CP violation with Majorana neutrinos in K meson decays

Abstract: We study the possibility of having CP asymmetries in the decay K^± → π^∓ℓ^±ℓ^± (ℓ = e, μ). This decay violates Lepton Number by two units and occurs only if there are Majorana particles that mediate the transition. Even though the absolute rate is highly suppressed by current bounds, we search for Majorana neutrino scenarios where the CP asymmetry arising from the lepton sector could be sizeable. This is indeed the case if there are two or more Majorana neutrinos with similar masses in the range around 10² MeV. In particular, the asymmetry is potentially near unity if two neutrinos are nearly degenerate, in the sense Δm_N ∼ Γ_N. The full decay, however, may be difficult to detect not only because of the suppression caused by the heavy-to-light lepton mixing, but also because of the long lifetime of the heavy neutrino, which would induce large space separation between the two vertices where the charge leptons are produced. This particular problem should be less serious in heavier meson decays, as they involve heavier neutrinos with shorter lifetimes.