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 8TeV. The data were collected in 2012 with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.8fb−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.
Highlights
The data from proton–proton collisions produced at the CERN LHC provide an excellent environment to investigate properties of the top quark, in the context of its production and decay, with unprecedented precision
The helicity parameters F0 and FL are fit simultaneously, but they are strongly anti-correlated due to the unitarity constraint FL + F0 + FR = 1, as indicated by the statistical correlation coefficient ρ0,L given in the table
Uncertainties related to lepton efficiency, multijet background estimations, and statistical uncertainties are considered uncorrelated between the e + jets and μ + jets analyses, while all other uncertainties are assumed to be fully correlated
Summary
The data from proton–proton (pp) collisions produced at the CERN LHC provide an excellent environment to investigate properties of the top quark, in the context of its production and decay, with unprecedented precision Such measurements enable rigorous tests of the standard model (SM), and deviations from the SM predictions would indicate signs of possible new physics [1,2,3,4]. The helicity fractions of W bosons in top quark decays were first measured at the Tevatron Collider [9,10,11] They have been measured at the LHC, using samples containing tt events obtained in pp collisions at 7 TeV, and having either one [12,13] or two [12] charged leptons in the final state. The measurement is performed using pp collisions at centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.8 fb−1 , collected during 2012 by the CMS detector
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