Quasi-Dirac neutrinos at the LHC

TL;DR

This paper explores how the ratio of same-sign to opposite-sign lepton events at the LHC can reveal the nature of quasi-Dirac neutrinos, providing insights into neutrino mass origins and potential experimental signatures.

Contribution

It demonstrates that for quasi-Dirac neutrinos, the lepton event ratio can vary between 0 and 1, challenging the standard Majorana/Dirac dichotomy and linking it to neutrino mass mechanisms.

Findings

The ratio $R_{ll}$ can take any value between 0 and 1 for quasi-Dirac neutrinos.

A correlation exists between $R_{ll}$ and decay rate ratios in the inverse seesaw model.

Heavy neutrino signatures could be within reach at the LHC.

Abstract

Lepton number violation is searched for at the LHC using same-sign leptons plus jets. The standard lore is that the ratio of same-sign lepton to opposite-sign lepton events, $R_{ll}$, is equal to $R_{ll}=1$ ($R_{ll}=0$) for Majorana (Dirac) neutrinos. We argue that for "quasi-Dirac" neutrinos, $R_{ll}$ can have any value between 0 and 1, the precise value being controlled by the mass splitting versus the width of the quasi-Dirac resonances. A measurement of $R_{ll}\neq 0,1$ would then contain valuable information about the origin of neutrino masses. We consider as an example the inverse seesaw mechanism in a left-right symmetric scenario, which is phenomenologically particularly interesting since all the heavy states in the high energy completion of the model could be within experimental reach. A prediction of this scenario is a correlation between the values of $R_{ll}$ and the ratio between the rates for heavy neutrino decays into standard model gauge bosons, and into three body final states $ljj$ mediated by off-shell $W_R$ exchange.