Dirac vs. Majorana HNLs (and their oscillations) at SHiP

TL;DR

This paper demonstrates that the SHiP experiment can distinguish between Dirac and Majorana Heavy Neutral Leptons (HNLs) using decay product angular distributions, and can even resolve HNL oscillations for tiny mass splittings.

Contribution

It introduces a method to differentiate Dirac and Majorana HNLs at SHiP via decay angular distributions and spin correlations, enabling model selection and oscillation measurement.

Findings

SHiP can distinguish Majorana from Dirac HNLs using decay angular distributions.

The analysis can resolve HNL oscillations for mass splittings around 10^{-6} eV.

SHiP has significant sensitivity to feebly interacting particles and their properties.

Abstract

SHiP is a proposed high-intensity beam dump experiment set to operate at the CERN SPS. It is expected to have an unprecedented sensitivity to a variety of models containing feebly interacting particles, such as Heavy Neutral Leptons (HNLs). Two HNLs or more could successfully explain the observed neutrino masses through the seesaw mechanism. If, in addition, they are quasi-degenerate, they could be responsible for the baryon asymmetry of the Universe. Depending on their mass splitting, HNLs can have very different phenomenologies: they can behave as Majorana fermions -- with lepton number violating (LNV) signatures, such as same-sign dilepton decays -- or as Dirac fermions with only lepton number conserving (LNC) signatures. In this work, we quantitatively demonstrate that LNV processes can be distinguished from LNC ones at SHiP, using only the angular distribution of the HNL decay products. Accounting for spin correlations in the simulation and using boosted decision trees for discrimination, we show that SHiP will be able to distinguish Majorana-like and Dirac-like HNLs in a significant fraction of the currently unconstrained parameter space. If the mass splitting is of order $10^{-6}$ eV, SHiP could even be capable of resolving HNL oscillations, thus providing a direct measurement of the mass splitting. This analysis highlights the potential of SHiP to not only search for feebly interacting particles, but also perform model selection.