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In this paper, we do three things. First, we outline the conditions under which the interaction rate of processes that change the internal state of a system of $N$targets scales as $N^2$. This is an effect distinct from coherent scattering but with the same scaling. Second, we compute example rates for such processes for various weakly interacting particles. Finally, we point to potential quantum observables for these processes that go beyond traditional energy exchange. Maximal coherence in inelastic processes is achieved when the targets are placed in an equal superposition of the ground and excited states. These coherent inelastic processes are analogous to Dicke superradiance, where cooperative effects reinforce the emission of radiation from matter, and we thus refer to them as interactions. We compute the superradiant interaction rates for the cosmic neutrino background ( $\mathrm{C}\nu \mathrm{B}$), dark matter scattering and absorption, and late-universe particles, such as reactor neutrinos, when the two-level system is realized by nuclear or electron spins in a magnetic field. The rates we find can be quite sizable on macroscopic yet small targets. For example, the $\mathrm{C}\nu \mathrm{B}$interacts with a rate of $\mathcal{O}\left(\mathrm{Hz}\right)$when scattering off a 10 cm liquid or solid-state density spin-polarized sphere, a $\mathcal{O}\left({10}^{21}\right)$enhancement compared to the incoherent inelastic contribution. For QCD axion dark matter, similar rates can be achieved with much smaller samples, $N\sim \mathcal{O}\left({10}^{15}\right){\left(\frac{m}{2\times {10}^{-8}\mathrm{eV}}\right)}^{-1/2}$, where $m$is the axion mass. Using the Lindblad formalism for open quantum systems, we show that these superradiant interactions can manifest as a source of noise on the system. This noise is tunable however and can serve as a signature of new physics, as the energy splitting controls the momentum transfer and hence, the amount of macroscopic coherence. These considerations point to new observables that go beyond traditional net energy exchange. These observables are sensitive to the of the excitation and deexcitation rates—instead of the net energy exchange rate which can be very suppressed—and can be viewed as introducing diffusion and decoherence to the system. While we postpone to upcoming work proposing a concrete protocol that extracts these effects from a macroscopic ensemble of atoms, the effects presented in this paper may point to a new class of ultra-low threshold detectors. Published by the American Physical Society2025 

Axion dark matter (DM) constitutes an oscillating background that violates parity and time-reversal symmtries. Inside piezoelectric crystals, where parity is broken spontaneously, this axion background can result in a stress. We call this new phenomenon “the piezoaxionic effect.” When the frequency of axion DM matches the natural frequency of a bulk acoustic normal mode of the piezoelectric crystal, the piezoaxionic effect is resonantly enhanced and can be read out electrically via the piezoelectric effect. We explore all axion couplings that can give rise to the piezoaxionic effect—the most promising one is the defining coupling of the QCD axion, through the anomaly of the strong sector. We also point our another, subdominant phenomenon present in all dielectrics, namely the “electroaxionic effect.” An axion background can produce an electric displacement field in a crystal which in turn will give rise to a voltage across the crystal. The electroaxionic effect is again largest for the axion coupling to gluons. We find that this model-independent coupling of the QCD axion may be probed through the combination of the piezoaxionic and electroaxionic effects in piezoelectric crystals with aligned nuclear spins, with near-future experimental setups applicable for axion masses between 10^−11  eV and 10^−7  eV, a challenging range for most other detection concepts. 

A<sc>bstract</sc> One contribution to any dark sector’s abundance comes from its gravitational production during inflation. If the dark sector is weakly coupled to the inflaton and the Standard Model, this can be its only production mechanism. For non-interacting dark sectors, such as a free massive fermion or a free massive vector field, this mechanism has been studied extensively. In this paper we show, via the example of dark massive QED, that the presence of interactions can result in a vastly different mass for the dark matter (DM) particle, which may well coincide with the range probed by upcoming experiments. In the context of dark QED we study the evolution of the energy density in the dark sector after inflation. Inflation produces a cold vector condensate consisting of an enormous number of bosons, which via interesting processes — Schwinger pair production, strong field electromagnetic cascades, and plasma dynamics — transfers its energy to a small number of “dark electrons” and triggers thermalization of the dark sector. The resulting dark electron DM mass range is from 50 MeV to 30 TeV, far different from both the 10⁻⁵eV mass of the massive photon dark matter in the absence of dark electrons, and from the 10⁹GeV dark electron mass in the absence of dark photons. This can significantly impact the search strategies for dark QED and, more generally, theories with a self-interacting DM sector. In the presence of kinetic mixing, a dark electron in this mass range can be searched for with upcoming direct detection experiments, such as SENSEI-100g and OSCURA.