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=8TeV

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 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.

The electro-weak couplings of the top and bottom quarks — Global fit and future prospects

“…The helicity fractions of the W boson produced in top-quark decay can be predicted to excellent precision [49]. The measurements by ATLAS and CMS [50][51][52][53] at √ s = 7 and 8 TeV reach a precision of several percent. The combination of precise predictions and measurements converts these measurements in true hadron collider precision measurements and in sensitive probes to new physics affecting the tbW vertex [23].…”

Polarized vector boson scattering in the fully leptonic WZ and ZZ channels at the LHC

“…Both CMS and ATLAS have measured the W polarization fractions in the W + jets [12,13] channel and in tt events [14,15].…”

From the bottom to the top—reconstruction oftt̄ events with deep learning

“…It is often chosen because of its significant BR (≈ 30% if electrons and muons are considered) and its relatively low background. Several important measurements in the lepton+jets channel require a reconstruction of the tt decay, such as measurements of the top-quark mass [2,3], the W-helicity in top-quark decays [6,7], the tt charge asymmetry [8], and the top-quark width [9].…”

“…The added value of the work documented in this paper is threefold: We quantify the performance in reconstructing tt events with a DNN, which has a wide range of applications, as outlined above. We compare the performance to the standard approach (kinematic fitting), which has been used for many analyses at the LHC [2,3,[6][7][8][9][25][26][27][28]. We consider the simple sequential optimisation procedure that we perform and document ready-to-use for data analysis at LHC experiments.…”