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Exploring W+jets Events with CMS at the LHC

Preliminary examination of W+jets events with CMS at the LHC, focusing on operation stages, detector information, trigger systems, event rates, trigger algorithms, and signal analysis.

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Exploring W+jets Events with CMS at the LHC

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  1. W + jets events with CMS at the LHC Kira Grogg University of Wisconsin - Madison Preliminary examination Kira Grogg, U. Wisconsin

  2. LHC Startup Stage 1 Initial commissioning 43x43156x156, 3x1010/bunch L=3x1028 - 2x1031 Starts in 2008 • Year one (+) operation • Lower intensity/luminosity: • Event pileup • Electron cloud effects • Phase 1 collimators • Equipment restrictions • Partial Beam Dump • 75 ns. bunch spacing (pileup) • Relaxed squeeze Shutdown Stage 2 75 ns operation 936x936, 3-4x1010/bunch L=1032 - 4x1032 -> ~1fb-1 Stage 3 25 ns operation 2808x2808,3-5x1010/bunch L=7x1032 - 2x1033 ->~10fb-1 Long Shutdown Phase 2 collimation Full Beam Dump Scrubbed Full Squeeze Stage 4 25 ns operation Push to nominal per bunch L=1034 -> ~100fb-1/yr Kira Grogg, U. Wisconsin

  3. Compact Muon Solenoid (CMS) Magnetic field: 4T Weight: 12,500 T Diameter: 15.0 m Length: 21.5 m Kira Grogg, U. Wisconsin

  4. CMS Geometry • Phi and Eta , pictures Kira Grogg, U. Wisconsin

  5. With a 40 MHz frequency (bunch crossings every 25 ns), not all events can be retained L1 trigger hardware selects 100 kHz of interesting events Background QCD event rate ~ MHz W+jets event rate ~40 Hz Triggers Electron/photon Jet/MET Muon L1Trigger My job Kira Grogg, U. Wisconsin

  6. Software trigger Multi-processor farm Starts with L1 data and reduces event rate to 100 Hz Finds an object (such as an electron) in a hierarchical series of algorithms Energy deposit Track reconstruction etc. We rate: ~2 Hz = 107 / yr (L = 1032 cm-2 s-1, 1 yr = 1 fb-1) High Level Trigger Kira Grogg, U. Wisconsin

  7. Characteristics of W+jets Electron & Neutrino Jets Previous W+jets studies as CDF Cross sections Jet transverse energy Simulating the W+jets Backgrounds to W+jets Plots for W+jets Electron pT Jet PT Missing ET Transverse W mass Analysis Outline Kira Grogg, U. Wisconsin

  8. Produced The jets • Partons radiate partons, which hadronize to form a jet • Look at a cone r = 0.5 surrounding highest pT observed particle Kira Grogg, U. Wisconsin

  9. Previous W+jets studies Affolder et al. , The CDF Collaboration, Phys. Rev. D 63, 072003 (2001) • Tevatron info: • p -pbar collisions • s = 1.96 TeV • L = 108 pb-1 to 320 pb-1 • Backgrounds to W+jets at Tevatron: • Top • QCD • W • Ze+e- • Measurements: • Select events with electron ET > 20 GeV and || < 1.2 ; ETmiss > 30 GeV • Require high PT (> 13 GeV) track near EM deposit • N jets, found using R =0.4 cone algorithm. ET < 15 GeV, |  | < 2.4  Selection efficiency ~20% Kira Grogg, U. Wisconsin

  10. Cross section measurements by jet number CDF W + n jets Cross Sections Renormalization scales: Kira Grogg, U. Wisconsin

  11. CDF W + n jets, Jet ET • Jet transverse energy Mismatch at higher ET Kira Grogg, U. Wisconsin

  12. W+ jets simulated with Alpgen v2.1 Fixed order matrix element calculations of cross sections Generates multi-parton processes in hadronic collisions. Underlying event simulated with Pythia v6.409 Generates event hadronization, parton shower, and IFSR Detector simulated using GEANT4 Toolkit for the simulation of the passage of particles through matter Simulation Hard scattering ALPGEN Underlying event PYTHIA Detector simulation GEANT4 Reconstruction of event CMSSW Kira Grogg, U. Wisconsin

  13. QCD produces many jets, some of which can be misidentified as an electron top Decays into W+b W tau decays to electron or jets WW (W e) (W jj) Zee lose an electron Backgrounds at LHC Kira Grogg, U. Wisconsin

  14. Isolated Non-isolated Electron Trigger Automatically trigger on electrons pt > 63 GeV Kira Grogg, U. Wisconsin

  15. Electron PT N-jets 1-3 Kira Grogg, U. Wisconsin

  16. PT of highest and 2nd highest pT Jet Kira Grogg, U. Wisconsin

  17. Missing Et Missing ET is roughly 1/2 mW Cut Kira Grogg, U. Wisconsin

  18. Transverse W mass mT of W calculated based on electron and missing ET information peaks near 80 GeV Cuts: elec pT >15 GeV ETmiss > 30 GeV Kira Grogg, U. Wisconsin

  19. Conclusions/next steps • W+jets is a good signal to look for soon after LHC startup • High cross section • Clean signal • Need to understand as background to new physics • Higgs, SUSY, top… • Need more simulations (esp. backgrounds) • Eventually, take real data Kira Grogg, U. Wisconsin

  20. BACKUP SLIDES Kira Grogg, U. Wisconsin

  21. “Iterative cone 5 calo jets” • Cone 5  use a cone of r = 0.5 • Where • Start with object seed (calorimeter tower) of largest ET ( > 1 GeV) • Calculate direction of “proto-jet” • using objects inside the cone • Use proto-jet as next seed • Iterate until ET/E< 1% and r < 0.01 between iterations • Add to list of jets, remove objects in the jet from seed list, and repeat procedure for the next jet Jet cone algorithm Kira Grogg, U. Wisconsin

  22. Kira Grogg, U. Wisconsin

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