Differential branching fraction and angular analysis of ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }{\mu }^{+}{\mu }^{-}$ decays

The differential branching fraction of the rare decay $\Lambda^{0}{b} \rightarrow \Lambda \mu^+\mu^-$ismeasuredasafunctionof$q^{2}$, the square of the dimuon invariant mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of $3.0 \mbox{ fb}^{-1}$, collected by the LHCb experiment. Evidence of signal is observed in the $q^2$ region below the square of the $J\mathrm{/}\psi$ mass. Integrating over $15<q^2<20Ge{V}^2\mathrm{/}{c}^4$ the branching fraction is measured as $d$\mathcal{B}$(\Lambda^{0}{b} \rightarrow \Lambda \mu^+\mu^-)/dq^2 = (1.18 ^{+ 0.09} {-0.08} \pm 0.03 \pm 0.27) \times 10^{-7} ( \mbox{GeV}^{2}/c^{4})^{-1}$, where the uncertainties are statistical, systematic and due to the normalisation mode, ${\mathrm{\Lambda }}_b^0\to J\mathrm{/}\psi \mathrm{\Lambda }$, respectively. In the $q^2$ intervals where the signal is observed, angular distributions are studied and the forward-backward asymmetries in the dimuon ($A_{\mathrm{F}\mathrm{B}}^l$) and hadron ($A_{\mathrm{F}\mathrm{B}}^h$) systems are measured for the first time. In the range $15<q^2<20Ge{V}^2\mathrm{/}{c}^4$ they are found to be $A^{l}{\rm FB} = -0.05 \pm 0.09 \mbox{ (stat)} \pm 0.03 \mbox{ (syst)}$and$A^{h}_{\rm FB} = -0.29 \pm 0.07 \mbox{ (stat)} \pm 0.03 \mbox{ (syst)}$.