Testing lepton flavour universality and activities of phase-II upgrade in CMS

<!--HTML--> I carried out my research activities for the PhD in Physics within the Compact Muon Solenoid (CMS) experiment team at the University and INFN of Pisa. CMS is one of the two multipurpose detectors at the Large Hadron Collider (LHC), credited for the discovery of Higgs Boson in 2012.<br><br>Within the CMS Pisa group, I am working in the Beyond Standard Model (BSM) physics searches, in particular, the study of Lepton Flavour Non-Universality (LFNU) in collaboration with other international groups. LFNU can be observed by investigating deviations from the Standard Model (SM) prediction of all the three charged leptons, that in the SM are supposed to have the same coupling constants for the electroweak interactions. For this investigation, I am looking at a specific decay channel $B_{c}^{+}$ $\to$ $J$/$\Psi$$\tau$+$v$$_{\tau}$; To measure the ratio R(J/$\Psi$), where: R(J/$\Psi$) = $_{B(B_{c}^{+} \rightarrow J/\Psi\mu+v \mu}^{B(B_{c}^{+} \rightarrow J/\Psi \tau+v\tau}$. I am studying this particular decay, since measurements of semi-leptonic decay of b hadron provide a direct handle to study the origin of LFNU and non-universal coupling. The analysis is ongoing and seems to converge soon. CMS Run3 data shall have some positive impact on the results but better statistical results are expected from high luminosity upgrades.<br><br>The LHC is undergoing an upgrade to accomplish the transition to the high-luminosity phase (HL-LHC), foreseen from ~2027. To withstand this high radiation flux, CMS is having major upgrades. I have actively worked on the upgrade of the inner pixel tracker of the CMS, which is located closest to the interaction point and susceptible to the highest amount of radiation. I have performed TCAD simulation studies, to study the effect of these high doses of radiation on the silicon pixels. Since the present tracker will be completely replaced with a new and high radiation resistant sub-detector, extensive R&D is ongoing. As part of this R & D, I am pursuing the studies of planar and 3D silicon pixels of varying dimensions for the innermost layer of the tracker. Modules are being irradiated at high fluences and beam test studies are being done before and after irradiation for evaluating the performances in realistic conditions.