Master thesis defense by Henriette Aarup Petersen – University of Copenhagen

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Master thesis defense by Henriette Aarup Petersen

The search for right-handed neutrinos with the ATLAS experiment

In this analysis it is explored what the ATLAS experiment can do to investigate the νMSM parameters through the prompt trilepton signature of a right-handed neutrino. The case for mixing of a single right- handed and muon neutrino is considered here. No opposite-sign same-flavour leptons is further required to focus on the Majorana nature of the neutrinos. This analysis uses Monte Carlo simulations for five HNL mass points (defined in terms off muon-sterile mixing angle and HNL mass) which match data sets recorded during 2015 and 2016 for a center-of-mass energy corresponding to sqrt(s) = 13 TeV and an integrated luminosity of 36 fb−1.

A cut-based analysis is investigated , as well as a more intelligent approach to signal selection  in the form of a supervised neural network. The neural network approach was found to yield a significant improvement in significance and it is expected to be the best long-term analysis approach. Furthermore, it is roughly evaluated which values of muon-sterile mixing angles can be excluded based on the given luminosity and significances obtained with the analysis presented. Good signal exclusion is obtained for lower masses but more statistics and/or signal extraction procedure is needed for higher masses.