Search for heavy neutrinos in $D_s$ decays with the CMS detector at LHC
This thesis presents a search for a sterile right-handed neutrino $N$ produced in $D_s$ meson decays, using proton-proton collisions collected by the CMS experiment at the LHC. The data set used for the analysis, the B-Parking data set, corresponds to an integrated luminosity of $41.7,textrm{fb}^{-1}$ and was collected during the 2018 data-taking period. The analysis is targeting the $D_s^+rightarrow N(rightarrowmu^{pm}pi^{mp})mu^{+}$ decays, where the final state muons can have the same electric charge allowing for a lepton flavor violating decay. To separate signal from background, a cut-based analysis is optimized using requirements on the sterile neutrino vertex displacement, muon and pion impact parameter, and impact parameter significance. The expected limit on the active-sterile neutrino mixing matrix parameter $|V_{mu}|^2$ is extracted by performing a fit of the $mupi$ invariant mass spectrum for two sterile neutrino mass hypotheses, 1.0 and 1.5 GeV. The analysis is currently blinded, following the internal CMS review process. The expected limit ranges between approximately $10^{-4}$ for a 1.0 GeV neutrino to $7times10^{-5}$ for a 1.5 GeV neutrino. This is competitive with the best existing results from collider experiments over the same mass range.
http://amsdottorato.unibo.it/10924/
http://amsdottorato.unibo.it/10924/1/DsToHnl_phdthesis_LeonardoLunerti.pdf