Lepton-number violation is the nonconservation of global lepton number by two units and, if observed, would indicate Majorana mass terms or other physics beyond the Standard Model. Experimental tests in colliders focus on event-level observables that can distinguish charge-conjugate processes and reveal LNV mediators. Theorists such as André de Gouvêa Northwestern University emphasize collider searches as complementary to nuclear neutrinoless double beta decay experiments because colliders probe higher mass scales and different operator structures with different background and model dependencies.
Collider signatures: same-sign leptons and jets
A primary observable is the production of same-sign dileptons accompanied by jets with little or no missing transverse energy. For example, a heavy Majorana neutrino produced in association with a charged lepton can decay back to a charged lepton and two jets, producing an l± l± jj topology that violates lepton number. Experimental groups including the CMS Collaboration at CERN and the ATLAS Collaboration at CERN have published searches for these topologies, setting limits on heavy Majorana neutrino couplings and masses. The key experimental handles are the charge of the two leptons, the invariant mass of the lepton-plus-jet system which can show a resonant peak, and the absence of large missing energy which reduces backgrounds from neutrino-emitting Standard Model processes. Backgrounds from misidentified leptons and charge misassignment require careful control and data-driven estimation.
Displaced vertices, kinematics, and complementary probes
If the LNV mediator is long lived, displaced vertices or delayed timing signals provide a low-background signature. LHCb Collaboration at CERN and the general-purpose detectors exploit precision tracking to locate secondary vertices millimeters to meters from the interaction point, which is a strong indicator of weakly coupled heavy neutral leptons or R-parity violating supersymmetric particles. Additional observables include detailed kinematic endpoints, angular correlations sensitive to the mediator spin, and charge asymmetries between positive and negative same-sign events. Alternative mechanisms such as doubly-charged Higgs bosons would produce same-sign lepton pairs with different invariant-mass distributions and branching fractions.
Observation of any of these observables would have profound consequences for neutrino mass models and baryogenesis via leptogenesis, and would reshape theoretical priorities. Experimental sensitivity depends on collider energy, integrated luminosity, detector technology, and geographic concentrations of expertise at facilities like CERN, where collaborative infrastructures enable these demanding searches. Noncollider experiments remain essential to provide complementary constraints on low-energy LNV operators.