First measurement of the forward rapidity dependence of $W$ boson transverse helicity fractions

Abstract

The transverse helicity fractions of $W$ bosons are measured as a function of the $W$ boson rapidity, $y_W$, in the range $0\le y_W\le 5$ using $W\to \mu {\nu }_{\mu }$ decays in $pp$ collisions at $\sqrt{s}$ = 13 TeV recorded by the LHCb experiment and corresponding to an integrated luminosity of $5.1$ fb${}^{-1}$. The fractions are extracted from a template fit to the muon transverse momentum and pseudorapidity. The results show a strong rapidity dependence and agree with next-to-leading-order Standard Model predictions, providing the first determination of the transverse helicity fractions of $W$ bosons in the forward region.

All figures and tables, along with any supplementary material and additional information, are available at https://lhcb-glance.cern.ch/alcm/public/analysis/full-details/6150