<p>The scattering and absorption rates of light dark matter with electron spin-dependent interactions depend on the target’s spin response. We show how this response is encoded by the target’s dynamical magnetic susceptibility, which can be measured using neutron scattering. We directly use existing neutron scattering data to compute the dark matter scattering rate in a candidate target material, finding close agreement with the previous first-principles calculation at MeV dark matter masses. Complementary experiments and measurements can extend the reach of this technique to other dark matter models and masses, and identify promising target materials for future experiments.</p>

United States

Asher Berlin

Alexander J. Millar

Tanner Trickle

Kevin Zhou

Determining spin-dependent light dark matter rates from neutron scattering

Millicharged particles are generic in theories of dark sectors. A cosmic or local abundance of them may be produced by the early universe, stellar environments, or the decay or annihilation of dark matter/dark energy. Furthermore, if such particles are light, these production channels result in a background of millicharged radiation. We show that light-shining-through-wall experiments employing superconducting RF cavities can also be used as “direct deflection” experiments to search for this relativistic background. The millicharged plasma is first subjected to an oscillating electromagnetic field of a driven cavity, which causes charge separation in the form of charge and current perturbations. In turn, these perturbations can propagate outwards and resonantly excite electromagnetic fields in a well-shielded cavity placed nearby, enabling detection. We estimate that future versions of the existing Dark SRF experiment can probe orders of magnitude of currently unexplored parameter space, including millicharges produced from the Sun, the cosmic neutrino background, or other mechanisms that generate a thermal abundance with energy density as small as ~ 10 that of the cosmic microwave background.

Surjeet Rajendran

Harikrishnan Ramani

Erwin Tanin

Direct deflection of Millicharged radiation

Millicharged dark matter particles can be efficiently captured by the Sun, where they annihilate into tau leptons, leading to the production of high-energy neutrinos. In contrast to the Earth, the high temperature of the Sun suppresses the fraction of millicharged particles that are bound to nuclei, allowing for potentially high annihilation rates. We recast existing constraints from the IceCube Neutrino Observatory and use this information to place new limits on the fraction of the dark matter that is millicharged. This analysis excludes previously unexplored parameter space for masses of <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>χ</mi></mrow></msub><mo>∼</mo><mo>(</mo><mn>5</mn><mi>–</mi><mn>100</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>, charges of <math><mrow><msub><mrow><mi>q</mi></mrow><mrow><mi>χ</mi></mrow></msub><mo>∼</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup><mi>–</mi><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></mrow></math>, and fractional abundances as small as <math><mrow><msub><mrow><mi>f</mi></mrow><mrow><mi>DM</mi></mrow></msub><mo>∼</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>5</mn></mrow></msup></mrow></math>.

Dan Hooper

High-energy neutrinos from millicharged dark matter annihilation in the Sun

Thermal relic dark matter below <math><mrow><mo>∼</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> is excluded by cosmic microwave background data if its annihilation to visible particles is unsuppressed near the epoch of recombination. Usual model-building measures to avoid this bound involve kinematically suppressing the annihilation rate in the low-velocity limit, thereby yielding dim prospects for indirect detection signatures at late times. In this work, we investigate a class of cosmologically viable sub-GeV thermal relics with late-time annihilation rates that are detectable with existing and proposed telescopes across a wide range of parameter space. We study a representative model of inelastic dark matter featuring a stable state <math><msub><mi>χ</mi><mn>1</mn></msub></math> and a slightly heavier excited state <math><msub><mi>χ</mi><mn>2</mn></msub></math> whose abundance is thermally depleted before recombination. Since the kinetic energy of dark matter in the Milky Way is much larger than it is during recombination, <math><msub><mi>χ</mi><mn>1</mn></msub><msub><mi>χ</mi><mn>1</mn></msub><mo>→</mo><msub><mi>χ</mi><mn>2</mn></msub><msub><mi>χ</mi><mn>2</mn></msub></math> upscattering can efficiently regenerate a cosmologically long-lived Galactic population of <math><msub><mi>χ</mi><mn>2</mn></msub></math>, whose subsequent coannihilations with <math><msub><mi>χ</mi><mn>1</mn></msub></math> give rise to observable gamma-rays in the <math><mo>∼</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi><mo>−</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math> energy range. We find that proposed MeV gamma-ray telescopes, such as e-ASTROGAM, AMEGO, and MAST, would be sensitive to much of the thermal relic parameter space in this class of models and thereby enable both discovery and model discrimination in the event of a signal at accelerator or direct detection experiments.

Gordan Krnjaic

Elena Pinetti

Reviving MeV-GeV indirect detection with inelastic dark matter

In the presence of axion dark matter, fermion spins experience an “axion wind” torque and an “axioelectric” force. We investigate new experimental probes of these effects and find that magnetized analogs of multilayer dielectric haloscopes can explore orders of magnitude of new parameter space for the axion-electron coupling. We also revisit the calculation of axion absorption into in-medium excitations, showing that axioelectric absorption is screened in spin-polarized targets, and axion wind absorption can be characterized in terms of a magnetic energy loss function. Finally, our detailed theoretical treatment allows us to critically examine recent claims in the literature. We find that axioelectric corrections to electronic energy levels are smaller than previously estimated and that the purported electron electric dipole moment due to a constant axion field is entirely spurious.

Alexander Millar

Physical signatures of fermion-coupled axion dark matter

Detection of axion dark matter heavier than an meV is hindered by its small wavelength, which limits the useful volume of traditional experiments. This problem can be avoided by directly detecting in-medium excitations, whose <math><mrow><mo>∼</mo><mi>meV</mi><mi>–</mi><mi>eV</mi></mrow></math> energies are decoupled from the detector size. We show that for any target inside a magnetic field, the absorption rate of electromagnetically coupled axions into in-medium excitations is determined by the dielectric function. As a result, the plethora of candidate targets previously identified for sub-GeV dark matter searches can be repurposed as broadband axion detectors. We find that a kg yr exposure with noise levels comparable to recent measurements is sufficient to probe parameter space currently unexplored by laboratory tests. Noise reduction by only a few orders of magnitude can enable sensitivity to the QCD axion in the <math><mrow><mo>∼</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>meV</mi><mi>–</mi><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math> mass range.

Absorption of Axion Dark Matter in a Magnetized Medium

The simplest cosmologies motivate the consideration of dark matter subcomponents that interact significantly with normal matter. Moreover, such strongly coupled relics may have evaded detection to date if upon encountering the Earth they rapidly thermalize down to terrestrial temperatures, <math><mrow><msub><mrow><mi>T</mi></mrow><mrow><mo>⊕</mo></mrow></msub><mo>∼</mo><mn>300</mn><mtext> </mtext><mtext> </mtext><mi>K</mi><mo>∼</mo><mspace></mspace><mn>25</mn><mtext> </mtext><mtext> </mtext><mi>meV</mi></mrow></math>, well below the thresholds of most existing dark matter detectors. This shedding of kinetic energy implies a drastic enhancement to the local density, motivating the consideration of alternative detection techniques sensitive to a large density of slowly moving dark matter particles. In this work, we provide a rigorous semianalytic derivation of the terrestrial overdensities of strongly coupled relics, with a particular focus on millicharged particles (MCPs). We go beyond previous studies by incorporating improved estimates of the MCP-atomic scattering cross section, new contributions to the terrestrial density of sub-GeV relics that are independent of the Earth’s gravitational field, and local modifications that can arise due to the cryogenic environments of precision sensors. We also generalize our analysis in order to estimate the terrestrial density of thermalized MCPs that are produced from the collisions of high-energy cosmic rays and become bound by the Earth’s electric field.

Hangwan Liu

Maxim Pospelov

Terrestrial density of strongly-coupled relics

Dark photons can oscillate into Standard Model (SM) photons via kinetic mixing. The conversion probability depends sensitively on properties of the ambient background, such as the density and electromagnetic field strength, which cause the SM photon to acquire an in-medium effective mass. Resonances can enhance the conversion probability when there is a level-crossing between the dark photon and background-dependent SM photon states. In this work, we show that the widely used Landau-Zener (LZ) approximation breaks down when there are multiple level-crossings due to a nonmonotonic SM photon potential. Phase interference effects, especially when the dark photon mass is close to an extremum of the SM photon effective mass, can cause deviations from the LZ approximation at the level of a few orders of magnitude in the conversion probability. We present an analytic approximation that is valid in this regime and that can accurately predict the conversion probabilities in a wide range of astrophysical environments.

Canada

Nirmalya Brahma

Katelin Schutz

Photon-dark photon conversion with multiple level crossings

Superconducting cavities can operate analogously to Weber bar detectors of gravitational waves, converting mechanical to electromagnetic energy. The significantly reduced electromagnetic noise results in increased sensitivity to high-frequency signals well outside the bandwidth of the lowest mechanical resonance. In this work, we revisit such signals of gravitational waves and demonstrate that a setup similar to the existing MAGO prototype, operating in a scanning or broadband manner, could have sensitivity to strains of <math><mrow><mo>∼</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>22</mn></mrow></msup><mo>−</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>18</mn></mrow></msup></mrow></math> for frequencies of <math><mrow><mo>∼</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>kHz</mi><mi>–</mi><mn>1</mn><mtext> </mtext><mi>GHz</mi></mrow></math>.

Spain

Diego Blas

France

Raffaele Tito D’Agnolo

Switzerland

Sebastian A. R. Ellis

Roni Harnik

Yonatan Kahn

Jan Schütte-Engel

Michael Wentzel

Electromagnetic cavities as mechanical bars for gravitational waves

In the presence of QCD axion dark matter, atoms acquire time-dependent electric dipole moments. This effect gives rise to an oscillating current in a nuclear spin-polarized dielectric, which can resonantly excite an electromagnetic mode of a microwave cavity. We show that with existing technology such a “polarization haloscope” can explore orders of magnitude of new parameter space for QCD-coupled axions. If any cavity haloscope detects a signal from the axion-photon coupling, an upgraded polarization haloscope has the unique ability to test whether it arises from the QCD axion.