1. 1 Direct Photon + Jet Events at the Compact Muon Solenoid with  s = 14 TeV Michael Anderson University of Wisconsin Preliminary Exam

2. 2 Outline  Standard Model   + jet Events  Large Hadron Collider  Compact Muon Solenoid Detector  Detector/Physics Simulation  Reconstructed Jets and Photons  Conclusions & Plans Simulations @Wisconsin

3. 3 Standard Model  12 elementary matter particles  4 force-carrying particles  1 so far undetected particle: Higgs Quarks (Fermions) Leptons (Fermions) Force Carriers (Bosons) Higgs Quarks only exist in colorless composites!

4. 4 Direct Photons Compton-like Annihilation  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,  ~ 1 nb for Et(  > 20 GeV  Useful in new physics searches: –Heavy/Composite quarks b’ -> b +  –Supersymmetry Non-direct  ’s example: final state radiation

5. 5 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

6. 6 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 < E t < 500 GeV  pp B. Abbot et al. “Determination of the Absolute Jet Energy Scale in the D0 Calorimeter.” Nucl Instr and Meth. A424 (1999)

7. 7 Large Hadron Collider  14 TeV proton-proton collider  Circumference of 27 km  Luminosity up to 10 34 cm -2 s -1  8T Magnets

8. 8 Proton interactions at LHC Luminosity L = particle flux/time Interaction rate Cross section  = “effective” area of interacting particles @start-up 10 28 - 10 31 cm -2 s -1

9. 9 Compact Muon Solenoid Solenoid (4T)Muon chambers Silicon Strip & Pixel Tracker Electromagnetic Calorimeter Hadronic Calorimeter Brass/Scintillator Forward calorimeter 6 m diameter

10. 10 Current CMS Surface Underground

11. 11 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

12. 12 Silicon Tracker  Measures pt & path of charged particles within |  | < 2.5  Strip Tracker –200 m 2 coverage –10  m precision measurements –11M electronic channels  Inner Pixel tracking system –66M channels  Used for finding isolated photons and rejecting jets

13. 13   =-ln(tan(  /2) Electromagnetic Calorimeter  Measures energy & position of electrons and photons within |  | < 3  PbWO 4 crystals, very dense (8.3 g/cm 3 ) –23 cm long (26 radiation lengths) –61K in the barrel, 22 x 22 mm 2 –15K in the endcaps, 28 x 28 mm 2  Ultimate precision of energy resolution: 0.5%

14. 14 50 GeV electrons After calibration ECAL Crystal Calibration 9 GeV beam uncalibrated Raw  0 mass  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

15. 15 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 HBHE Barrel Resolution

16. 16 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

17. 17 Level-1 TriggerHFHCALECAL RPCCSCDT Pattern Comparator Trigger Regional Calorimeter Trigger 4  e, J, E T, H T, E T miss Calorimeter Trigger Muon Trigger max. 100 kHz L1 Accept Global Trigger Global Muon Trigger Global Calorimeter Trigger Local DT Trigger Local CSC Trigger DT Track Finder CSC Track Finder 40 MHz pipeline, latency < 3.2  s  Every event is ~1MB each  Identifies potential photons, jets…  Hardware implemented  Reduces rate from 40 MHz -> 100 kHz  Processes each event in 3  s

18. 18 Level-2 e/  Trigger Pixels Tracker Strips ETET   Create “super-clusters” from clusters of energy deposits using Level-1 ECAL information –Must be in area specified by Level-1 trigger –Must have E T 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

19. 19 Outline  Standard Model   + jet Events  Large Hadron Collider  Compact Muon Solenoid Detector  Detector/Physics Simulation  Reconstructed Jets and Photons  Conclusions & Plans Simulations @Wisconsin

20. 20 Computing PIC Barcelona FZK Karlsruhe CNAF Bologna IN2P3 Lyon T1 T0 FNAL Chicago T1 CMS data sizes and computing needs require a worldwide approach to Physics analysis. FNAL is US CMS national computing center. T2 UW Madison 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 Tier 2 centers Data Analysis Local Data Distribution Wisconsin is a T2 site RAL Oxford

21. 21 Simulation Workflow   +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! ALPGEN Hard scattering PYTHIA Hadronization GEANT4 Detector simulation CMSSW Reconstruction of event

22. 22 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 R9 R19 Number of Events / bin

23. 23 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 wider for meson decay

24. 24 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

25. 25 Event Selection Start with events corresponding to ∫L  100 pb -1 where  gen  not in ECAL  gap, and Et(  ) > 15 GeV CutReason Events After cut Start480k Photon  < 1.479 Select barrel photons (well measured)  R Nearest Track > 0.1 (track pt > 10 GeV) Isolation (reject fake photons) Photon R9 > 0.9Narrow Energy Deposit (Shower shape & rejects conversions)

26. 26 Photon ,   Relatively flat in  and   Crack in ECAL  : 1.479 - 1.653  Photons near this  harder to find/reconstruct   Generated Reconstructed # Events / bin Keep |  |<1.479

27. 27 Et gen – Et rec Et gen Longer tail on positive side GeV Photon Et  Et of the generated & reconstructed photon match well  Difference in Et ~< 10%  Used default vertex (0,0,0) Generated Reconstructed # Events / bin Keep 15 GeV < Et

28. 28 Photon Shower-Shape  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 E (3x3 crystals) E (SuperCluster) RR # Events / bin Keep 0.94 < R9

29. 29 Nearest Track to Photon RR Nearest Track  R Nearest Track p t GeV # Events / bin Most in overflow Keep 0.1 <  R  Want isolated photons  Nearest Track where –Track pt > 10 GeV –Will lower to 1.5 GeV in future   R =  (  ) 2 + (  ) 2

30. 30 Photon Energy Resolution  Select clean  ’s for calibration  (Et gen –Et rec )/ Et gen  After cuts, fewer events in positive tail  ~24% loss in # of events after R9 cut Before cuts After cuts # Events / bin

31. 31 Event Selection Start with events corresponding to ∫L  100 pb -1 where  gen  not in ECAL  gap, and Et(  ) > 15 GeV CutReason Events After cut Start480k Photon  < 1.479 Select barrel photons (well measured) 330k (67%)‏  R Nearest Track > 0.1 (track pt > 10 GeV) Isolation (reject fake photons) 310k (65%)‏ Photon R9 > 0.9Narrow Energy Deposit (Shower shape & rejects conversions) 240k (49%)‏

32. 32 Jet Reconstruction  Cone algorithms: –Find seed crystals (starting positions for cone) –Move cone around until E T in 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 =  (  ) 2 + (  ) 2

33. 33 Jet ,   Jet alg: Iterative Cone –R = 0.5 –(Will try out others)  Highest-p t Jet with –p t > 10 GeV –0.8  < |  jet -   | < 1.2   ~93% of events have reconstructed jet that meet this criteria Generated Reconstructed # Events / bin

34. 34 # Events / bin Jet Calibration  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! GeV (pt genJet – pt recJet ) pt genJet Generated  Et Generated Jet pt Reconstructed Jet pt # Events / bin (Et gen  – Et rec  ) Et gen  Calibration will shift mean

35. 35 Conclusion & Plans  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 

36. 36 Extras

37. 37 Calibration Procedure  True jet-parton calibration variable  Can be approximated by  These would be determined in bins of p t,g

38. 38 Supersymmetry  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 pp ~~ G ~ G ~  

39. 39 Prev.  + 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.

40. 40 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).

41. 41 Photon + Jet  Number of events with direct photons for three regions of  and ∫L = 10 fb -1 ∫L = 10 fb -1

42. 42 Muon System  3 technologies, all self-triggering –drift-tubes(DT), cathode strip chambers(CSC), resistive plate chambers (RPC)  25000 m 2 of active detection planes –100  m position precision in DT and CSC  About 1M electronic channels DTCSCRPC

43. 43 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.