Muon collider probes of Majorana neutrino dipole moments and masses

TL;DR

This paper evaluates the potential of future colliders, especially muon colliders, to detect Majorana neutrino dipole moments and masses, highlighting their sensitivity and advantages over existing bounds.

Contribution

It provides a detailed analysis of collider sensitivities to Majorana neutrino dipole moments and mass matrix entries, emphasizing the unique capabilities of muon colliders.

Findings

Hadron colliders are less sensitive than astrophysical bounds for dipole moments.

Muon colliders can reach sensitivities matching current laboratory bounds at high energies.

Muon colliders could significantly improve bounds on Majorana neutrino mass matrix entries.

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, $\lambda_{e\mu}$, by considering same-sign dilepton final states. We find that hadron colliders, even the proposed FCC-hh, are sensitive only to $|\lambda_{e\mu}|\gtrsim 10^{-9}\mu_B$ (with $\mu_B$ 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 $|\lambda_{e\mu}|\sim 5\cdot 10^{-9}\mu_B$ for energy $\sqrt{s}\simeq 3$ TeV, to $\sim 10^{-12}\mu_B$ for $\sqrt{s}\simeq 50$ TeV, matching the current laboratory bounds for $\sqrt{s}\simeq 30$ 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 $m_{e\mu}$ and $m_{\mu\mu}$, two of the entries of the Majorana neutrino mass matrix. These bounds could be as strong as $\sim 50$ keV, still far above the neutrino mass scale.