Dark Force Detection in Low Energy e-p Collisions

TL;DR

This paper investigates a light dark-force boson $X$ with $m_X<100\,\mathrm{MeV}$ and $\alpha_X\sim10^{-8}$ by proposing a high-luminosity, low-energy $e^-p$ collider experiment at the JLab FEL, where $X\to e^+e^-$ can be reconstructed in $e^-p\to e^-p\,X\to e^-p\,e^+e^-$. It analyzes four coupling structures (scalar, pseudoscalar, vector, axial-vector), derives indirect constraints from lepton $g-2$ and direct beam-dump limits, and develops a detailed signal–background framework for the $e^-p\to e^-p\,e^+e^-$ channel, including a matrix element method that can enhance sensitivity by about a factor of three. The study finds that with about $1~\text{ab}^{-1}$ of data and ~1 MeV $e^+e^-$ invariant-mass resolution, one can discover or constrain significant regions of the $m_X$–$\alpha_X$ plane accessible to such a setup, while displaced-vertex searches could extend reach to longer lifetimes. If realized, this approach would provide a complementary probe to beam-dump experiments, leveraging full event reconstruction and high luminosity to explore a compelling dark-force parameter space in the MeV–GeV range and potentially reveal the nature of dark-sector interactions.

Abstract

We study the prospects for detecting a light boson X with mass m_X < 100 MeV at a low energy electron-proton collider. We focus on the case where X dominantly decays to e+ e- as motivated by recent "dark force" models. In order to evade direct and indirect constraints, X must have small couplings to the standard model (alpha_X < 10^-8) and a sufficiently large mass (m_X >10 MeV). By comparing the signal and background cross sections for the e- p e+ e- final state, we conclude that dark force detection requires an integrated luminosity of around 1 inverse attobarn, achievable with a forthcoming JLab proposal.