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Neutrinos with energy of order 10 MeV, such as from pion decay-at-rest sources, are an invaluable tool for studying low-energy neutrino interactions with nuclei—previously enabling the first measurement of coherent elastic neutrino-nucleus scattering. Beyond elastic scattering, neutrinos and dark matter in this energy range also excite nuclei to its low-lying nuclear states, providing an additional physics channel. Here, we consider neutral-current inelastic neutrino-nucleus and dark matter (DM)-nucleus scattering off $^{40}$Ar, $^{133}$Cs, and $^{127}$I nuclei that are relevant to a number of low-threshold neutrino experiments at pion decay-at-rest facilities. We carry out large scale nuclear shell model calculations of the inelastic cross sections considering the full set of electroweak multipole operators. Our results demonstrate that GamowTeller transitions provide the dominant contribution to the cross section and that the long-wavelength limit provides a reasonable approximation to the total cross section for neutrino sources. We show that future experiments will be sensitive to this channel, and thus these results provide additional neutrino and DM scattering channels to explore at pion decay-at-rest facilities

Here we present a novel probe of light (≲ O(100) MeV) dark matter (DM) using pion decayat-rest experiments. Dark sector particles produced during pion decay can be detected when they scatter in a distant detector. The decay of nuclei excited by the inelastic scattering of DM is an unexploited channel which has significantly lower background compared to similar searches using the elastic scattering channel. Using this channel, we demonstrate an increased sensitivity to a dark photon portal DM model compared to the existing constraints. The sensitivity of the DM parameter space is not restricted by the detector threshold as in the elastic channel. With existing experiments world-leading constraints on this parameter space have been obtained, reaching the thermal relic benchmark for scalar DM. Future experiments will be able to reach the thermal relic benchmark for fermionic DM.

Searches for axionlike particles (ALPs) are motivated by the strong CP problem in particle physics and by unexplained dark matter in astrophysics. In this paper, we discuss novel ALP searches using monoenergetic nuclear deexcitation photons from a beam dump, using the isotope decay-at-rest (IsoDAR) experiment as an example. We show that IsoDAR can set limits that close a gap in traditional QCD axion searches using the ALP-photon coupling, as well as provide sensitivity to large regions of new parameter space in models where ALPs couple to nucleons and electrons. We also show how isotope decay-at-rest experiments may be designed to improve potential ALP production and optimize detection sensitivity.

Stopped-pion experiments that measure coherent elastic neutrino-nucleus scattering (CE$\nu$NS) are sensitive to sterile neutrinos via disappearance. Using timing and energy spectra to perform flavor decomposition, we show that the delayed electron neutrino component provides an independent test of short-baseline anomalies that hint at ∼ eV-mass sterile neutrinos. Dedicated experiments will be sensitive to nearly the entire sterile neutrino parameter space consistent with short-baseline data.

We propose a new approach to search for light dark matter (DM) in the range of keV-GeV via inelastic nucleus scattering at large-volume neutrino detectors such as Borexino, DUNE, JUNO, and Hyper-K. The approach uses inelastic nuclear scattering of cosmic-ray boosted DM, enabling a low-background search for DM in these experiments. The large neutrino detectors with higher threshold can be used since the nuclear deexcitation lines are $\mathcal{O}$(10) MeV. Using a hadrophilic dark-gauge-boson-portal model as a benchmark, we show that the nuclear inelastic channels generally provide better sensitivity than the elastic scattering for a large region of light DM parameter space.

We consider the nuclear absorption of dark matter as an alternative to the typical indirect detection search channels of dark matter decay or annihilation. In this scenario, an atomic nucleus transitions to an excited state by absorbing a pseudoscalar dark matter particle and promptly emits a photon as it transitions back to its ground state. The nuclear excitation of carbon and oxygen in the Galactic Center would produce a discrete photon spectrum in the $\mathcal{O}(10)$ MeV range that could be detected by gamma-ray telescopes. Using the $\texttt{BIGSTICK}$ large-scale shell-model code, we calculate the excitation energies of carbon and oxygen. We constrain the dark matter-nucleus coupling for current COMPTEL data, and provide projections for future experiments AMEGO-X, e-ASTROGAM, and GRAMS for dark matter masses from $\sim$ 10 to 30 MeV. We find the excitation process to be very sensitive to the dark matter mass and find that the future experiments considered would improve constraints on the dark matter-nucleus coupling within an order of magnitude.