Constraining neutrino electromagnetic properties by germanium detectors

TL;DR

This paper develops a detailed atomic physics method to analyze germanium detector responses at sub-keV energies, setting new limits on neutrino electromagnetic properties and guiding future experiments.

Contribution

It introduces a robust ab initio multiconfiguration relativistic random phase approximation method for understanding germanium detector responses at very low energies.

Findings

Limits on neutrino millicharge derived from existing data.

Constraints on neutrino magnetic moment and charge radius squared.

Projected sensitivities for future low-threshold experiments.

Abstract

The electromagnetic properties of neutrinos, which are either trivial or negligible in the context of the Standard Model, can probe new physics and have significant implications in astrophysics and cosmology. The current best direct limits on the neutrino millicharges and magnetic moments are both derived from data taken with germanium detectors with low thresholds at keV levels. In this paper, we discuss in detail a robust, ab initio method: the multiconfiguration relativistic random phase approximation, that enables us to reliably understand the germanium detector response at the sub-keV level, where atomic many-body physics matters. Using existing data with sub-keV thresholds, limits on reactor antineutrino's millicharge, magnetic moment, and charge radius squared are derived. The projected sensitivities for next generation experiments are also given and discussed.