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Study of Z + Jets in the electron channel with CMS at the LHC

Study of Z + Jets in the electron channel with CMS at the LHC. Christos Lazaridis University of Wisconsin-Madison Preliminary Examination. http://hep.wisc.edu/~lazaridis/files/2007_11/28. Outline. The Standard Model Large Hadron Collider Compact Muon Solenoid Z + Jets Previous results

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Study of Z + Jets in the electron channel with CMS at the LHC

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  1. Study of Z + Jetsin the electron channel with CMS at the LHC Christos Lazaridis University of Wisconsin-Madison Preliminary Examination http://hep.wisc.edu/~lazaridis/files/2007_11/28

  2. Christos Lazaridis Preliminary Examination Outline • The Standard Model • Large Hadron Collider • Compact Muon Solenoid • Z + Jets • Previous results • Monte Carlo studies • Summary/Future Plans

  3. The Standard Model • Quarks • u and d quark make nucleons in atom • Leptons • electrons complete the atoms • Force Carriers: • γ: electromagnetism • W/Z : weak interaction • gluons: color (nuclear) interaction • Higgs boson to give W & Z mass • Not discovered yet Particle Interactions: Christos Lazaridis Preliminary Examination

  4. Z + Jets • Verification of the Standard Model at high Q2 • A QCD regime never probed before! • Events at scale where pQCD expected to be valid: Z mass scale • Look for new physics in the Z + Jets spectrum • Unseen resonance in the electron pair invariant mass spectrum(?) • Due to high cross section, useful as a high-statistics detector calibration tool • Z + Jets are irreducible backgrounds • for interesting SM processes(e.g. top production) • for new physics searches(e.g. Higgs) Christos Lazaridis Preliminary Examination

  5. Large Hadron Collider • p-p collider • 14 TeV total collision energy • 27 km circumference • protons travel around circumference in 90 μsec • 25 ns bunch crossing • 8 μm bunch transverse radius Christos Lazaridis Preliminary Examination

  6. Magnets • Superconducting NbTi magnets • Operating @1.9K • 1232 dipoles bend proton beam around ring, B = 8T • Quadrupoles focus beam in transverse plane Arrows show directionof magnetic field Christos Lazaridis Preliminary Examination

  7. Christos Lazaridis Preliminary Examination Compact Muon Solenoid Design • Mass: 12.500 T • Length: 21.5 m • Diameter: 15.0 m • Solenoid • field: 4 Tesla • diameter: 6 m • length: 12.5 m • 2.66 GJ stored energy

  8. Christos Lazaridis Preliminary Examination Compact Muon Solenoid Built ←Surface Underground↓ Endcap Discs: Designed, assembled & installed by Wisconsin

  9. Christos Lazaridis Preliminary Examination Tracker • Tracker coverage extends to || < 2.5 • Resolution: δpT/pT≈ (15pT (TeV)0.5)% • Silicon pixel detectors used closestto the interaction region • Silicon strip detectors used in barrel and endcaps • cover an area of 210m2

  10. Christos Lazaridis Preliminary Examination Electromagnetic Calorimeter • Measures e/ energy and position to || < 3 • ~76,000 lead tungstate crystals • Short radiation length • Small Moliere radius • Resolution:

  11. Hadronic Calorimeter • HCAL samples showers to measure their energy/position • Barrel/Endcap region (|| < 3) • Brass/scintillator layers Resolution: • Forward region (3 < || < 5) • Steel plates/quartz fibers Resolution: Christos Lazaridis Preliminary Examination

  12. Detecting electrons and jets • Electrons • Leave a track • Deposit energy in the ECAL • Jets • Leave a track • Deposit energy in the ECAL/HCAL A collimated spray of high energy hadrons Christos Lazaridis Preliminary Examination

  13. Christos Lazaridis Preliminary Examination Proton-proton collisions at LHC Startup:1028 – 1031 cm-2s-1 Luminosity L = Particle Flux/Time Interaction Rate:

  14. Christos Lazaridis Preliminary Examination CMS Trigger • Level 1 Trigger: • Hardwired processors • High Level Trigger: • Farm of processors • L1 Trigger Requirements: • Output: 100 kHz (50 kHz for initial running) • Latency: 3 μsec • Data collection, decision, propagation • HLT designed to output 100 Hz Trigger Rejection ~4x105

  15. Christos Lazaridis Preliminary Examination Level 1 Trigger • Information from Calorimeters and Muon detectors • Electron/photon triggers • Jet/MET triggers • Muon triggers • Highly complex • Trigger primitives: ~5000 electronics boards • Regional/Global: 45 crates, 630 boards • Flexibility • Most algorithms implemented in re-programmable FPGAs • The 18 crates of Regional Calorimeter Trigger: • Receive input from the CMS calorimeters • Combined with the GCT outputs to the GT the 4 highest isolated/non-isolated electrons, jets, τ-jets, missing ET and total ET per event

  16. RCT Trigger Supervisor • An online framework toconfigure, test, operate andmonitor the CMS Trigger • Each subsystem is represented by a “cell” • Cells communicate via XML-formatted commands My contribution so far: • A control panel for the RCTthat permits • Specific Crate/Card selection • Command execution Crate Selection Card Selection Command Selection Christos Lazaridis Preliminary Examination

  17. Christos Lazaridis Preliminary Examination Calorimeter Trigger Towers    

  18. Christos Lazaridis Preliminary Examination Electron Trigger Algorithms

  19. Jets Trigger Algorithms • Jets • Not a “cone” but a “square” algorithm • 12x12 Trigger tower ΣET > Threshold sliding in 4x4 steps AND • Central 4x4 ET > others TriggerTowers Christos Lazaridis Preliminary Examination

  20. Christos Lazaridis Preliminary Examination Z+Jets Characteristics • Total Z production cross section: 56 nb • Z → e+e- cross section: 1.5 nb • Total cross section x Branching Ratio ( ~ 3%) • 104 events/10pb-1 at LHC startup luminosity (1031 cm-2s-1) • Looking for • 2 electrons • N jets, N = 0...5 • Within detector coverage • Reconstructing • Z Invariant Mass • Z PT

  21. CDF Z+4 Jet events CDF has found two Z+4 Jet events in 1.7 fb-1 of data CDF Public Note 8827 Christos Lazaridis Preliminary Examination

  22. CDF Z→e+e- peak • Requirements: • Two electrons with ET > 25 GeV • At least one central electron: |ηe|<1 • Second electron central or forward • |ηe|< 1 or 1.2 < |ηe|< 2.8 • Z mass window: 66 < Mee < 116 GeV/c2 • ΔR (e,jet) > 0.7 • Jets found using the MidPoint algorithm with R = 0.7 • pT,jet > 30 GeV/c and |yjet| < 2.1 Mee Invariant Mass Events with at least one jet CDF Public Note 8827 Christos Lazaridis Preliminary Examination

  23. Z+Jets Cross Section MCFM: Monte Carlo for FeMtobarn processes Z + Jets Inclusive Cross Section CMS will have large data samples at high jet pT. Will probe proton PDF and investigate scale choices Data/Theory Ratio • Shaded bands show systematic uncertainties • Dashed lines: PDF uncertainties • Dash-dot lines: scale uncertainties CDF Public Note 8827 Christos Lazaridis Preliminary Examination

  24. Christos Lazaridis Preliminary Examination Monte Carlo Studies • Dataset used: • Z+N jets, N = 0…5 • Generated with: • ALPGEN 2.12 • PYTHIA 6.409 • Detector simulation with GEANT4 8.2.p01 • Detector electronics simulation and event reconstruction and physics objects provided by CMS framework Matrix element generator: multi-parton processes in hadronic collisions General purpose MC generator: for hadronization and showering Simulation of the passage of particles through matter

  25. Christos Lazaridis Preliminary Examination Selecting Ze+e- events #jets pt Total events CS (pb) Ze+e- 0 0-100 704016 1.50x103 234264 1 0-100 36449 3.10x102 12231 100-300 9039 1.00x101 2932 2 0-100 19290 9.00x101 6485 100-300 2693 9.40x100 933 3 0-100 74466 6.80x101 24680 100-300 24316 1.30x101 8063 4 0-100 33083 1.40x101 11031 100-300 1038 4.30x100 322 5 0-100 1178 8.80x100 388 100-300 5966 5.00x100 1983

  26. Offline Electron Reconstruction • Create “super-clusters” from clusters of energy deposits using Level-1 E/M calorimeter information • In area specified by Level-1 trigger • ET > threshold • Match super-clusters to hits in pixel detector • Electrons leave a hit (track) • Photons do not • Combine with full tracking information • Track seeded with pixel hit • Final cuts made to isolate electrons ET Tracker Strips ET/pT cut Pixels pT Christos Lazaridis Preliminary Examination

  27. Christos Lazaridis Preliminary Examination Trigger on Electrons for Z + 0-5 jets Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets Automatic L1 trigger for electrons above 63 GeV (x102) max value of 63 GeV (6 bits) # electrons/10 pb-1 Isolated Electron Et

  28. Christos Lazaridis Preliminary Examination Highest Et Reconstructed Electrons Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets (x102) (x102) # electrons/10 pb-1 # electrons/10 pb-1 electron • Electrons with: • ET > 15 GeV • -2.4 < η < 2.4 electron

  29. Christos Lazaridis Preliminary Examination Highest Et Electrons η/φ Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets (x102) η1st # electrons/10 pb-1 (x102) φ1st # electrons/10 pb-1 Electron η Electron φ

  30. Christos Lazaridis Preliminary Examination ΔR Matching to Generated Electrons # electrons No match

  31. Christos Lazaridis Preliminary Examination Electron Finding Efficiency Entries: Overflows: 141530 135 Matched MCe 185 All MCe

  32. Christos Lazaridis Preliminary Examination Z Invariant Mass (x102) With 10 pb-1 of datawe should see ~10Z + 5 Jets events! # evts/10 pb-1 • 2 isolated electrons with pt > 25 GeV • -2.4 < η < 2.4 • Z+0 Jets fit: • Range 80-100 GeV Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets Z+4 Jets Z+5 Jets Mee

  33. Christos Lazaridis Preliminary Examination Reconstructed pt(Z) (x102) • Measure differential cross section of Z production • Compare with MC predictions # evts/10 pb-1 Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets

  34. Christos Lazaridis Preliminary Examination Offline Jet Reconstruction • Iterative cone • Draw a cone ΔR = 0.5 around a seed with Et > threshold • The computed direction seeds a new cone • Iterate until the cone position is stable • Stable cone ≡ a jet; towers are removed from the list of input objects • no jet merging • Other algorithms: • Midcone: provides jet merging • KT: Agrees well with theory R

  35. Christos Lazaridis Preliminary Examination Highest Et Jets Second Jet Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets • Iterative Cone • Rcone = 0.5 • Pt > 15 GeV • -2.4 < η < 2.4 • Some jets (e.g. in Z+0 jets events) are underlying soft QCD radiation Highest Jet (x102) (x102) # jets/10 pb-1 # jets/10 pb-1

  36. Christos Lazaridis Preliminary Examination Signal Identification Starting with: 104 Z→e+e-events (with 10 pb-1) 2 Electrons Et > 15 GeV, |η| < 2.4 Above trigger threshold: Efficient for W and Z events 0.6 x 104 events (60 % of initial) N Jets; N = 0...5, Et > 15 GeV, |η| < 2.4 Jet ET higher than QCD 4.8 x 103 events (48 % of initial) 80 < MT,ee < 100 Mass of the e+e- pair should be close to the Z mass Final Count: 4 x 103 events (40 % of initial)

  37. Christos Lazaridis Preliminary Examination Summary/Future Plans • Z + Jets is a high cross section channel • 104 events/10pb-1 (LHC startup luminosity) • Measure Z + Jets total and differential cross sections • Can be used for detector calibration • Clear Z→e+e- peak • Plans: • Work on the RCT, key component in Z+Jets triggering • Triggers • Study/Improve efficiencies for electrons & jets • Tune Monte Carlo generators and PDFs • Discover new physics!

  38. Christos Lazaridis Preliminary Examination Backup Slides

  39. Christos Lazaridis Preliminary Examination Muon System • Muon chambers identify muons and provideposition information for track matching • Drift Tubes in the central barrel region • Cathode Strip Chambers in the endcap region • Resistive Parallel Plate Chambers in both the barrel and endcaps

  40. Calorimeter Trigger Algorithms:Electrons • Electron (Hit Tower + Max)‏ • 2-tower ΣET + Hit tower H/E • Hit tower 2x5-crystal strips > 90% ET in 5x5 (Fine Grain)‏ • Isolated Electron (3x3 Tower)‏ • Quiet neighbors: all towers pass Fine Grain & H/E cut • One group of 5 EM ET < Threshold Christos Lazaridis Preliminary Examination

  41. Christos Lazaridis Preliminary Examination Removing Electrons From Jets Collection • Keep jets that don’t have a nearby “electron” within a cone ΔR<0.15… Keep >0.15 …where “electron” is required to havethe ratio 0.85< Pt,electron/Pt,Jet < 1.15

  42. Lund String Fragmentation • Used by PYTHIA to describe hadronization and jet formation • Color “string" stretched between q and qbar moving apart • String breaks to form 2 color singlet strings until only on mass-shell hadrons remain Christos Lazaridis Preliminary Examination

  43. Z Production: Rate   • At 1033: • ~108 events/10fb-1 Z (50 Hz - 500Hz for full luminosity)‏ • 10^6 events/10fb^1 Z (for L 10^31)‏ • 10^3 events/10pb^1 Z (for L 10^31)‏ • Z+jet: ~ 0.1 Hz • Z production ~2.5nb • More stuff here   (nb)‏   Events/sec ( L = 1033 cm-2s-1)‏       s 10 1 √s (TeV)‏ Christos Lazaridis Preliminary Examination

  44. Christos Lazaridis Preliminary Examination Previous Studies D0 (1)‏ • hep-ex/0506025 – A. Bellavance • hep-ex/0608052 – V.M. Abazov et. al. • Tevatron D0 Results • 1.96 TeV ppbar collisions • Z → e+e- selection cuts: • 2 electromagnetic objects • ET > 25 GeV • Pseudorapidity η< |1.05| (central calorimeter only)‏ • Jets Identification: • Cone Algorithm • ET > 20 GeV • η < |2.4|

  45. Christos Lazaridis Preliminary Examination Previous Studies D0 (2)‏ D0 (0.4fb-1) Cross Section vs Jet Multiplicity MCFM: Monte Carlo for FeMtobarn processes ME-PS: LO Matrix Element calculations using Pythia for parton showering Pythia predicts fewer events at high jet multiplicity because of missing higher order contributions at the hard-scatter level

  46. Christos Lazaridis Preliminary Examination Previous Studies D0 (3)‏ D0 (0.4fb-1) Highest pT jet distribution ME-PS: LO Matrix Element calculations using Pythia for parton showering Z/γ* + ≥ 1 jet Z/γ* + ≥ 2 jets Z/γ* + ≥ 3 jets

  47. Christos Lazaridis Preliminary Examination Previous Studies • LHC will probe higher Z pT shedding light on PDF uncertainties Tevatron Plots Can be used to scale the highest Jet PT • Shaded bands are uncertainties for fixed strong coupling strength and varying PDF parameters (hep-ph/0512167: Parton Distributions and the Strong Coupling: CTEQ6AB PDFs - J. Pumplin, A. Belyaev, J. Huston, D. Stump, W.K. Tung)‏

  48. Christos Lazaridis Preliminary Examination Higgs • Standard electroweak theory predicts W± and Z of zero mass • Experiment tells us that the weak force is short-range, so its carriers must be massive • The Higgs field gives mass to these three bosons by spontaneously breaking the SU(2)xSU(1) symmetry • The Higgs coupling to other particles determines their masses • Stronger coupling  Higher mass • Couples to self • Small problem: • Theory cannot predict the Higgs mass

  49. Christos Lazaridis Preliminary Examination Expected Higgs Mass • LEP results from a direct Higgs search set a limit on the Higgs mass ~115GeV/c2 • On January 2007, indirect Tevatron evidence were predicting mH=85+39-28 GeV/c2

  50. Christos Lazaridis Preliminary Examination General Higgs Production • Vector boson fusion • Lower rate • Lower background • Gluon-gluon fusion • Highest Higgs production rate • High QCD background Associated W/Z production ttbar fusion

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