SENSEI: Search for Millicharged Particles produced in the NuMI Beam

TL;DR

This paper reports a search for millicharged particles produced in proton beam collisions, setting new constraints on their properties and demonstrating the potential of low-threshold detectors in beam-dump experiments.

Contribution

It introduces a novel search for millicharged particles using low-threshold detectors at the NuMI beam, establishing world-leading constraints and highlighting future experimental prospects.

Findings

No ionization events detected, constraining millicharged particles.

Set new limits on particle masses between 30 MeV and 380 MeV.

Demonstrated the effectiveness of low-threshold detectors in beam experiments.

Abstract

Millicharged particles appear in several extensions of the Standard Model, but have not yet been detected. These hypothetical particles could be produced by an intense proton beam striking a fixed target. We use data collected in 2020 by the SENSEI experiment in the MINOS cavern at the Fermi National Accelerator Laboratory to search for ultra-relativistic millicharged particles produced in collisions of protons in the NuMI beam with a fixed graphite target. The absence of any ionization events with 3 to 6 electrons in the SENSEI data allow us to place world-leading constraints on millicharged particles for masses between 30 MeV to 380 MeV. This work also demonstrates the potential of utilizing low-threshold detectors to investigate new particles in beam-dump experiments, and motivates a future experiment designed specifically for this purpose.