A Study of Semileptonic $B$ Decays in Orbitally Excited $D$ Mesons at LHCb

In this thesis, a study of semi-inclusive semileptonic $B$ decays in excited $D$ mesons has been presented; in particular, the analysis has been focused on orbitally excited $P$-wave $D^{**}$ mesons and on higher mass resonances found in the data sample. The theoretical framework to study inclusive semileptonic $B$ meson decays is the Operator Product Expansion (OPE) approach, while exclusive decay modes are described different models, such as the so-called ISGW2 and LLSW; on the other hand, excited $D$ mesons properties are predicted by the Heavy Quark Effective Theory (HQET). The final measurements has been perfomed on a data sample collected at LHCb experiment (Chapter 2) in the (2011-2012) data taking period, corresponding to a total integrated luminosity of $3 fb^{-1}$. This data sample has been subjected to two different selection stages, calibrated on a MonteCarlo (MC) sample: a pre-selection of $B \to D^{*} \mu \nu_{\mu} X$ decays, and a selection of $B \to D^{*} \pi_{**} \mu \nu_{\mu} X$ events with the aid of a MultiVariate Analysis (MVA). After this preselection, the $D^{*}\pi_{**}$ invariant mass spectrum has been fitted in order to extract excited $D$ meson yields and thus measure branching ratios for semileptonic $B$ meson decays; in addition, the $m(K\pi\pi)-m(K\pi)$ invariant mass distribution has been fitted for measuring the number of events in the normalization channel ($B \to D^{*} \mu \nu{\mu} X$). Two of the four neutral $D^{**}$ states ($D_{1}(2420)^{0}$ and $D_{2}(2460)^{0}$) have been observed; the relative statistical significances are equal to $19.1\sigma$ and $11.5\sigma$ respectively. The masses and the widths of these two narrow states has been found to be consistent with the PDG values. No evidence for neutral broad $D^{**}$ resonances ($D_{0}^{*}(2400)^{0}$ and $D_{1}(2430)^{0}$) and charged $D^{**}$ have been found. It was not possible to separate, with the current statistics and selection criteria, the non-resonant $B \to D^{*} \pi_{**} \mu \nu_{\mu}$ decays from the combinatorial background. Two additional broad structures have been found in the higher mass region of the $D^{*}\pi_{**}$ spectrum: the first one, called $D_{1}^{*}(2650)^{0}$ with a statistical significance of $5.1\sigma$, while the other one, called $D_{J}^{*}(2760)^{0}$, with a statistical significance of $3.8\sigma$. This is the first evidence for semileptonic $B$ decays into excited $D$ mesons other than $D^{**}$. These structures have also been observed with higher significance by the BaBar and LHCb experiments, by analyzing prompt $D$ meson production from $e^{+}e^{-}$ and $pp$ collisions respectively, that are characterized by much higher cross sections; further resonances observed in these works, such as $D_{J}(2580)^{0}$, $D_{J}(2740)^{0}$ and $D_{J}(3000)^{0}$, were not seen in this analysis of semileptonic decays.