Muon Capture Constraints on Sterile Neutrino Properties

TL;DR

This paper investigates how muon capture experiments constrain the properties of sterile neutrinos, especially those with large magnetic moments and specific decay modes, ruling out certain models proposed to explain anomalies in neutrino experiments.

Contribution

It provides new experimental constraints on sterile neutrino models with large magnetic moments based on muon capture data, and discusses potential ways to evade these constraints.

Findings

Radiative muon capture measurements exclude certain sterile neutrino models.

Constraints are complementary to those from neutrino scattering experiments.

Extensions of the model could potentially bypass current constraints.

Abstract

We show that ordinary and radiative muon capture impose stringent constraints on sterile neutrino properties. In particular, we consider a sterile neutrino with a mass between 40 to $80 {\rm MeV}$ that has a large mixing with the muon neutrino and decays predominantly into a photon and light neutrinos due to a large transition magnetic moment. Such a model was suggested as a possible resolution to the puzzle presented by the results of the LSND, KARMEN, and MiniBooNE experiments. We find that the scenario with the radiative decay to massless neutrinos is ruled out by measurements of the radiative muon capture rates at TRIUMF in the relevant mass range by a factor of a few in the squared mixing angle. These constraints are complementary to those imposed by the process of electromagnetic upscattering and de-excitation of beam neutrinos inside the neutrino detectors induced by a large transition magnetic moment. The latter provide stringent constraints on the size of the transitional magnetic moment between muon, electron neutrinos and $N$. We also show that further extension of the model with another massive neutrino in the final state of the radiative decay may be used to bypass the constraints derived in this work.