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.
Distributions of (left) $\tau ^-$ decay time and (right) BDT output for $\Lambda ^0_ b $ $\rightarrow$ $\Lambda ^+_ c $ $\tau ^-$ $\overline{\nu } _\tau$ candidates. Projections of the three-dimensional fit results are overlaid. The various fit components are described in the legend. |
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Distributions of $ q^2$ for $\Lambda ^0_ b $ $\rightarrow$ $\Lambda ^+_ c $ $\tau ^-$ $\overline{\nu } _\tau$ candidates having a BDT output value (left) below and (right) above 0.66. Projections of the three-dimensional fit are overlaid. The various fit components are described in the legend. |
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Distribution of the $\Lambda ^+_ c $ $\pi ^-$ $\pi ^+$ $\pi ^-$ invariant mass for the $\Lambda ^0_ b \rightarrow \Lambda ^+_ c D ^-_ s (X)$ control sample, with $ D ^-_ s \rightarrow \pi ^- \pi ^+ \pi ^- $. The components contributing to the fit model are indicated in the legend. |
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Distribution of the (top left) minimum $\pi ^+$ $\pi ^-$ mass combination formed from the pion triplet, (top right) maximum $\pi ^+$ $\pi ^-$ mass combination and (bottom) $\Lambda ^+_ c $ $\pi ^-$ $\pi ^+$ $\pi ^-$ mass, for simulated samples of $\Lambda ^0_ b $ $\rightarrow$ $\Lambda ^+_ c $ $\tau ^-$ $\overline{\nu } _\tau$ (blue points) and $\Lambda ^0_ b $ $\rightarrow$ $\Lambda ^+_ c $ $ D ^-_ s $ (X) (red line) decays. |
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Distribution of the $\tau $ decay time for $\Lambda ^0_ b $ $\rightarrow$ $\Lambda ^+_ c $ $\tau ^-$ $\overline{\nu } _\tau$ candidates with (top) BDT output value below 0.66 (bottom) BDT output value above 0.66. The various fit components are described in the legend. |
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Distribution of the $\Lambda ^+_ c $ $\pi ^-$ $\pi ^+$ $\pi ^-$ invariant mass for all candidates in the normalization channel, after removal of the $\Lambda ^{*+}_ c $ contributions. The fit components are indicated in the legend. The signal is described by a Crystal Ball (CB) function, and the background by an exponential term. |
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Animated gif made out of all figures. |
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Relative systematic uncertainties in ${\cal{K}}(\Lambda ^+_ c )$. |
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Created on 23 March 2024.