Constraints on millicharged particles with low threshold germanium detectors at Kuo-Sheng Reactor Neutrino Laboratory

TL;DR

This paper investigates constraints on millicharged particles produced in reactors and cosmic interactions, using low-threshold germanium detectors to derive new limits on their mass and charge, relevant for dark matter and beyond Standard Model physics.

Contribution

It provides the first experimental limits on millicharged particles using low-threshold germanium detectors at a reactor site, considering various production mechanisms.

Findings

Established new upper limits on millicharged particle charge and mass.

Identified potential signatures with enhanced differential cross sections.

Demonstrated the detector's sensitivity at 300 eV threshold for such particles.

Abstract

Relativistic millicharged particles ($\chi_q$) have been proposed in various extensions to the Standard Model of particle physics. We consider the scenarios where they are produced at nuclear reactor core and via interactions of cosmic-rays with the earth's atmosphere. Millicharged particles could also be candidates for dark matter, and become relativistic through acceleration by supernova explosion shock waves. The atomic ionization cross section of $\chi_q$ with matter are derived with the equivalent photon approximation. Smoking-gun signatures with significant enhancement in the differential cross section are identified. New limits on the mass and charge of $\chi_q$ are derived, using data taken with a point-contact germanium detector with 500g mass functioning at an energy threshold of 300~eV at the Kuo-Sheng Reactor Neutrino Laboratory.