, Triggering and forward physics at the LHC, July 2007

  7. Boundary conditionsPhysics at a proton-proton collider x1p x2p   q q W W q H p p q   Most interactions are due to interactions at large distance between incoming protons → small momentum transfer, particles in the final state have large longitudinal, but small transverse momentum p pT = p sin  pT  plane perpendicular to the beam “Interesting events”: Processes with large transverse momentum pT Proton beam can be seen as beam of quarks and gluons with a wide band of energies Hard scatter between these proton constituents Monika Grothe, Triggering and forward physics at the LHC, July 2007

  8. Boundary conditionsEvent pile-up Because of very high particle density in LHC proton bunches: Event pile-up per bunch crossing Consequence: High particle density in detector H  ZZ, Z   cleanest "golden" signature But at L = 1034 cm-2 s-1 overlapped by O(25) non-elastic events And this (not the Higgs though) repeats every 25 ns Monika Grothe, Triggering and forward physics at the LHC, July 2007

  9. Recap: Boundary conditions • Boundary conditions for triggering at the LHC: • Affordability limits read-out bandwidth and storage capacity • Huge cross section at LHC energies • Rate reduction of overall 7 orders of magnitude needed • High particle density in detector because of event pile-up • Discovery physics with relatively high ET/pT Base trigger on selection of high ET/pT objects Monika Grothe, Triggering and forward physics at the LHC, July 2007

  10. Trigger architectureat the LHC/CMS Monika Grothe, Triggering and forward physics at the LHC, July 2007

  11. Trigger architectureMulti level trigger Monika Grothe, Triggering and forward physics at the LHC, July 2007

  12. Trigger architectureMulti level trigger (II) 107 channels Decision within a few s L1 In: 1 GHz L1 Out: 100 kHz 1000 units 103 x 103 switch fabric Decision within a few 100 ms HLT In: 100 kHz HLT Out: 100 Hz 4x106 MIPS L1: trigger decision algorithms implemented in fast, custom-made electronics only very much reduced information on event at its disposal HLT: implemented as software algorithms run on a processor farm in principle full event information at it disposal Monika Grothe, Triggering and forward physics at the LHC, July 2007

  13. Trigger architecturePipelines on L1 Speed of light in air: 0.3m/ns Outer diameter of CMS detector ~7m, hence particle needs up to 23ns to cause signal in CMS muon chamber At LHC 25ns btw bunch crossings It is impossible to form a trigger decision within 25 ns of each bunch crossing Since in principle every bunch crossing could result in an interesting event, need way to store them till trigger came to a decision: pipelines g L1 trigger latency = depth of pipeline = maximum time available for L1 trigger decision Monika Grothe, Triggering and forward physics at the LHC, July 2007

  14. Recap: Trigger architecture • General trigger architecture at the LHC/CMS: • Pipelined, multi-level trigger • L1: latency a few s, coarse granularity information, custom-made electronics • HLT: latency a few 100 ms, full event info, computer farm Monika Grothe, Triggering and forward physics at the LHC, July 2007

  15. Trigger algorithms for Level-1 and Higher Levels Triggerat the LHC/CMS Monika Grothe, Triggering and forward physics at the LHC, July 2007

  16. L1 trigger algorithmsInformation used for L1 algorithms Monika Grothe, Triggering and forward physics at the LHC, July 2007

  17. L1 trigger algorithmsWhy no track information on L1 ? Monika Grothe, Triggering and forward physics at the LHC, July 2007

  18. L1 trigger algorithmsExample: L1 calorimeter trigger Trigger towers: smallest unit trigger electronics looks at L1 calorimeter trigger searches for trigger tower clusters with maximum ET Small cluster: L1 electron or photon Big cluster: L1 jet Monika Grothe, Triggering and forward physics at the LHC, July 2007

  19. L1 trigger algorithmsL1 electrons/photons and jets L1 electron or photon: L1 jet: Sliding 3x3 region window Region = 4x4 trigger towers Monika Grothe, Triggering and forward physics at the LHC, July 2007

  20. L1 trigger algorithmsAlgo example: Isolated electron/photon isolated L1 electron or photon Algorithm implemented in asics Monika Grothe, Triggering and forward physics at the LHC, July 2007

  21. L1 trigger algorithmsExample L1 trigger table (L = 2x1033 cm-2 s-1)  3 safety factor  50 kHz (expected start-up DAQ bandwidth) Purity is not the issue, but 100% efficiency for “interesting events” while respecting bandwidth limits Background to e/ are mainly  in em-rich jets Background to jets are jets - huge rate of QCD jets Monika Grothe, Triggering and forward physics at the LHC, July 2007

  22. HLT trigger algorithmsHLT electron algorithm “Level 2” step: Starting from L1 em object information as seed, reconstruct clusterof ECAL crystals in which electron has deposited its energy reconstruct electron energy and position from cluster “Level 2.5” step: Match cluster with hits in the pixel detector “Level 3” step: Starting from pixel detector seed, reconstruct full track information 1 trigger tower = 5x5 crystals e efficiency vs jet rejection for L2.5 pixel matching: ECAl barrel ECAL endcap CMS pixel detector coverage Monika Grothe, Triggering and forward physics at the LHC, July 2007

  23. Recap: Trigger algorithms • Trigger algorithms on L1 and HLT at LHC/CMS: • Algorithms on L1 based on calorimeter and muon system information, tracking only on HLT • Main objective of L1 trigger: Keep output rate under control, i.e. purity is not the issue, but maximum signal efficiency is • Example L1 calo trigger: Looks for local maxima in ET, large cluster = L1 jet , small cluster: L1 electron/photon • Example HLT electron trigger: Reduction of jet background by factor 10 by requiring matching pixel track stub • Allocated trigger bandwidth per trigger condition summarized in L1 and HLT trigger menus • Trigger menus determine physics reach of experiment Monika Grothe, Triggering and forward physics at the LHC, July 2007

  24. Pushing the limits: Triggering on forward physics Monika Grothe, Triggering and forward physics at the LHC, July 2007

  25. Triggering on forward physicsForward physics Experimental definition: All processes in which particles are produced at small polar angles. • = 90o   = 0 •  = 10o    2.4 • = 170o    -2.4 • = 1o    5.0 edge of coverage of central CMS/ATLAS detectors Monika Grothe, Triggering and forward physics at the LHC, July 2007

  26. Triggering on forward physicsExample of forward physics:Diffraction Monika Grothe, Triggering and forward physics at the LHC, July 2007

  27. Great. And why bother at the LHC ? Diffraction as tool for discovery physics ! ? Monika Grothe, Triggering and forward physics at the LHC, July 2007

  28. shields color charge of other two gluons Vacuum quantum numbers “Double Pomeron exchange” Triggering on forward physicsSuppose 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 (to good approx) JPC = 0++ SM Higgs with ~120 GeV: gg  H, H  b bbar highest BR But signal swamped by gg  jet jet Best bet with CMS: H   Monika Grothe, Triggering and forward physics at the LHC, July 2007

  29. beam dipole dipole p’ roman pots p’ roman pots Triggering on forward physicsDiffraction as tool for discovery physics:CEP pp  pXp with X = H(~120 GeV) b bbar In non-diffractive production hopeless, signal swamped by QCD di-jet background  Selection rules: central system is JPC = 0++(to good approx) I.e. a particle produced with proton tags has known quantum #  For light (~120 GeV) Higgs: Proton tagging improves S/B for SM Higgs dramatically CEP may be discovery channel in certain regions in MSSM  CP quantum numbers and CP violation in Higgs sector directly measurable from azimuthal asymmetry of the protons Needed: Proton spectrometer using the LHC beam magnets Detect diffractively scattered protons inside of beam pipe Monika Grothe, Triggering and forward physics at the LHC, July 2007

  30. Triggering on forward physicsCMS + TOTEM (+ FP420) CMS IP T1/T2, Castor ZDC RPs@150m RPs@220m possibly detectors@420m • 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 (2008) Possible addition FP420: Silicon tracking detectors and fast timing Cherenkov detectors, integrated into a LHC cryostat at 420m from IP Monika Grothe, Triggering and forward physics at the LHC, July 2007

  31. Triggering on forward physicsThe difficulty of triggering on a 120GeV Higgs 120 GeV Higgs decays preferably into 2 b-jets with ~60 GeV each 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) 2 x 1033 cm-2 s-1 L1jet trigger rates 100 Rate (kHz) 10 L1 output bandwidth: 100 kHz 1 Is it possible to lower the CMS jet trigger thresholds significantly by combining central CMS jet trigger condition with condition on forward detectors ? 60 L1 ET threshold (GeV) Note: Usable in L1 trigger only 220m proton taggers, 420m too far away from IP for signal to arrive within L1 latency of 3.2 s Attention: Cumulative rate shown Total number of events with ET above threshold and function of threshold

  32. Triggering on forward physicsA dedicated forward detectors L1 trigger stream → 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 forward detectors 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 Demonstrated that for luminosities up to 2x 1033 cm-1 s-1 including 220m detectors into the L1 trigger provides a rate reduction sufficient to lower the 2-jet threshold substantially to 40GeV while still meeting the CMS L1 bandwidth limits Monika Grothe, Triggering and forward physics at the LHC, July 2007

  33. Triggering on forward physicsEfficiency of forward detectors L1 stream for diffractive events 420m Efficiency 220m 420+420m 420+220m H(120 GeV) → b bbar L1 trigger threshold [GeV] Efficiency pp  p jj X 2-jet trigger no fwd detectors condition Events per pb-1 Central exclusive production pp pHp with H (120GeV)  bb: Can gain another 10% from  trigger single-arm 220m single-arm 420m 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 Monika Grothe, Triggering and forward physics at the LHC, July 2007

  34. Triggering on forward physicsExperimental challenge:Pile-up background ! Monika Grothe, Triggering and forward physics at the LHC, July 2007

  35. Triggering on forward physicsPile-up background (II) TOTEM d(epeXp)/dxL [nb] det@420 xL=P’/Pbeam= 1-x Diff events characterized by low fractional proton momentum loss diffractive peak Number of PU events with protons within acceptance of near-beam detectors on either side: ~2 % with p @ 420m ~6 % with p @ 220m Coincidence of non-diffractive event with protons from pile-up events in the near-beam detectors:  fake double-Pomeron exchange signature Non-diffractive event with signature in the central CMS detector identical to some DPE signal event: At 2x 1033 cm-2s-1 10% of these non-diffractive events will be mis-identified as DPE event. This is independent of the specific signal. Monika Grothe, Triggering and forward physics at the LHC, July 2007

  36. Triggering on forward physicsHandles against pile-up background Can be reduced on the High Level trigger: 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 within 3mm Further offline cuts possible: Condition that no second vertex be found within 3mm vertex window left open by fast timing detectors Exploiting difference in multiplicity between diff signal and non-diff background ; 12 s = M2 incl QCD di-jets + PU CEP H(120) bb (jets) (p tagger) CEP of H(120 GeV) → b bbar and H(140 GeV) → WW: S/B of unity for a SM Higgs M(2-jets)/M(p’s) Monika Grothe, Triggering and forward physics at the LHC, July 2007

  37. Recap: Triggering on forward physics • CMS trigger menus now foresee a dedicated forward detectors trigger stream: • Trigger at LHC designed for high pT physics, trigger thresholds generally too high for forward physics which has by definition lower pT • Combining central CMS detector trigger conditions with condition on Totem 220m proton tagger allows to lower in particular L1 jet trigger thresholds substantially while respecting trigger output rate limits • HLT forward detectors trigger algorithms reduce background from pile-up substantially and could use 420m proton tagger information • Central exclusive production of a low mass Higgs boson is physics channel profits substantially from new stream Monika Grothe, Triggering and forward physics at the LHC, July 2007

  38. The thus designed dedicated forward detectors trigger stream forms an essential part of an extension of its baseline program that CMS wishes to implement: Diffractive and forward physics with CMS + Totem *at nominal LHC optics * possibly also including FP420 Monika Grothe, Triggering and forward physics at the LHC, July 2007

  39. The CMS + Totem (+ FP420) program Objective: 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. Areas covered, in addition to diffraction as tool for discovery physics in central exclusive production: • Diffraction in the presence of a hard scale: “Looking at the proton through a lense that filters out anything but the vacuum quantum numbers • Diffractive structure functions • Soft rescattering effects/underlying event and rapidity gap survival factor • Low xBJ structure of the proton • Saturation, color glass condensates • Rich program of and p physics • Validation of cosmic ray air shower MC CERN/LHC 2006-039/G-124 M. Grothe, J. Mnich primary editors from CMS side Monika Grothe, Triggering and forward physics at the LHC, July 2007

  40. CMS + TOTEM (+ FP420)Unprecedented kinematic coverage Castor Castor TOTEM det @420 d(epeXp)/dxL [nb] ZDC ZDC xL=P’/Pbeam= 1-x CMS Castor thungsten/quartz Cherenkov calorimeter TOTEM T2: GEM tracking detector CMS ZDC thungsten/quartz Cherenkov calorimeter TOTEM Silicon tracking det. housed in Roman pots Monika Grothe, Triggering and forward physics at the LHC, July 2007

  41. Recap: CMS + Totem (+ FP420) program • CMS + Totem intend to carry out a joint program on diffractive and forward physics • Unprecedented kinematic coverage • LoI submitted to LHCC Dec 2006 • LoI identifies and addresses for the first time at the LHC central experimental questions for carrying out a program that spans the full lifetime of the LHC • FP420 as possible extension of program currently under review in CMS (and ATLAS), document on results of extensive R&D effort over the past several years in preparation, decision by end of this year Monika Grothe, Triggering and forward physics at the LHC, July 2007

  42. Outlook:A Level-1 track trigger for the SLHC Monika Grothe, Triggering and forward physics at the LHC, July 2007

  43. Outlook: L1 track triggerTracking in the L1 trigger: CMS ideas • Not done at the LHC because of difficulty of high density of low pT tracks in tracker • At the SLHC, the increase of luminosity x10, to 1035 cm-2 s-1, will degrade efficiency of LHC algorithms in keeping the L1 output rate under control Tracking trigger on L1 the solution ? • Example electron trigger: On HLT, matching of Calo electron candidate with pixel detector hits reduces background by a factor 10 • Tracking trigger challenge: • Find only high pT track stubs and match them with L1 electron and muon objects • Because of high occupancy at the SLHC, need to do so while keeping read-out data from detector at a minimum • On-detector hit correlator needed to reduce combinatorics • Suggested solution: Stacked pixel layers -- C. Foudas & J. Jones A track like this wouldn’t trigger: <5mm w=1cm ; l=2cm  rL y Search Window rB x Monika Grothe, Triggering and forward physics at the LHC, July 2007

  44. Outlook: L1 track triggerTracking in the L1 trigger: CMS ideas (II) Optical fibre to OptoTX card Optical Transceiver Cooling System Correlator ASIC Flip bonded sensors Thermal Epoxy Kevlar-Carbon Fibre Laminate Support Structure • Use closely spaced stacked pixel layers • Angle of track bisecting sensor layers defines pT • Track stub: Combination of hits in the 2 layers which are at most 1 pixel apart • Pipelined column-parallel readout architecture where each pixel in a column forms a single cell in the pipeline • Self-timed, asynchronous system with self-triggering pixels • One module of 2 stacked layers a few millimeters apart would allow track stub reco • Two modules. e.g. at 10cm and 20cm from the beam line, would allow full track reco J. Jones et al, A pixel detector for L1 triggering at SLHC, LECC2005 J. Jones et al, Stacked tracking for CMS at SLHC, LECC 2006 Monika Grothe, Triggering and forward physics at the LHC, July 2007

  45. Grand summary • Triggering at the LHC is not an easy job • Pipelined multi-level trigger architecture with algorithms that identify high ET/pT objects • Extension of the CMS trigger menus: A dedicated forward detectors trigger stream • Extension of the CMS baseline physics program: A joint CMS + Totem (possibly + FP420) program on diffractive and forward physics • Motivation - Capitalize on diffraction as tool for discovery physics: Central exclusive Higgs production • A possible upgrade of the CMS L1 trigger for the SLHC: Conceptual design for a Level-1 track trigger at the SLHC Monika Grothe, Triggering and forward physics at the LHC, July 2007

  46. Backup Monika Grothe, Triggering and forward physics at the LHC, July 2007

  47. HLT trigger algorithmsHigh level triggers strategy In CMS all trigger decisions beyond Level-1 are performed in a Filter Farm running ~normal CMS reconstruction software on “PCs” The filter algorithms are setup in several steps HLT does partial event reconstruction “on demand” seeded by the L1 objects found, using full detector resolution Algorithms are essentially offline quality but optimized for fast performance Monika Grothe, Triggering and forward physics at the LHC, July 2007

  48. L1 trigger rates 250 GeV jets80 GeV tt 30-40 GeV for m or e20 GeV each for gg • Trigger cuts determine physics reach! • Efficiency for Hgg and H4 leptons = >90% (in fiducial volume of detector) • Efficiency for WH and ttH production with Wln = ~85% • Efficiency for qqH with Htt (t1/3 prong hadronic) = ~75% • Efficiency for qqH with Hinvisible or Hbb = ~40-50% Monika Grothe, Triggering and forward physics at the LHC, July 2007

  49. Trigger Mapping onto Cal surface Area covered by 1 jet Monika Grothe, Triggering and forward physics at the LHC, July 2007

  50. How does it look in real life ? Monika Grothe, Triggering and forward physics at the LHC, July 2007

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