Collider Prospects for the Neutrino Magnetic Moment Portal

TL;DR

This work investigates collider prospects for the active-to-sterile neutrino magnetic moment portal within a dimension-6 EFT framework, focusing on future lepton colliders (CEPC, FCC-ee, CLIC, MuC). It comprehensively analyzes production channels (2-2, 2-3, 2-4) and decay modes (N→νγ, νZ, ℓW), deriving channel-dependent sensitivities to the transition moment d_γ across a range of sterile-neutrino masses m_N and collider energies. Across LEP, CEPC/FCC-ee, and MuC/CLIC, the study demonstrates that active-to-sterile dipole interactions can be probed down to d_γ ≃ 10^-7 GeV^-1 at TeV-scale colliders, corresponding to Wilson coefficients of order unity with Λ ~ 20 TeV, especially in the MuC/CLIC regime where multiple channels contribute. The results emphasize the importance of channel choice, flavor structure, and collider energy in shaping sensitivity, and show that MuC can provide a powerful test of UV theories in the strong-coupling regime through these dipole operators.

Abstract

The transition magnetic moment between active and sterile neutrinos is theoretically well-motivated scenario beyond the Standard Model, which can be probed in cosmology, astrophysics, and at terrestrial experiments. In this work, we focus on the latter by examining such an interaction at proposed lepton colliders. Specifically, in addition to revisiting LEP, we consider CEPC, FCC-ee, CLIC, and the muon collider, motivated by the potential realization of any of them. Within the effective field theory framework, we present parameter regions that can be probed, highlighting the dependence on the lepton flavor interacting with the sterile neutrino. By including several new processes with large sterile neutrino production cross sections at high-energy lepton colliders, we find that the expected sensitivity for the active-to-sterile neutrino transition magnetic moment can reach $d_γ\simeq \mathcal{O}(10^{-7})$ GeV$^{-1}$.

Figures (11)

• Figure 1: Feynman diagrams corresponding to the s-channel (left panel) and t-channel (right panel) 2-2 process for sterile neutrino production at lepton colliders. In addition to sterile neutrino, $N$, SM neutrino is produced as well. The black blobs represent the insertion of the dimension-6 operators (see \ref{['eq:da']}).
• Figure 2: Representative diagrams for 2-3 (left and middle panels) and 2-4 (right panel) processes for sterile neutrino production at the proposed MuC.
• Figure 3: Upper (lower) panels show cross sections with $c_B/\Lambda^2 = 1/(30 \text{ TeV})^{2}$ (after generator-level cuts discussed in \ref{['sec:Event cuts']}) for various sterile neutrino production channels, evaluated for $e^+e^-$ ($\mu^+\mu^-$) collider at low (high) center-of-mass energy $\sqrt{s}$. Notice the relative suppression of the cross sections corresponding to the 2-3 and 2-4 processes (colors other than blue) at smaller $\sqrt{s}$ as well as their relevance for $\sqrt{s}\gtrsim 1$ TeV. The narrow dip in panel (a) arises from absence of sterile neutrino production via the $Z$ resonance for $d_Z=0$$(c_W/c_B=0.286)$.
• Figure 4: Branching ratios for $N\rightarrow\nu\gamma$, $N\rightarrow\nu Z$ and $N\rightarrow \ell^{\pm}W^{\mp}$ decay channels are shown for four benchmark points considered in this work.
• Figure 5: Branching ratios for $N\rightarrow\nu\gamma$, $N\rightarrow\nu Z$ and $N\rightarrow \ell^{\pm}W^{\mp}$ decay channels are shown as a function of $c_W/c_B$ for two different sterile neutrino masses $m_N=100\text{ GeV}$ and $m_N=1000\text{ GeV}$.