Search for exotic resonances in the decay $B^+ \rightarrow J/\psi \; \omega K^+$ in the LHCb experiment at CERN

“The expression \textit{exotic resonances} indicates those states whose characteristics don’t fit in the ordinary mesons nor baryons scheme.\\ In this thesis an analysis of the $J/\psi \; \omega$ invariant mass in the decay $B^+ \rightarrow (J/\psi \rightarrow \mu^+ \mu^-) \; (\omega \rightarrow \pi^+ \pi^- \pi^0) \; K^+$ is performed in order to search for such resonances, in particular the X(3872), previously observed in this final state by the BaBar experiment in 2010 at the Stanford Linear Accelerator Center [10]. The nature of this particle is still under study: different theoretical models were proposed. Thus, further experimental measurements are needed in order to improve the understanding of its characteristics. The present analysis is performed on data collected by the LHCb experiment at the Large Hadron Collider at CERN during 2011 and 2012. It is structured as follows: $$ $$• the events are reconstructed and selected in order to reduce as much as possible the background contamination;$$ $$• the Monte Carlo dataset is analysed in order to study the characteristics of the signal events and the expected resolutions;$$ $$• a statistical background subtraction is performed and the $J/\psi \; \omega$ invariant mass is analysed;$$ $$• additional contributions from other decays are searched for and eliminated where possible;$$ $$• the efficiency of the selection chain is calculated from MC data and used to correct the $J/\psi \; \omega$ data distribution;$$ $$• the efficiency-corrected $J/\psi \; \omega$ mass spectrum is compared to BaBar's results;$$ $$• a double peak structure is observed in the low mass region of the $J/\psi \; \omega$ invariant mass spectrum, and it is attributed to the two charmonium states $\chi_{c0}(2P)$ and X(3872);$$ $$• the relative yields of the two contributions are evaluated and compared to those reported by BaBar;$$ $$• future perspectives are illustrated.