Observation of the suppressed decay $\Lambda \ _b \ ^0 \to p \pi^- \mu^+ \mu^-$

Abstract

The suppressed decay $\Lambda \ _b \ ^0 \to p \pi^- \mu^+ \mu^-$, excluding the $J/ \psi$ and $\psi(2S) \to \mu^+ \mu^-$ resonances, is observed for the first time with a significance of 5.5 standard deviations. The analysis is performed with proton-proton collision data corresponding to an integrated luminosity of 3 $fb^{−1}$ collected with the LHCb experiment. The $\Lambda \ _b \ ^0 \to p \pi^- \mu^+ \mu^-$ branching fraction is measured relative to the $\Lambda \ _b \ ^0 \to J/ \psi (\to \mu^+ \mu^-)p \pi^-$ branching fraction giving
$\frac{\mathcal{B}(\Lambda_b^0 \to p \pi^*- \mu^+ \mu^-)}{\mathcal{B}(\Lambda_b^0 \to J/ \psi (\to \mu^+ \mu^-)p \pi^-)} = 0.044 \pm 0.012 \pm 0.007$,
where the first uncertainty is statistical and the second is systematic. This is the first observation of a $b \to d$ transition in a baryonic decay.