Angular moments of the decay ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }{\mu }^{+}{\mu }^{-}$ at low hadronic recoil

An analysis of the angular distribution of the decay $\Lambda_b^0 \rightarrow \Lambda \mu^{+} \mu^{-}$ is presented, using data collected with the LHCb detector between 2011 and 2016 and corresponding to an integrated luminosity of approximately $5f{b}^{-1}$. Angular observables are determined using a moment analysis of the angular distribution at low hadronic recoil, corresponding to the dimuon invariant mass squared range $15<q^2<20Ge{V}^2\mathrm{/}{c}^4$. The full basis of observables is measured for the first time. The lepton-side, hadron-side and combined forward-backward asymmetries of the decay are determined to be \begin{align} A_{FB}^{l} & = -0.39 \pm 0.04,\rm{stat} \pm 0.01, \rm{syst}, \nonumber\ A_{FB}^{h} & = -0.30 \pm 0.05,\rm{stat} \pm 0.02, \rm{syst}, \nonumber\ A_{FB}^{lh} & = +0.25 \pm 0.04,\rm{stat} \pm 0.01, \rm{syst}. \nonumber \end{align} The measurements are consistent with Standard Model predictions.