Muon collider probes of Majorana neutrino dipole moments and masses

Abstract

Majorana neutrinos may have transitional dipole moments, which violate lepton number as well as lepton flavour. We estimate the sensitivity of future colliders to the electron-muon neutrino dipole moment, λ_eμ, by considering same-sign dilepton final states. We find that hadron colliders, even the proposed FCC-hh, are sensitive only to |λ_eμ| ≳ 10⁻⁹μ_B (with μ_B the Bohr magneton), a value two-three orders of magnitude larger than current bounds from astrophysics and low-energy neutrino-scattering experiments. In the case of a future muon collider, we show that the sensitivity varies from |λ_eμ| ~ 5 ∙ 10⁻⁹μ_B for energy \( \sqrt{s} \) ≃ 3 TeV, to ∼ 10⁻¹²μ_B for \( \sqrt{s} \) ≃ 50 TeV, matching the current laboratory bounds for \( \sqrt{s} \) ≃ 30 TeV. The singular advantage of the muon collider signal would be a direct, clean identification of lepton number and flavour violation. We also show that a muon collider would improve by orders of magnitude the direct bounds on m_eμ and m_μμ, two of the entries of the Majorana neutrino mass matrix. These bounds could be as strong as ∼ 50 keV, still far above the neutrino mass scale.