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Direct Measurement of the W boson Decay Width at DØ Neeti Parashar

Direct Measurement of the W boson Decay Width at DØ Neeti Parashar. Northeastern University Boston, Massachusetts U.S.A. DPF Meeting, College of William and Mary, VA, USA. 24 May, 2002. Outline. Introduction Event Selection Monte Carlo Simulation Background Estimation

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Direct Measurement of the W boson Decay Width at DØ Neeti Parashar

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  1. Direct Measurement of the W boson Decay Width at DØNeeti Parashar Northeastern University Boston, Massachusetts U.S.A. DPF Meeting, College of William and Mary, VA, USA. 24 May, 2002

  2. Outline • Introduction • Event Selection • Monte Carlo Simulation • Background Estimation • W Width Determination • Systematic Uncertainty • Conclusions

  3. Introduction • W decay width, W, is an important parameter in the Standard Model (SM) • In pp collisions, W’s are • producedby process:ud  W • decayleptonic:W  l; l = e,   hadronic:W  q’q; q’ or q = u, d, s,c or b (not t) • Considering 3 color charges, total of 9 decay channels yield a total width of ~ 9(W  e) • Including QCD corrections, the SM prediction for the full width is given by GeV

  4. Introduction Indirect measurement • Assumes W coupling to lepton to determine (We) • (W)/(Z) from theory and Br(Z0 ee) from LEP • Measured by UA1, UA2, LEP, CDF, DØ • DØ result (2000): • CDF result (1995): GeV GeV Motivation for Direct Measurement • a test of SM • a probe for possible new physics

  5. Direct Measurement • The total decay width, W, of the W could be measured by the transverse mass mT of We • mT spectrum is sensitive to the W decay width, particularly in the high end • Theoretical input of (W)/(Z) not required • Radiative corrections to W can be observed

  6. Event Selection Weevent selection 1994-1995 Run I data, 85 pb-1 • “Loose” Electrons • Fiducial region:||< 1.1in central calorimeter (CC) • High EM fraction:fem> 0.9 • Isolation of EM cluster:fiso< 0.15 • Shower shape consistent with that expected for e: 2 < 100 • “Tight” Electrons • “Loose” Electrons AND • Track match:trk < 5 • 4-variable likelihood(2,trk,fem,fiso) 4vl < 0.25 • MissingET> 25 GeV • pT(W) < 15 GeV

  7. Event Selection A typical W  e event in Run I W transverse mass • (45, 200) 2447 • (75, 200) 6286 • (90, 200) 319 • Total W candidates = 24487 • W candidates in mT bins

  8. Zee event selection To study Electron Efficiency & Monte Carlo (MC) simulation • Both electrons are in CC region and “tight” • pt(e) > 25 GeV • Invariant mass of dielectron pair 60 GeV < mee < 120 GeV • Total candidates = 1997

  9. Monte Carlo • A fast Monte Carlo, also used in the W mass analysis • Three main steps • W(Z) boson production • For MRSA, β = 0.00852  0.00013 • For CTEQ4M, β = 0.00852  0.00012 • W(Z) boson decay • Include W e • Detector simulation • Parameterized simulation • Parameters need to be constrained from real data (Zee, π0, etc)

  10. Background Three main backgrounds to We sample • Multijet events • Energy is mis-measured (missing Et) • The other passes electron cuts (fake electron) • Z  ee • One electron is missing or mis-measured • HERWIG Z events and GEANT simulation • MC sample passed through W selection cuts • Normalized, total = 102 • W e • Same signature as We, but at low pt • Included in MC

  11. Multijet Background • Method (Data based) • Loose sample: N = Ns + Nb • Tight (data) sample: Nt = eNs + jNb • Background in data Nb= j(eN- Nt)/(e-j) • The difference between loose and tight samples is the e requirement cuts • Efficiencies • e: electron efficiency • Use Zee data • e=0.8633 ±0.0118 • j: jet efficiency • Use non-isolated data without missing Et requirement • Calculate j in low missing Et region (0-10 GeV) • j = 0.0583 ±0.0016

  12. Multijet Background • Inclusive number 368±32 • In 90 <mT< 200 GeV25.4±2.2 • Background distribution vs. mT • Smooth background with exponential function in tail

  13. WDetermination • Fast MC simulation • Generate W e events with different W • Run through W selection cuts • Normalize MC to data with background subtracted • Add background • QCD background • Zee background • Calculate likelihood for different W • Calculate likelihood data vs. MC • Find Wat maximum likelihood • Find statistical uncertainty

  14. WDetermination For fitting region mT(90,200) with 319 events W =2.231+0.145/-0.138 GeV 2=22.6/22 and Kolmogorov-Smirnov test, k = 0.434

  15. Uncertainties

  16. Conclusions DØ Direct W width measurement (hep-ex/0204009, submitted to PRD) Consistent with SM CDF+D0 direct measurement (preliminary) Preliminary world average GeV GeV GeV GeV

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