A Direct Detection Search for Hidden Sector New Particles in the 3-60 MeV Mass Range

TL;DR

This paper proposes a cost-effective experiment using the PRad setup to search for hidden sector particles and the X17 particle in the 3-60 MeV mass range, aiming to explore unexplored parameter space and address dark matter puzzles.

Contribution

It introduces a novel direct detection method with the PRad apparatus to search for hidden sector particles in the 3-60 MeV range, improving sensitivity and background control.

Findings

Sensitivity of 8.9×10⁻⁸ to 5.8×10⁻⁹ in ε²

Single-setting coverage of the 3-60 MeV mass range

Utilizes a well-established, cost-effective experimental setup

Abstract

In our quest to understand the nature of dark matter and discover its non-gravitational interactions with ordinary matter, we propose an experiment using a \pbo ~calorimeter to search for or set new limits on the production rate of i) hidden sector particles in the $3 - 60$ MeV mass range via their $e^+e^-$ decay (or $\gamma\gamma$ decay with limited tracking), and ii) the hypothetical X17 particle, claimed in multiple recent experiments. The search for these particles is motivated by new hidden sector models and dark matter candidates introduced to account for a variety of experimental and observational puzzles: the small-scale structure puzzle in cosmological simulations, anomalies such as the 4.2$\sigma$ disagreement between experiments and the standard model prediction for the muon anomalous magnetic moment, and the excess of $e^+e^-$ pairs from the $^8$Be M1 and $^4$He nuclear transitions to their ground states observed by the ATOMKI group. In these models, the $1 - 100$ MeV mass range is particularly well-motivated and the lower part of this range still remains unexplored. Our proposed direct detection experiment will use a magnetic-spectrometer-free setup (the PRad apparatus) to detect all three final state particles in the visible decay of a hidden sector particle allowing for an effective control of the background and will cover the proposed mass range in a single setting. The use of the well-demonstrated PRad setup allows for an essentially ready-to-run and uniquely cost-effective search for hidden sector particles in the $3 - 60$ MeV mass range with a sensitivity of 8.9$\times$10$^{-8}$ - 5.8$\times$10$^{-9}$ to $\epsilon^2$, the square of the kinetic mixing interaction constant between hidden and visible sectors. This updated proposal includes our response to the PAC49 comments.