Differential branching fraction and angular analysis of Λ(formula presented.)decays

Abstract: The differential branching fraction of the rare decay Λ _b ⁰ → 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 < q ²< 20 GeV ²/c ⁴ the differential branching fraction is measured asdℬ(formula presented.)±0.03±0.27× ^{10−7 GeV2/ c4−1} ,$$ \mathrm{d}\mathrm{\mathcal{B}}\left({\varLambda}_b^0\to \varLambda {\mu}^{+}{\mu}^{-}\right)/d{q}^2=\left({1.18}_{-0.08}^{+0.09}\pm 0.03\pm 0.27\right)\times {10}^{-7}{\left({\mathrm{GeV}}^2/{c}^4\right)}^{-1}, $$where the uncertainties are statistical, systematic and due to the normalisation mode, Λ _b ⁰ → J/ψΛ, 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 ^ℓ) and hadron (A _FB ^h) systems are measured for the first time. In the range 15 < q ²< 20 GeV ²/c ⁴ they are found to be(formula presented.)=−0.29±0.07stat±0.03syst.$$ \begin{array}{l}{A}_{\mathrm{FB}}^{\ell }=-0.05\pm 0.09\left(\mathrm{stat}\right)\pm 0.03\left(\mathrm{syst}\right)\;\mathrm{and}\hfill \\ {}{A}_{\mathrm{FB}}^h=-0.29\pm 0.07\left(\mathrm{stat}\right)\pm 0.03\left(\mathrm{syst}\right).\hfill \end{array} $$[Figure not available: see fulltext.]