Measurement of the D* longitudinal polarization in B0→D*−τ+ντ decays

The longitudinal polarization fraction of the D meson is measured in B0 → D−τþντ decays, where the τ lepton decays to three charged pions and a neutrino, using proton-proton collision data collected by the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV and corresponding to an integrated luminosity of 5 fb−1. The D polarization fraction FD L is measured in two q2 regions, below and above 7 GeV2=c4, where q2 is defined as the squared invariant mass of the τντ system. The FD L values are measured to be 0.52 0.07 0.04 and 0.34 0.08 0.02 for the lower and higher q2 regions, respectively. The first uncertainties are statistical and the second systematic. The average value over the whole q2 range is FD L ¼ 0.41 0.06 0.03. These results are compatible with the Standard Model predictions.