Probing New Physics with Underground Accelerators and Radioactive Sources

TL;DR

This paper proposes using underground accelerators and radioactive sources to produce and detect weakly coupled light particles, exploring new parameter space relevant to the proton charge radius puzzle in neutrino detectors.

Contribution

It introduces a novel experimental approach to produce and detect weakly coupled scalars using underground facilities and radioactive decay, expanding the search for new physics.

Findings

Can probe scalar masses between 250 keV and 1 MeV

Sensitive to couplings between 10^{-11} and 10^{-7}

Potential to address the proton charge radius puzzle

Abstract

New light, weakly coupled particles can be efficiently produced at existing and future high-intensity accelerators and radioactive sources in deep underground laboratories. Once produced, these particles can scatter or decay in large neutrino detectors (e.g Super-K and Borexino) housed in the same facilities. We discuss the production of weakly coupled scalars $\phi$ via nuclear de-excitation of an excited element into the ground state in two viable concrete reactions: the decay of the $0^+$ excited state of $^{16}$O populated via a $(p,\alpha)$ reaction on fluorine and from radioactive $^{144}$Ce decay where the scalar is produced in the de-excitation of $^{144}$Nd$^*$, which occurs along the decay chain. Subsequent scattering on electrons, $e(\phi,\gamma)e$, yields a mono-energetic signal that is observable in neutrino detectors. We show that this proposed experimental set-up can cover new territory for masses $250\, {\rm keV}\leq m_\phi \leq 2 m_e$ and couplings to protons and electrons, $10^{-11} < g_e g_p < 10^{-7}$. This parameter space is motivated by explanations of the "proton charge radius puzzle", thus this strategy adds a viable new physics component to the neutrino and nuclear astrophysics programs at underground facilities.