Probing active-sterile neutrino transition magnetic moments at LEP and CEPC

TL;DR

This paper investigates how sterile neutrinos with transition magnetic moments interact with photons and how experiments at LEP and future colliders like CEPC can constrain these interactions, providing new limits on the dipole portal couplings.

Contribution

It introduces a comprehensive analysis of constraints on sterile neutrino dipole couplings from LEP data and explores the potential of CEPC to probe previously unconstrained parameter space.

Findings

Z decay measurements at LEP set strong constraints on the dipole couplings.

Future CEPC can explore new parameter space beyond current experimental limits.

Different interaction scenarios with gauge fields affect the sensitivity of collider searches.

Abstract

We consider the sterile neutrino, that is also know as heavy neutral lepton (HNL), interacting with the Standard Model (SM) active neutrino and photon via a transition magnetic moment, the so-called dipole portal, which can be induced from the more general dipole couplings which respect the full gauge symmetries of the SM. Depending on the interactions with ${\rm SU}(2)_L$ and ${\rm U(1)}_Y$ field strength tensors ${\cal W}_{\mu \nu}^a$ and $B^{\mu \nu}$, we consider four typical scenarios and probe the constraints on the couplings with photon $d_\gamma$ at LEP using the analyses to search monophoton signature and the measurement of $Z$ decay. We find that in the considered scenarios assuming the coupling with $Z$ boson $d_Z\neq 0$, the measurement of $Z$ decaying into photon plus invisible particles can provide stricter constraints than the monophoton searches at the LEP1. The complementary constraints to existing experiments can be provided by the LEP. We also investigate the sensitivity on the dipole portal coupling $d_\gamma$ from the monophoton searches at future electron colliders, such as CEPC, and find that CEPC can explore the previously unconstrained parameter space by current experiments.