Differential branching fraction and angular analysis of Λ {/b 0} → Λμ + μ - decays

The differential branching fraction of the rare decay Λ 0 b → Λµ+µ − is measured as a function of 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 fb−1 , collected by the LHCb experiment. Evidence of signal is observed in the q 2 region below the square of the J/ψ mass. Integrating over 15 < q2 < 20 GeV2/c4 the branching fraction is measured as dB(Λ 0 b → Λµ+µ −)/dq 2 = (1.18 + 0.09 − 0.08 ± 0.03 ± 0.27) × 10−7 ( GeV2 /c4 ) −1 , where the uncertainties are statistical, systematic and due to the normalisation mode, Λ 0 b → J/ψ Λ, respectively. In the q 2 intervals where the signal is observed, angular distributions are studied and the forward-backward asymmetries in the dimuon (A` FB) and hadron (Ah FB) systems are measured for the first time. In the range 15 < q2 < 20 GeV2/c4 they are found to be A ` FB = −0.05 ± 0.09 (stat) ± 0.03 (syst) and A h FB = −0.29 ± 0.07 (stat) ± 0.03 (syst)