Measurement of the polarisation of W bosons produced in top-quark decays using dilepton events at <math altimg='si1.svg'><msqrt><mrow><mi>s</mi></mrow></msqrt><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>13</mn></math> TeV with the ATLAS experiment

International audience; 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 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 tt&#175; 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 cos⁡θ⁎ variable, where θ⁎ 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 cos⁡θ⁎ angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be f0=0.684&#177;0.005(stat.)&#177;0.014(syst.), fL=0.318&#177;0.003(stat.)&#177;0.008(syst.) and fR=−0.002&#177;0.002(stat.)&#177;0.014(syst.), in agreement with the Standard Model prediction.