Measurement of the W helicity in top pair production with dileptons at 7 TeV:   TOP-12-015

The top quark, the heaviest quark in the standard model (SM), decays with a short life time of  about

5 × 10-25 s before hadronization, permitting the study of top quark properties as a ’bare’ quark. It decays predominantly to a W boson and a b quark, t → W+ b.

The W is a massive vector boson, therefore it can have longitudinal, left-handed, or right-handed polarization states. It provides a tool to test the SM predictions, and any deviation from the SM it may arise a new physics beyond the SM.

We present a measurement of the W boson helicity fractions in t-tbar events with the full dataset collected by the CMS detector in 2011 at a center-of-mass energy of 7 TeV. This corresponds to an integrated luminosity of 4.6 fb-1. The study is based on the dilepton decay channel of top pair events, in which both W bosons decay leptonically. Only events containing electrons and/or muons in the final state are considered. We measure the fractions of left-handed, longitudinal, and right-handed W boson to be FL = 0.288 ± 0.035(stat) ± 0.040(sys), F0 = 0.698 ± 0.057(stat) ± 0.063(sys) and FR = 0.014 ± 0.027(stat) ± 0.042(sys). The results are consistent with the standard model predictions.

TOP-12-015 Figure1:The distribution of cosθ* found in data events is compared to the fit results. The ratio between data and simulation is shown at the bottom of the distribution.