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 $\phi$ 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.Comment: 5 pages, 2 figure

PeV-Scale Dark Matter as a Thermal Relic of a Decoupled Sector

In this letter, we consider a class of scenarios in which the dark matter is part of a heavy hidden sector that is thermally decoupled from the Standard Model in the early universe. The dark matter freezes-out by annihilating to a lighter, metastable state, whose subsequent abundance can naturally come to dominate the energy density of the universe. When this state decays, it reheats the visible sector and dilutes all relic abundances, thereby allowing the dark matter to be orders of magnitude heavier than the weak scale. For concreteness, we consider a simple realization with a Dirac fermion dark matter candidate coupled to a massive gauge boson that decays to the Standard Model through its kinetic mixing with hypercharge. We identify viable parameter space in which the dark matter can be as heavy as ~1-100 PeV without being overproduced in the early universe.Comment: 4 pages + appendices, 2 figure