Probing new physics with underground accelerators and radioactive sources

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 ϕ 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 ¹⁶O populated via a (p, α) reaction on fluorine and from radioactive ¹⁴⁴Ce decay where the scalar is produced in the de-excitation of ¹⁴⁴Nd*, which occurs along the decay chain. Subsequent scattering on electrons, e(ϕ, γ)e, yields a mono-energetic signal that is observable in neutrino detectors. We show that this proposed experimental setup can cover new territory for masses 250 keV≤m_ϕ≤2m_e and couplings to protons and electrons, 10^-11≤g_eg_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.