The decay $\Lambda_b^0 \to \Lambda_c^+ p \overline{p} \pi^$ is observed using $pp$ collision data collected with the LHCb detector at centreofmass energies of $\sqrt{s}=$ 7 and 8 TeV, corresponding to an integrated luminosity of 3 $fb^{1}$. The ratio of branching fractions between $\Lambda_b^0 \to \Lambda_c^+ p \overline{p} \pi^$ and $\Lambda_b^0 \to \Lambda_c^+ \pi^$ decays is measured to be \begin{equation*} \frac{\mathcal{B}(\Lambda_b^0 \to \Lambda_c^+ p \overline{p}\pi^)}{\mathcal{B}(\Lambda_b^0 \to \Lambda_c^+ \pi^)} = 0.0540 \pm 0.0023 \pm 0.0032. \end{equation*} Two resonant structures are observed in the $ \Lambda_c^+ \pi^$ mass spectrum of the ${\Lambda_b^0 \to \Lambda_c^+ p\overline{p} \pi^}$ decays, corresponding to the $\Sigma_c(2455)^0$ and $\Sigma_c^{*}(2520)^0$ states. The ratios of branching fractions with respect to the decay $\Lambda_b^0 \to \Lambda_c^+ p \overline{p} \pi^$ are \begin{align*} \frac{\mathcal{B}(\Lambda_b^0 \to \Sigma_c^0 p\overline{p})\times\mathcal{B}(\Sigma_c^0\to \Lambda_c^+ \pi^)}{\mathcal{B}(\Lambda_b^0 \to \Lambda_c^+ p \overline{p}\pi^)} = 0.089\pm0.015\pm0.006, \frac{\mathcal{B}(\Lambda_b^0 \to \Sigma_c^{*0} p\overline{p})\times\mathcal{B}(\Sigma_c^{*0}\to \Lambda_c^+ \pi^)}{\mathcal{B}(\Lambda_b^0 \to \Lambda_c^+ p \overline{p}\pi^)} = 0.119\pm0.020\pm0.014. \end{align*} In all of the above results, the first uncertainty is statistical and the second is systematic. The phase space is also examined for the presence of dibaryon resonances. No evidence for such resonances is found.
Invariant mass distributions of the (a) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c p \overline p \pi ^ $ and (b) $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \pi ^ $ candidates. Fit results are overlaid as a solid blue line. For (a), the red dotted line represents the signal component and the green dotted line the background due to random combinations. For (b), the red dotted line is the signal component, the green dotted line is the random combination background, the purple dashed line is the contribution from $\Lambda ^0_ b \rightarrow \Lambda ^+_ c \rho ^ $ and the brown dasheddotted line represents the contribution from $\Lambda ^0_ b \rightarrow \Lambda ^+_ c K ^ $. 
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Invariant mass of the $\Lambda ^+_ c $ $\pi ^$ system from the decay ${\Lambda ^0_ b \rightarrow \Lambda ^+_ c p \overline p \pi ^ }$. The $\Sigma_c^0$ and $\Sigma_c^{*0}$ resonances are indicated. The fit to the data is shown as a blue continuous line, with the background component shown as a green dotted line, the $\Sigma_c^{0}$ shape shown as a dashed red line, and the $\Sigma_c^{*0}$ shape shown as a dashdotted magenta line. 
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Backgroundsubtracted mass spectrum of the $\Lambda ^+_ c \pi ^ p $ system from the decay ${\Lambda ^0_ b \rightarrow \Lambda ^+_ c p \overline p \pi ^ }$ in (a) the full $\Lambda ^+_ c \pi ^ $ mass spectrum, (b) the signal region of the $\Sigma_c^0$ resonance, and (c) the signal region of the $\Sigma_c^{*0}$ resonance. In all figures, the black points are data and the red points are simulated events where the $\Lambda ^0_ b $ baryon decays to the $\Lambda ^+_ c $ $ p $ $\overline p $ $\pi ^$ final state (a) based on a uniformphasespace model, (b) through the $\Sigma_c^0$ resonance and (c) through the $\Sigma_c^{*0}$ resonance. No evident peaking shapes are visible. 
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Animated gif made out of all figures. 
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Summary of systematic uncertainties and correction factors to the ratio of branching fractions measurement. All uncertainties are given as a percentage of the ratio of branching fractions. 
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Created on 10 July 2020.