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Direct Photon-Jet Events at CMS with Ös = 14 TeV: Preliminary Exam Outline

This preliminary exam outline discusses g+jet events at CMS, focusing on the Standard Model, reconstructed jets, photons, and future computing steps. Topics include physics analysis, particle detection, Large Hadron Collider details, calibration techniques, and computing centers.

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Direct Photon-Jet Events at CMS with Ös = 14 TeV: Preliminary Exam Outline

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  1. Direct Photon + Jet Events at CMS with Ös = 14 TeV Michael Anderson University of Wisconsin Preliminary Exam

  2. Outline • Standard Model • g + jet Events • Large Hadron Collider • Compact Muon Solenoid Detector • Computing • Reconstructed Jets and Photons • Future Steps Computing @Wisconsin

  3. Standard Model Higgs Quarks (Fermions) Force Carriers (Bosons) • 12 elementary matter particles • 4 force-carrying particles • 1 so far undetected particle: Higgs Leptons (Fermions)

  4. Direct Photons  + Jet • Why Photons? • Photons don’t fragment • Energy & position can be measured accurately • Provide good probe of hard-scattering process • Provides direct measure of pdf for gluons • g+Jet has high cross-section,s ~ 2*108 mb • Photons also can be used in new physics searches • Gauge Mediated SUSY Breaking • Prompt photon & extra dimension models Compton-like Annihilation Signals involve’s + missing energy

  5. Photon + Jet • Good for Calibration! • High resolution on prompt  energy ~1% • Jet &  balance in  • Can use data to make jet energy corrections • Photon energy calibrated from 0 decays • Can then be used to calibrate all jet algorithms g p p

  6. Calibrating Jets • The jet energy losses can be divided into categories: • response of the calorimeter to different particles, • non-linearity response of the calorimeter to the particle energies, • un-instrumented regions of the detector, • energy radiated outside the jet clustering algorithm, • multiple interactions and underlying event • +jet calibration used by D0 for better than 3% accuracy for jets with 20 GeV < Et < GeV B. Abbot et al. “Determination of the Absolute Jet Energy Scale in the D0 Calorimeter.” Nucl Instr and Meth. A424 (1999)

  7. Large Hadron Collider • 14 TeV proton-proton collider • Circumference of 27 km • Luminosity up to 1034 cm-2s-1 • 8T Magnets

  8. General-Purpose Detectors • ATLAS • Weight: 7,000 T • Diameter: 25 m • Length: 46 m • CMS • Weight: 12,500 T • Diameter: 15.0 m • Length: 21.5 m

  9. Proton interactions at LHC @start-up 1028 - 1031 cm-2s-1 Luminosity L = particle flux/time Interaction rate Cross section  = “effective” area of interacting particles

  10. CMS 4T solenoid Muon chambers Forward calorimeter Silicon Strip & Pixel Tracker Electromagnetic Calorimeter PbWO4 Crystals Hadronic calorimeterBrass/Scintillator

  11. Current CMS Underground Surface

  12. Particle Detection in CMS • Photons: • “Super Cluster” of Energy in ECAL • No nearby track • Jets • Energy deposit in ECAL & HCAL • With tracks • Detailed Reconstruction discussed on later slide

  13. Silicon Tracker • Measures pt & path of charged particles within |h| < 2.5 • Strip Tracker • 200 m2 coverage • 10m precision measurements • 11M electronic channels • Inner Pixel tracking system • 66M channels • Used for rejecting electrons in this analysis

  14. Electromagnetic Calorimeter • Measures energy & position of electrons and photons within |h| < 3 • PbWO4 crystals • 61K in the barrel, 22 x 22 mm2 • 15K in the endcaps, 28 x 28 mm2 • Ultimate precision of energy resolution: 0.5% • Preshower detector for endcaps • Silicon sensors, 4300 modules, 137K channels

  15. Hadron Calorimeter • HB and HE: brass & scintillator with WLS fiber readout • Coverage to  < 3,  x  =0.087x0.087 (coarser for  >1.8) • Hadron Outer calorimeter (tail catcher) outside solenoid • Hadron Forward: steel & quartz fiber: coverage 3 <  < 5 Approx. 10K channels,Hybrid PhotoDiode readout for all but HF (PMT) HB HE

  16. Trigger • Level 1: Hardware trigger operating at beam crossing rate • Level 2: • Reconstruction done using High-Level Trigger (HLT) -- computer farm • Reduces rate from Level-1 value of up to 100 kHz to final value of ~100 Hz • Slower, but determines energies and track momenta to high precision

  17. Level-1 Trigger Calorimeter Trigger Muon Trigger • Every event is ~1MB each • Identifies potential photons, jets… • Hardware implemented • Reduces rate from 40 MHz -> 100 kHz • Processes each event in 3 s RPC CSC DT HF HCAL ECAL Local CSC Trigger Local DT Trigger RegionalCalorimeterTrigger PatternComparator Trigger CSC TrackFinder DT TrackFinder GlobalCalorimeterTrigger 40 MHz pipeline, latency < 3.2 ms Global Muon Trigger e, J, ET, HT, ETmiss 4 m Global Trigger max. 100 kHz L1 Accept

  18. Computing • Tier 0 at CERN • Record raw data and Data Summary Tapes (DST) • Distribute these to T1’s • Tier 1 centers • Pull data from T0 to T1 and store • Make data available to T2 UW Madison RAL Oxford T1 FNAL Chicago T1 T2 FZK Karlsruhe T1 T0 • Tier 2 centers • DST analysis…. • Local data distribution • Wisconsin is a T2 site T1 T1 CNAF Bologna T1 IN2P3 Lyon CMS data sizes and computing needs require a worldwide approach to Physics analysis. FNAL is US CMS national computing center. PIC Barcelona

  19. Simulation Workflow ALPGEN Hard scattering • g+jets simulated with Alpgen v2.1 • Fixed order matrix element simulated event generator • Generates multi-parton and boson + multi-parton processes in hadronic collisions. • Jet simulation with Pythia v6.409 • Generates event hadronization, parton shower, and I/FSR, underlying event • Detector simulated using GEANT4 • Toolkit for the simulation of the passage of particles through matter • Reconstruction with CMS software PYTHIA Underlying event GEANT4 Detector simulation CMSSW Reconstruction of event

  20. Photon Reconstruction • Photons are built from ECAL SuperClusters • Hybrid clustering algorithm in barrel • Island algorithm in endcaps • All SuperClusters are candidate Photons • “Photon object” contains: • energy in 5x5 crystals • Ratio of energy in 3x3 crystal to SuperCluster energy (R9) • ratio of the energy in center crystal to 3x3 crystals (R19) • presence or not of a matched pixel seed R9 R19

  21. Reducing Fake Photons • Jet can fake photon • Leading neutral mesons in jet (like 0-> and ->) can make narrow deposit of electromagnetic energy • Charged mesons and electrons rejected by tracking system • Strategies to minimize this fake  background: • Reconstruct multiple  energy deposits - see if are decay products of a neutral meson • Shower is often wider for meson decay • Reject ->e+e- conversions (multiple ’s from meson decay more likely to have conversions) • Isolation: require no high energy particles near it

  22. Reducing Fake Photons

  23. Cuts Starting with: 481,910 +jeteventswhere gen not in ECAL  gap, and Et > 15 GeV Photon  < 1.479 Select barrel photons 325,268 events ( 67%)‏ R Nearest Track > 0.1(track pt > 10 GeV) Isolation 311,609 events (65%)‏ Photon R9 > 0.9 Narrow Energy Deposit(Shower shape & rejects conversions) Final Count: 235,594 events (49%)‏

  24. Gen & Reco Photon: h, f • Relatively flat in h and f • Crack in ECAL h : 1.479 - 1.653 • Photons near this h harder to find/reconstruct h f

  25. Gen & Rec Photon: Et • Et of the generated & reconstructed photon match well • Difference in Et ~< 10% • Used default vertex (0,0,0) GeV Etgen – Etrec Etgen Longer tail on positive side

  26. Reco Photon: R9 E (3x3 crystals) E (SuperCluster) • R9 = • Photon shower is narrow • Photons that convert to e+e- pair in Tracker have lower R9 • ~73% of all photons have R9 > 0.94 • R9 used to select well measured photons R9

  27. Reco Photon: Nearest Track • Nearest Track where • Track pt > 10 GeV • R = Ö()2 + ()2 • Want isolated photons Nearest Track R R Nearest Track pt GeV

  28. Gen & Rec Photon: DEt/Etgen • (Etgen–Etrec)/ Etgen • After cuts, fewer events in positive tail • ~24% loss in # of events after R9 cut Before R9 cut After R9 cut

  29. Jet Reconstruction • Jet algorithms: • Iterative Cone • Seed Et > 1 GeV • DR = 0.5 or 0.7 • Good for new physics searches and photon+Jet balancing • Other algorithms useful also • Midpoint Cone • Merges jets based on overlap Threshold: 0.75 • Kt …

  30. Gen & Rec Jet: h, f • Jet alg: Iterative Cone • R = 0.5 • (Will try out others) • Highest-pt Jet with • pt > 10 GeV • 0.8p < |fjet-fg | < 1.2p • ~93% of events have reco jet that meet this criteria

  31. Gen & Rec Jet • Et of reco photon matches well with gen jet pt • But reco jet pt lower by ~ 22 GeV • Even the MC must be calibrated GeV (ptgen  – ptrec g) ptgen  (ptgenJet – ptrecJet) ptgenJet

  32. Conclusions • Photon + Jet events have high cross section • can get 1 fb-1 data in 1 year • Measure Photon+Jet Cross section • Probe hard scattering processes • Measure gluon pdf • Tune MC generators • Calibration of Jet energies is possible and effective using prompt photons • Does not require dependence on monte-carlo jet simulations

  33. Next Steps • Use L1 Trigger selection • Use HLT selected objects (photons, jets…) • Validate using RECO objects • Create a sample of good direct photon + jet events • Calibrate jet algorithms • Do physics with photons!

  34. Extras

  35. LHC Commissioning • Establish colliding beams as quickly & safely as possible • Planned turn on in 2008 • L = 1031 -> ~1 fb-1 in year

  36. Muon System • 3 technologies, all self-triggering • drift-tubes(DT), cathode strip chambers(CSC), resistive plate chambers (RPC) • 25000 m2 of active detection planes • 100m position precision in DT and CSC • About 1M electronic channels DT CSC RPC

  37. Problem h Regions • Just events whereDEt/Etgen > 0.1 • Spikes at module boundaries • biggest at h = 1.479 • Cuts over 50% of these events Before R9 cut h After R9 cut h

  38. Higgs -> g g • Inclusive Search for the Higgs Boson in the H → γγ Channel, M. Pieri, S. Bhattacharya, I. Fisk, J. Letts, V. Litvin, J.G. Branson CMS Note 2006/112 • Studied 6 types of background and used various higgs masses Note multiplication • Tried to optimize signal 2 different ways: with cuts and with neural nets • Here are their results with cuts:

  39. LHC Details In the LEP tunnel • pp s =14 TeV L=1034 cm-2 s-1=10 mb-1MHz • crossing rate 40 MHz • Heavy ions (1 month/year)

  40. Computing M.I.T. Wisconsin* Purdue Nebraska Caltech* UC San Diego* Florida* T0 at CERN, T1 at Fermilab as US CMS national center (“super” Tier 1 with twice the resources of other Tier1s). Keep many of the trigger streams at FNAL. T2 at UCSD, Caltech, UFlorida, Wisconsin, MIT, Nebraska and Purdue as regional US CMS centers. ( + Brazil + ?) Not every country has a T1. There are strong “T3” institutions springing up also.

  41. Production of Higgs Gluon Fusion Bremsstrahlung Top Fusion Vector Boson Fusion

  42. Higgs Decay to Photons • Branching Fraction of ~10-3 in range 120-150 GeV

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