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

Abstract

The W boson helicity fractions from top quark decays in tt 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 fb−1. 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 F0 = 0.681 ± 0.012 (stat) ± 0.023 (syst), FL = 0.323 ± 0.008 (stat) ± 0.014 (syst), and FR = −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.