Master thesis defense by Sissel Bay Nielsen

Prospects of Sterile Neutrino Search with the FCC-ee

A proposed future circular e+e- collider, the FCC-ee, is suggested to search for sterile neutrinos. The Neutrino Minimal Standard Model, vMSM, is a model of sterile neutrinos, that accommodates explanations for several phenomena of physics beyond the Standard Model. This thesis presents an overview of the theoretical motivation for vMSM, an outline of the experimental conditions at the FCC-ee, and a review of previous accelerator bounds for sterile neutrinos.

Two studies of sterile neutrinos with masses at the electroweak scale are introduced, an analysis of long lived sterile neutrinos, and an analysis of short lived sterile neutrinos. Both analyses include background studies and sensitivity estimates for the FCC-ee detector.

The study of long lived sterile neutrinos is based on a search for detectable displaced vertices with 1012 Z decays, obtaining a search reach on the mixing angle |θ|2 as small as 10-11. The study of short lived sterile neutrinos is a Monte Carlo study with a cut-based analysis, where a detector parametrisation of the FCC-ee detector, inspired by the CLIC detector design, is used. Background events were generated corresponding to a total number of 1.43x108 Z events, corresponding to a luminosity of L = 3.33 fb-1. A search reach on the mixing angle with coupling to the muon neutrino on |θμN|2 ~ 10-7 – 10-6 was achieved for sterile neutrino masses from 10 to 80 GeV.