Direct Photon + Jet Events at the Compact Muon Solenoid with Ö s = 14 TeV

1. Direct Photon + Jet Events at the Compact Muon Solenoid 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 • Detector/Physics Simulation • Reconstructed Jets and Photons • Conclusions & Plans Simulations @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) Quarks only exist in colorless composites!

4. Direct Photons Non-direct ’s example:final state radiation Compton-like • Emerge directly from hard scattering • Why look for direct photons? • Energy & position can be measured accurately • Provide good probe of hard-scattering process • Provides direct measure of pdf for gluons • Direct ’s have high cross-section,s ~ 1 nb for Et( > 20 GeV • Useful in new physics searches Annihilation

5. New Physics with  + jet ~ ~ • Supersymmetry • p p ->  + b + missing Et (neutralino) • Gauge-Mediated Supersymmetry Breaking • p p ->  +  + missing Et (gravitino) • Technicolor • New fermions carry technicolor quantum number • Technicolor meson: T ->  + b + jet ~ - - ~ ~ ~ ~ ~

6. Jets • Quarks exist in colorless composites • Thus fragment to into hadrons • Detector needs to be calibrated for jets • Want to accurately measure 4-momentum of scattered parton • 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, • multiple interactions and underlying event

7. 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 • +jet calibration used by D0 for better than 3% accuracy for jets with 20 GeV < Et < 500 GeV g p p B. Abbot et al. “Determination of the Absolute Jet Energy Scale in the D0 Calorimeter.” Nucl Instr and Meth. A424 (1999)

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

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. Compact Muon Solenoid Solenoid (4T) Muon chambers Forward calorimeter 6 m diameter Silicon Strip & Pixel Tracker Electromagnetic Calorimeter Hadronic CalorimeterBrass/Scintillator

11. Compact Muon Solenoid ← Surface assembly hall CMS is pieced together underground ↓ Solenoid Endcap Discs: Designed, assembled & installed by Wisconsin

12. Particle Detection in CMS • Photons: • Deposit of of Energy in ECAL • No nearby track • Jets • Energy deposit in ECAL & HCAL • With tracks • Detailed reconstruction 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 =-ln(tan(/2) • Inner Pixel tracking system • 66M channels • Used for finding isolated photons and rejecting jets

14. Electromagnetic Calorimeter  =-ln(tan(/2) • Measures energy & position of electrons and photons within |h| < 3 • PbWO4 crystals, very dense (8.3 g/cm3) • 23 cm long (26 radiation lengths) • 61K in the barrel, 22 x 22 mm2 • 15K in the endcaps, 28 x 28 mm2 • Ultimate precision of energy resolution: 0.5%

15. ECAL Crystal Calibration Aftercalibration 50 GeV electrons • 9 GeV beam of 0’s used to calibrate ECAL crystals • Energy spectra for 50 GeV electron beam centered on different crystals after calibration • Width of 0.67% consistent with statistical expectation 9 GeV beam uncalibrated Raw p0 mass

16. Hadron Calorimeter • HCAL Barrel and HCAL Endcap: brass & scintillator • Coverage to  < 3 •  x  =0.087x0.087 • Hadron Outer calorimeter (tail catcher) outside solenoid • Hadron Forward: steel & quartz fiber: coverage 3 <  < 5 • Approx 10K channels Barrel Resolution HB HE

17. Trigger • Level 1: Hardware trigger operating at 40MHz beam crossing rate • Brings event rate down to 100 kHz • 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

18. 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

19. Level-2 e/ Trigger ET • Create “super-clusters” from clusters of energy deposits using Level-1 ECAL information • Must be in area specified by Level-1 trigger • Must have ET greater than some threshold • Match super-clusters to hits in pixel detector • Photons don’t create a hit • Electrons do! • Combine with full tracking information • Track seeded with pixel hit • Final cuts made to isolate photons Tracker Strips Pixels 

20. Outline • Standard Model • g + jet Events • Large Hadron Collider • Compact Muon Solenoid Detector • Detector/Physics Simulation • Reconstructed Jets and Photons • Conclusions & Plans Simulations @Wisconsin

21. Computing CMS data sizes and computing needs require a worldwide approach to Physics analysis. FNAL is US CMS national computing center. • Tier 0 at CERN • Record raw data & reconstruction • Distribute to T1’s • Tier 1 centers • Store data from T0’s • Make available to T2’s UW Madison RAL Oxford T1 FNAL Chicago T1 T2 FZK Karlsruhe T1 T0 T1 T1 CNAF Bologna T1 IN2P3 Lyon • Tier 2 centers • Data Analysis • Local Data Distribution • Wisconsin is a T2 site PIC Barcelona

22. 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 Initial/Final State Radiation, underlying event • Detector simulated using GEANT4 • Toolkit for the simulation of the passage of particles through matter • Reconstruction with CMS software • Same software will be used on real data! PYTHIA Hadronization GEANT4 Detector simulation CMSSW Reconstruction of event

23. Photon Reconstruction • Photons are built from ECAL SuperClusters • 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 Number of Events / bin R9 R19

24. 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 • Shower is often widerfor meson decay

25. Reducing Fake Photons • Strategies to minimize fake  background • Reconstruct multiple  energy deposits - see if are decay products of a neutral meson • Select narrow energy deposits • Reject ->e+e- conversions (multiple ’s from meson decay more likely to have conversions) • Isolation: require no high energy particles near it

26. Event Selection

27. Photon h, f GeneratedReconstructed • Relatively flat in h and f • Crack in ECAL h : 1.479 - 1.653 • Photons near this h harder to find/reconstruct # Events / bin Keep||<1.479 h # Events / bin f

28. Photon Et Keep15 GeV < Et • Et of the generated & reconstructed photon match well • Difference in Et ~< 10% • Used default vertex (0,0,0) # Events / bin GeneratedReconstructed GeV Etgen – Etrec Etgen # Events / bin Longer tail on positive side

29. Photon Shower-Shape 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 in +jet have R9 > 0.94 • R9 used to select well measured photons Keep0.94 < R9 # Events / bin R9

30. Nearest Track to Photon Keep0.1 < R • Want isolated photons • Nearest Track where • Track pt > 10 GeV • Will lower to 1.5 GeV in future • R = Ö()2 + ()2 Nearest Track R # Events / bin Most in overflow R Nearest Track pt # Events / bin GeV

31. Photon Energy Resolution • Select clean ’s for calibration • (Etgen–Etrec)/ Etgen • After cuts, fewer events in positive tail • ~24% loss in # of events after R9 cut Before cuts # Events / bin After cuts # Events / bin

32. Event Selection

33. Jet Reconstruction • Cone algorithms: • Find seed tower (starting positions for cone) • Move cone around until ETin cone is maximized • Determine the merging of overlapping cones • Example: • Iterative Cone • Seed Et > 1 GeV • R = 0.5 or 0.7 • Good for new physics searches and photon+Jet balancing R = Ö()2 + ()2

34. Jet h, f GeneratedReconstructed • 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 reconstructed jet that meet this criteria # Events / bin h GeneratedReconstructed # Events / bin f

35. Jet Calibration Generated  EtGenerated Jet ptReconstructed Jet pt • Et of reconsructed photon matches well with generated jet pt • But reconstructed jet pt lower by ~ 22 GeV • Even the MC must be calibrated • Well-reconstructed ’s can be used to calibrate jets! # Events / bin GeV (Etgen  – Etrec g) Etgen  (ptgenJet – ptrecJet) ptgenJet # Events / bin Calibration will shift mean

36. Conclusion & Plans g • Photon + Jet events have high cross section •  ~ 1 nb (about 500,000 events in 100 pb-1) • Calibration of Jet energies is possible and effective using direct photons • Does not depend on monte-carlo jet simulations • Future plans • Probe hard scattering processes • Measure gluon pdf • Tune MC generators • Search for new physics

37. Extras

38. Calibration Procedure • True jet-parton calibration variable • Can be approximated by • These would be determined in bins of pt,g

39. Supersymmetry ~ ~ ~ ~ G G • Suggests new symmetry of nature • Every known Standard Model (SM) particle has a SUSY pair particle • SM fermions have SUSY boson partner • SM bosons have SUSY fermion partner • No SUSY particles discovered yet • SUSY particles have higher mass • Thus a broken symmetry! • Predicts massive, stable, weakly interacting particles • One signal for Gauge-Mediated SUSY involves: • decay of next-to-lightest SUSY particle to photon + lightest SUSY particle p p  

40. Prev. g + Jet Studies • Konoplianikov, V. et. al. “Jet Calibration using g+Jet Events in the CMS Detector.” CMS NOTE 2006/042 • D0 Collaboration, Abbot et al., “Determination of the absolute jet enrgy scale in the D0 Calorimeters.” Nuclear Instruments & Methods A 424 (1999) • D0 Collaboration, B. Abbott et al., "High-pT jets in pbar p collisions at √s=630 and 1800 GeV." Phys.Rev. D64 (2001)032003. • D0 Collaboration, B. Abbott et al., "Isolated Photon Cross Section in ppbar Collisions at √s = 1.8 TeV." Phys.Rev.Lett. 84 (2000)2786. • CDF Collaboration, F. Abe et al., "Properties of photon plus two-jet events in pbar p collisions at sqrt[s]=1.8 TeV." Phys.Rev. D57 (1998)67. • ISR-AFS Collaboration, T. Akesson et al., "Direct-photon plus away-side jet production in pbar p collisions at √s = 63 GeV and a determination of the gluon distribution." Zeit.Phys. C34 (1987)293.

41. 3 Generations • Measured cross-section for e+e- > Z/ > q q as function of center of mass energy. • Width of the peak gives a precise measurement of the rate at which Z decays, which in turn specifies the number of neutrino types (number of generations of matter).

42. Photon + Jet ∫L = 10 fb-1 • Number of events with direct photons for three regions of  and ∫L = 10 fb-1

43. 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

44. Muons Through Matter • Stopping power (= ⟨−dE/dx⟩) for positive muons in copper as a function of βγ = p/Mc over nine orders of magnitude in momentum. Solid curves indicate the total stopping power.