Study of semitauonic and semimuonic $\Lambda_b^0 \rightarrow \Lambda_c^{*+}l^-\bar \nu_l$ decays at LHCb

In this thesis the $\Lambda_b^0 \rightarrow \Lambda_c^{*+}\tau^-\bar \nu_\tau$ and $\Lambda_b^0 \rightarrow \Lambda_c^{*+}\mu^-\bar \nu_\mu$ decays have been investigated. In particular, the first measurement of the ratio $R(\Lambda_c^{*}) =\mathcal{B}(\Lambda_b^0 \rightarrow \Lambda_c^{*+}\tau^-\bar \nu_\tau)/\mathcal{B}(\Lambda_b^0 \rightarrow \Lambda_c^{*+}\mu^-\bar \nu_\mu)$ has been performed using a sample of LHC $pp$ collision data corresponding to 2 fb$^{-1}$ of integrated luminosity, recorded by the LHCb detector in the 2012. Only the excited state $\Lambda_c^{+}(2625)$ have been reconstructed, particularly in $\Lambda_c^{*+} \rightarrow \Lambda_c^{+}(\rightarrow pK^{-}\pi^+)\pi^+\pi^-$ decay mode and only the muonic decays of $\tau$ are considered, $\tau^- \rightarrow \mu^-\bar\nu_\mu \nu_\tau$. Moreover, the differential decay rate $(1/\Gamma) d\Gamma/dq^2(\Lambda_b \rightarrow \Lambda_c^*\mu\bar\nu_\mu)$, considering the unfolding of the raw spectrum for the $q^2$ resolution and the selection efficiency, has been determined. Finally, it is presented a new method to find the b-hadron momentum in a semileptonic decays of this one at a hadron collider. This is based on a multivariate regression algorithm which exploits the information that can be extracted by b-hadron flight vectors to infer its momentum.