Search for lepton flavour violation at the LHC
IDPASC Portugal - PHD Programme 2016
2016 / 2017
Experimental Particle Physics
Instituto Superior Técnico
Laboratório de Instrumentação e Física Experimental de Partículas
Lepton flavor violation (LFV) is allowed in the context of the SM in the presence of massive neutrinos. LFV in the charged-lepton sector is however expected to occur within the SM at tiny rates, that remain experimentally inaccessible. On the other hand, various new physics scenarios, beyond the SM, predict LFV processes with rates close to attainable experimental sensitivity. Observation of LFV would therefore be an unambiguous signature of physics beyond the SM. The search for the tau decay to 3 muons LFV process at CMS using Run2 data is the subject of this thesis. The search builds up on the experience acquired in the group, in precision measurements and searches for other rare processes involving multi-lepton final states. Despite the clean signature, the search is nonetheless challenging (stemming from soft trailing lepton and need for good detection efficiencies). Heavy flavour and vector boson decays are explored as distinct tau sources. A baseline dedicated trigger algorithm has been successfully deployed online, which is being used to collect the dataset, starting summer 2016. Possible improvements to the algorithm will be investigated, as part of the work program, which will otherwise focus on the offline analysis. Multivariate analysis techniques and data-based calibrations will be devised and implemented, in the search for this ultra-rare, smoking-gun process. Remarks: - LIP has a leading involvement in this physics research area. The proposed supervisor of this thesis is the co-ordinator of the overall CMS-wide heavy flavour physics analysis group (PAG). - The overall analysis effort is being prepared and will be pursued by LIP in partnership with an US institution, within the CMS collaboration. - Solid computing skills (C++) are a plus. Availability to travel to CERN for short periods is expected, in order to participate in data taking, and in PAG and analysis team meetings.