Quasi-Dirac neutrinos at the LHC

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 clarify under which conditions the ratio R ll can assume values different from 0 and 1, and we argue that the precise value 0 < R ll < 1 is controlled by the mass splitting versus the width of the quasi-Dirac resonances. A measurement of R ll = 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.