Observation of the decay ${\Lambda }_b^0\to {\Lambda }_c^{+}{\tau }^{-}{\overline{\nu }}_{\tau }$

Abstract

The first observation of the semileptonic $b$-baryon decay ${\Lambda }_b^0\to {\Lambda }_c^{+}{\tau }^{-}{\overline{\nu }}_{\tau }$, 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 $\mathcal{B}({\Lambda }_b^0\to {\Lambda }_c^{+}{\tau }^{-}{\overline{\nu }}_{\tau })=(1.50\pm 0.16\pm 0.25\pm 0.23)\%$ is obtained, where uncertainties are statistical, systematic and from the external branching fraction of the normalisation channel ${\Lambda }_b^0\to {\Lambda }_c^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$. The ratio of semileptonic branching fractions $\mathcal{R}({\Lambda }_c^{+})\equiv \mathcal{B}({\Lambda }_b^0\to {\Lambda }_c^{+}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}({\Lambda }_b^0\to {\Lambda }_c^{+}{\mu }^{-}{\overline{\nu }}_{\mu })$ is derived to be $0.242\pm 0.026\pm 0.040\pm 0.059$, where the external branching fraction uncertainty from the channel ${\Lambda }_b^0\to {\Lambda }_c^{+}{\mu }^{-}{\overline{\nu }}_{\mu }$ contributes to the last term. This result is in agreement with the Standard Model prediction.

"All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2021-044.html (LHCb public pages). arXiv admin note: text overlap with arXiv:1708.08856

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