The search for heavy Majorana neutrinos

The Majorana nature of neutrinos can be experimentally verified only via lepton-number violating processes involving charged leptons. We study 36 lepton-number violating (LV) processes from the decays of tau leptons and pseudoscalar mesons. These decays are absent in the Standard Model but, in presence of Majorana neutrinos in the mass range ∼ 100 MeV to 5 GeV, the rates for these processes would be enhanced due to their resonant contribution. We calculate the transition rates and branching fractions and compare them to the current bounds from direct experimental searches for ΔL = 2 tau and rare meson decays. The experimental non-observation of such LV processes places stringent bounds on the Majorana neutrino mass and mixing and we summarize the existing limits. We also extend the search to hadron collider experiments. We find that, at the Tevatron with 8 fb⁻¹ integrated luminosity, there could be 2σ (5σ) sensitivity for resonant production of a Majorana neutrino in the μ^±μ^±modes in the mass range of ∼10–180 GeV (10–120 GeV). This reach can be extended to ∼10–375 GeV (10–250 GeV) at the LHC of 14 TeV with 100 fb⁻¹. The production cross section at the LHC of 10 TeV is also presented for comparison. We study the μ^±e^± modes as well and find that the signal could be large enough even taking into account the current bound from neutrinoless double-beta decay. The signal from the gauge boson fusion channel W⁺W⁺→ℓ⁺₁ℓ⁺₂ at the LHC is found to be very weak given the rather small mixing parameters. We comment on the search strategy when a τ lepton is involved in the final state.