In this paper, we do three things. First, we outline the conditions under which the interaction rate of inelastic processes that change the internal state of a system of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi></math> targets scales as <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>N</mi><mn>2</mn></msup></math>. This is an effect distinct from coherent elastic 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 superradiant interactions. We compute the superradiant interaction rates for the cosmic neutrino background (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathvariant="normal">C</mi><mi>ν</mi><mi mathvariant="normal">B</mi></mrow></math>), 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 <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathvariant="normal">C</mi><mi>ν</mi><mi mathvariant="normal">B</mi></mrow></math> interacts with a rate of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><mi>Hz</mi><mo stretchy="false">)</mo></math> when scattering off a 10 cm liquid or solid-state density spin-polarized sphere, a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>21</mn></mrow></msup><mo stretchy="false">)</mo></mrow></math> enhancement compared to the incoherent inelastic contribution. For QCD axion dark matter, similar rates can be achieved with much smaller samples, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>N</mi><mo>∼</mo><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>15</mn></mrow></msup><mo stretchy="false">)</mo><msup><mrow><mo stretchy="false">(</mo><mfrac><mrow><mi>m</mi></mrow><mrow><mn>2</mn><mo stretchy="false">×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo stretchy="false">−</mo><mn>8</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></mfrac><mo stretchy="false">)</mo></mrow><mrow><mo>−</mo><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msup></mrow></math>, where <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>m</mi></math> 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 sum 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.

Canada

Asimina Arvanitaki

United States

Savas Dimopoulos

Marios Galanis

Superradiant interactions of the cosmic neutrino background, axions, dark matter, and reactor neutrinos

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.

Davide Racco

Olivier Simon

Jedidiah Thompson

Dark QED from inflation

Black hole superradiance is a powerful probe of light, weakly coupled hidden sector particles. Many candidate particles, such as axions, generically have self-interactions that can influence the evolution of the superradiant instability. As pointed out in [A. Gruzinov, arXiv:1604.06422.] in the context of a toy model, much of the existing literature on spin-0 superradiance does not take into account the most important self-interaction-induced processes. These processes lead to energy exchange between quasi-bound levels and particle emission to infinity; for large self-couplings, superradiant growth is saturated at a quasi-equilibrium configuration of reduced level occupation numbers. In this paper, we perform a detailed analysis of the rich dynamics of spin-0 superradiance with self-interactions, and the resulting observational signatures. We focus on quartic self-interactions, which dominate the evolution for most models of interest. We explore multiple distinct regimes of parameter space introduced by a nonzero self-interaction, including the simultaneous population of two or more bound levels; at large coupling, we confirm the basic picture of quasiequilibrium saturation and provide evidence that the “bosenova” collapse does not occur in most of the astrophysical parameter space. Compared to gravitational superradiance, we find that gravitational wave “annihilation” signals and black hole spin-down are parametrically suppressed with increasing interactions, while new gravitational wave “transition” signals can take place for moderate interactions. The novel phenomenon of scalar wave emission is less suppressed at large couplings, and if the particle has Standard Model interactions, then coherent, monochromatic axion wave signals from black hole superradiance may be detectable in proposed axion dark matter experiments.

Masha Baryakhtar

Robert Lasenby

Black hole superradiance of self-interacting scalar fields