Observation of the decay $ \Lambda_b^0\rightarrow \Lambda_c^+\tau^-\overline{\nu}_{\tau}$

TL;DR

This paper reports the first observation of the semileptonic decay of the $\Lambda_b^0$ baryon to a $\Lambda_c^+$, a tau lepton, and a neutrino, measuring its branching fraction and ratio to muonic decay, consistent with Standard Model predictions.

Contribution

It provides the first experimental measurement of the $\Lambda_b^0 \rightarrow \Lambda_c^+ \tau^- \overline{\nu}_{\tau}$ decay, including branching fraction and ratio to muon channel, using LHCb data.

Findings

Branching fraction $\approx 1.50\%$ with uncertainties.

Branching ratio $\approx 0.242$ with uncertainties.

Observation significance of $6.1\sigma$.

Abstract

The first observation of the semileptonic $b$-baryon decay $ \Lambda_b^0 \rightarrow \Lambda_c^+ \tau^-\overline{\nu}_{\tau}$, with a significance of $6.1\,\sigma$, is reported using a data sample corresponding to 3 fb$^{-1}$ of integrated luminosity, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV at the LHC. The $\tau^-$ lepton is reconstructed in the hadronic decay to three charged pions. The branching fraction ${\mathcal{B}}(\Lambda_b^0 \rightarrow \Lambda_c^+\tau^-\overline{\nu}_{\tau}) = (1.50 \pm 0.16\pm 0.25\pm 0.23)\%$ is obtained, where uncertainties are statistical, systematic and from the external branching fraction of the normalisation channel $\Lambda_b^0\rightarrow \Lambda_c^+\pi^-\pi^+\pi^-$. The ratio of semileptonic branching fractions ${\mathcal{R}}( \Lambda_c^+)\equiv {\mathcal{B}}( \Lambda_b^0 \rightarrow \Lambda_c^+ \tau^-\overline{\nu}_{\tau})/{\mathcal{B}}( \Lambda_b^0 \rightarrow \Lambda_c^+ \mu^-\overline{\nu}_{\mu})$ is derived to be $0.242 \pm 0.026 \pm 0.040\pm 0.059$, where the external branching fraction uncertainty from the channel $\Lambda_b^0\rightarrow \Lambda_c^+\mu^-\overline{\nu}_{\mu}$ contributes to the last term. This result is in agreement with the Standard Model prediction.