Muon collider probes of Majorana neutrino dipole moments and masses

A bstract 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 −9 μ 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 −9 μ B for energy $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> ≃ 3 TeV, to ∼ 10 − 12 μ B for $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> ≃ 50 TeV, matching the current laboratory bounds for $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> ≃ 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.