Master Thesis defense by Merlijn van Emmerik

Title: Constraints on Heavy Neutral Leptons via Charged Lepton Flavor Violating Quarkonia Decays

Abstract: This work focuses on the constraints of heavy neutral leptons (HNLs), introduced via the Seesaw- Type-I mechanism in the minimal extension of standard model neutrinos. Such hypothetical particles mediate neutrino oscillations, where two HNLs are considered to be in a quasi-Dirac pair in this work. Moreover, constraints on the weak-mixing angle between heavy-active states and the subsequent HNL masses are established via the charged lepton flavor violating (cLFV) modes of heavy quarkonium decay of J/ψ(1S) and Υ(1S) to τ+e−.

These decays can induce HNLs beyond tree-level for which both s-channel and box-type diagrams are considered at 1-loop in perturbation theory. The former is done via the previously computed Z and electromagnetic form-factors and the latter via explicit computation, successfully providing parameter limits from processes which have thus far not been considered. The result is additionally compared to two Low energy Effective Field theories (LEFT) available.