A measurement of the V+A fraction of W bosons from top quark decay

Joel Heinrich ¹, Aafke Kraan ¹, Evelyn Thomson²

Susana Cabrera²

¹ University of Pennsylvania
² IFIC(CSIC-University of Valencia)
E-mail authors

We search for possible effects from physics beyond the standard model in the decay of the top quark. Neglecting the b-quark mass, the V-A charged weak current interaction forces the top quark to decay into W⁺ bosons with longitudinal and left-handed helicity, and the anti-top quark to decay into W^- bosons with longitudinal and right-handed helicity. Effects from new massive particles or non-standard model couplings in the t-W-b vertex may create W⁺ bosons with right-handed helicity and W^- bosons with left-handed helicity, which is an apparent V+A weak current. For subsequent W boson decays to an "l &nu " final state, we can estimate the W boson helicity from the invariant mass squared of the lepton and the b-jet from top quark decay.

We analyze two CDF run II data samples with an integrated luminosity of 695 pb^-1 in the lepton+jets channel and 750 pb^-1 in the dilepton channel. For the lepton+jets channel, we require one electron or muon with E_T>20 GeV, MET > 20 GeV, at least three jets with E_T>15 GeV, and at least one SVX b-tagged jet. The lepton+jets event selection is almost identical to that used for the measurement of the pair production cross-section in the lepton+jets channel with at least one SVX b-tagged jet, with the exception being that the requirement on total transverse energy, H_T>200 GeV, is dropped. We select 304 events with exactly 1 SVX b-tagged jet and 75 events with exactly 2 SVX b-tagged jets. For the dilepton channel, we require two oppositely charged leptons (electrons or muons) with E_T>20 GeV, MET > 25 GeV, at least two jets with E_T>15 GeV, and total transverse energy of leptons, MET and jets H_T>200 GeV. We do not require b-tag information in the dilepton channel. The dilepton event selection is identical to that used for the measurement of the pair production cross-section in the dilepton channel. We select 64 events.

The V+A fraction, f_V+A , of W bosons from top quark decay under the assumption that the top mass is 175 GeV/c² is measured to be:

f_V+A = -0.06 ± 0.25

In the absence of a signal, an upper limit is set on the V+A fraction of W bosons from top quark decay with value:

f_V+A < 0.29 at 95% C.L.

To compare with other previous published results for the measurement and 95% CL of the measured fraction f_V+A, we translate our result into the fraction f+ of right-handed W+ bosons (left-handed W- bosons) by remembering that f_V+A = f+/0.3 (if indeed f0 = 0.7), and we obtain f+ = 0.02 ± 0.07 and f+ < 0.09 at 95% CL.

Documentation

CDF conference note 8280 note describes this analysis. (public).
Internal note CDF-8229 (requires a CDF password)

Tables and Plots

Description	eps/gif
Lepton+jets data with exactly 1 SVX b-tagged jet. The invariant mass squared of the lepton and the b-jet is shown. Fitted result from ALL samples, with f_V+A=-0.06.
Lepton+jets data with exactly 2 SVX b-tagged jets. The invariant mass squared of the lepton and the highest E_T b-jet is shown for different ranges of the invariant mass squared of the lepton and second highest E_T b-jet. Fitted result from ALL samples, with f_V+A=-0.06.
Dilepton data. The invariant mass squared of the lepton and the highest E_T jet is shown for different ranges of the invariant mass squared of the lepton and second highest E_T jet. There are two entries per event, one entry for each lepton. Fitted result from ALL samples, with f_V+A=-0.06.
Lepton+jets with exactly 1 SVX b-tagged jet. The M_lb² distribution is shown for ALPGEN ttbar MC with f_V-A=1.0, with f_V+A=1.0, and for background (ALPGEN Wbbbar+1p and data model of QCD). Available for all on one plot, and separately!
Lepton+jets with exactly 2 SVX b-tagged jets. The M_lb² distribution is shown for ALPGEN ttbar MC with f_V-A=1.0, with f_V+A=1.0, and for background (ALPGEN Wbbbar+1p). The x-axis is for the invariant mass squared of the lepton and the highest E_T b-jet and the y-axis is for the lepton and second highest E_T b-jet.
Dilepton. The M_lb² distribution is shown for ALPGEN ttbar MC with f_V-A=1.0, with f_V+A=1.0, and for background (Drell-Yan, diboson, fakes). The x-axis is for the invariant mass squared of the lepton and the highest E_T jet and the y-axis is for the lepton and second highest E_T jet.
Likelihood DATA (ALL samples and dilepton and lepton+jets on one plot)
Likelihood DATA (ALL samples)
Likelihood DATA (Lepton+jets sample)
Likelihood DATA (Dilepton sample)
Table of nuisance parameters in likelihood fit
Table of systematics on measurement of f_V+A
Table with top quark mass dependence of f_V+A. Note that our quoted values assume a top quark mass of 175 GeV/c²
Table of systematics on 95% CL: gaussian convolution scheme
Check of 95% CL Coverage: the CL contours as a function of f_V+A
FITTED/PULL values for f_V+A for six hypothesis of INPUT f_V+A: ALL samples
Results of MC pseudo-experiments using observed number of events - ALL sample
Results of MC pseudo-experiments using observed number of events - lepton+jets with exactly 1 SVX b-tag
Results of MC pseudo-experiments using observed number of events - lepton+jets with exactly 2 SVX b-tags
Results of MC pseudo-experiments using observed number of events - lepton+jets sample
Results of MC pseudo-experiments using observed number of events - dilepton sample