Master defense by Jonathan L. Schubert

Title: Interference Between Majorana States in the “Neutrinoless Double Beta Decay” at Colliders

Abstract: Heavy neutral leptons (HNLs) are hypothetical particles able to ex- plain several puzzles of fundamental physics, chiefly neutrino oscillations. Being sterile with respect to Standard Model interactions, these particles admit Majorana masses.

This mass term allows for processes violating the Standard Model lepton number. Hence, such processes become a “smoking gun” signature of HNLs, pursued by many experiments. In this work we demonstrate that if HNLs are the sole origin of neutrino masses, destructive interference between Majorana states suppresses the same sign di-lepton signal at colliders.

In the phenomenologically interesting case of large HNL couplings, such a suppression stems from mutually cancelling contribu- tions of several HNLs akin to their contributions to neutrino masses. Nevertheless, the signal can be much larger than naively expected. We also identify parts of the parameter space of such realistic HNL models where the lepton number violating sig- nal is maximised at the Large Hadron Collider and Future Circular Collider (LHC) in the hadron-hadron phase (FCC-hh). Our results are obtained within the effective W approximation which allows for analytic treatment and gives clear dependence on the model parameters. Subsequently, they are rescaled to detector level Monte- carlo simulations.

For the LHC, the parameter bounds on the HNL’s mixing to the Standard Model leptonic sector expected from our results are not competitive with Electroweak Precision Tests, improving only slightly on other collider based searches. For the FCC-hh we find a potential improvement on indirect bounds for HNLs with masses in the TeV range, with strong improvement on other collider based searches. Although approximate, our results are expected to be good estimates of discovery potential. Exact exclusion limits on HNL parameters warrant further investigation.