Measurement of the polarisation of Bosons Produced in Top-quark Decays Using Dilepton Events at TeV with the ATLAS experiment

A measurement of the polarisation of W bosons produced in top-quark decays is presented, using proton–proton collision data at a centre-of-mass energy of $\sqrt{s}=13$ TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139 fb$^{−1}$. The measurement is performed selecting $t\overline{t}$ events decaying into final states with two charged leptons (electrons or muons) and at least two b-tagged jets. The polarisation is extracted from the differential cross-section distribution of the $\mathrm{cos}{\theta }^{⁎}$ variable, where ${\theta }^{⁎}$ is the angle between the momentum direction of the charged lepton from the W boson decay and the reversed momentum direction of the b-quark from the top-quark decay, both calculated in the W boson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the $\mathrm{cos}{\theta }^{⁎}$ angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be ${f}_{0}=0.684±0.005\phantom{\rule{0.2em}{0ex}}\left(\mathrm{stat}.\right)±0.014\phantom{\rule{0.2em}{0ex}}\left(\mathrm{syst}.\right)$, ${f}_{L}=0.318±0.003\phantom{\rule{0.2em}{0ex}}\left(\mathrm{stat}.\right)±0.008\phantom{\rule{0.2em}{0ex}}\left(\mathrm{syst}.\right)$ and ${f}_{R}=-0.002±0.002\phantom{\rule{0.2em}{0ex}}\left(\mathrm{stat}.\right)±0.014\phantom{\rule{0.2em}{0ex}}\left(\mathrm{syst}.\right)$, in agreement with the Standard Model prediction.