Measurement of the W boson helicity fractions in the decays of top quark pairs to lepton + jets final states produced in pp collisions at sqrt(s) = 8 TeV

TL;DR

This paper measures the fractions of different helicity states of W bosons from top quark decays in proton-proton collisions at 8 TeV, providing the most precise results consistent with the Standard Model predictions.

Contribution

It presents the most accurate measurements to date of W boson helicity fractions in top quark decays at 8 TeV, using CMS data and combining electron and muon channels.

Findings

Measured helicity fractions are F[0]=0.681, F[L]=0.323, F[R]=-0.004.

Results agree with Standard Model predictions.

Provides the most precise helicity measurements to date.

Abstract

The W boson helicity fractions from top quark decays in t t-bar events are measured using data from proton-proton collisions at a centre-of-mass energy of 8 TeV. The data were collected in 2012 with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.8 inverse femtobarns. Events are reconstructed with either one muon or one electron, along with four jets in the final state, with two of the jets being identified as originating from b quarks. The measured helicity fractions from both channels are combined, yielding F[0] = 0.681 +/- 0.012 (stat) +/- 0.023 (syst), F[L] = 0.323 +/- 0.008 (stat) +/- 0.014 (syst), and F[R] = -0.004 +/- 0.005 (stat) +/- 0.014 (syst) for the longitudinal, left-, and right-handed components of the helicity, respectively. These measurements of the W boson helicity fractions are the most accurate to date and they agree with the predictions from the standard model.