Measurement of the differential branching fraction of the decay ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$

Abstract

The differential branching fraction of the decay ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$ is measured as a function of the square of the dimuon invariant mass, $q^2$. A yield of $78\pm 12$ ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$ decays is observed using data, corresponding to an integrated luminosity of 1.0\,fb${}^{-1}$, collected by the LHCb experiment at a centre-of-mass energy of 7\,TeV. A significant signal is found in the $q^2$ region above the square of the $J/\psi$ mass, while at lower-$q^2$ values upper limits are set on the differential branching fraction. Integrating the differential branching fraction over $q^2$, while excluding the $J/\psi$ and $\psi (2S)$ regions, gives a branching fraction of $\BF($Λ_b^0\rightarrowΛμ^+μ^-$)=(0.96\pm 0.16\stat\pm 0.13\syst\pm 0.21 (\mathrm{norm}))\times 10^{-6}$, where the uncertainties are statistical, systematic and due to the normalisation mode, $$Λ_b^0\rightarrow J/psiΛ$, respectively.

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