Strongly interacting massive particles <math><mi>π</mi></math> have been advocated as prominent dark matter candidates when they regulate their relic abundance through odd-numbered <math><mn>3</mn><mi>π</mi><mo>→</mo><mn>2</mn><mi>π</mi></math> annihilation. We show that successful freeze-out may also be achieved through even-numbered interactions <math><mi>X</mi><mi>X</mi><mo>→</mo><mi>π</mi><mi>π</mi></math> once bound states <math><mi>X</mi></math> among the particles of the low-energy spectrum exist. In addition, <math><mi>X</mi></math>-formation hosts the potential of also catalyzing odd-numbered <math><mn>3</mn><mi>π</mi><mo>→</mo><mn>2</mn><mi>π</mi></math> annihilation processes, turning them into effective two-body processes <math><mi>π</mi><mi>X</mi><mo>→</mo><mi>π</mi><mi>π</mi></math>. Bound states are often a natural consequence of strongly interacting theories. We calculate the dark matter freeze-out and comment on the cosmic viability and possible extensions. Candidate theories can encompass confining sectors without a mass gap, glueball dark matter, or <math><msup><mi>ϕ</mi><mn>3</mn></msup></math> and <math><msup><mi>ϕ</mi><mn>4</mn></msup></math> theories with strong Yukawa or self-interactions.

China

Austria

Xiaoyong Chu

Marco Nikolic

Josef Pradler

Successful Freeze-Out of Strongly Interacting Dark Matter with Even-Numbered Interactions

Thermal freeze-out offers an attractive explanation of the dark matter density free from fine-tuning of initial conditions. For dark matter with a mass below tens of MeV, photons, electrons, and neutrinos are the only available direct Standard Model annihilation products. Using a full three-sector abundance calculation, we determine the minimal mass of dark matter, allowing for an arbitrary branching into electrons/photons and neutrinos that is compatible with current cosmological observations. The analysis takes into account the heat transfer between the various sectors from annihilation and elastic scattering, representing the first fully self-consistent analysis that tracks the respective sectors’ temperatures. We thereby provide accurate thermal annihilation cross sections, particularly for velocity-dependent cases, and deduce the sensitivity of current and upcoming CMB experiments to MeV thermal dark matter. In the latter context, we also establish the fine-tuned parameter region where a tiny admixture of neutrinos in the final states rules in MeV-scale <math><mi>p</mi></math>-wave annihilating DM into electrons. Finally, we show that a sub-% millicharged dark matter with an interaction strength that interferes with 21-cm cosmology is still allowed when freeze-out is supplemented with annihilation into neutrinos. For all cases considered, we provide concrete particle physics models and supplement our findings with a discussion of other relevant experimental results.

Minimal mass of thermal dark matter and the viability of millicharged particles affecting 21-cm cosmology

Axion-photon conversion is a prime mechanism to detect axion-like particles that share a coupling to the photon. We point out that in the vicinity of neutron stars with strong magnetic fields, magnetars, the effective photon mass receives comparable but opposite contributions from free electrons and the radiation field. This leads to an energy-dependent resonance condition for conversion that can be met for arbitrary light axions and leveraged when using systems with detected radio component. Using the magnetar SGR J1745-2900 as an exemplary source, we demonstrate that sensitivity to <math><mo>|</mo><msub><mrow><mi>g</mi></mrow><mrow><mi>a</mi><mi>γ</mi></mrow></msub><mo>|</mo><mo>∼</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>12</mn></mrow></msup><mspace width="0.2em"></mspace><mrow><mi>Ge</mi><msup><mrow><mi>V</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> or better can be gained for <math><msub><mrow><mi>m</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>≲</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>6</mn></mrow></msup><mspace width="0.2em"></mspace><mrow><mi>eV</mi></mrow></math>, with the potential to improve current constraints on the axion-photon coupling by more than one order of magnitude over a broad mass range. With growing insights into the physical conditions of magnetospheres of magnetars, the method hosts the potential to become a serious competitor to future experiments such as ALPS-II and IAXO in the search for axion-like particles.

Italy

Kyrylo Bondarenko

Netherlands

Alexey Boyarsky

CERN

United States

Anastasia Sokolenko

Neutron stars as photon double-lenses: Constraining resonant conversion into ALPs

We consider electrically neutral complex-vector particles <math><mi>V</mi></math> below the GeV mass scale that, from a low-energy perspective, couple to the photon via higher-dimensional form factor interactions. We derive ensuing astrophysical constraints by considering the anomalous energy loss from the Sun, Horizontal Branch, and Red Giant stars as well as from SN1987A that arise from vector pair production in these environments. Under the assumption that the dark states <math><mi>V</mi></math> constitute dark matter, the bounds are then complemented by direct and indirect detection as well as cosmological limits. The relic density from freeze-out and freeze-in mechanisms is also computed. On the basis of a UV-complete model that realizes the considered effective couplings, we also discuss the naturalness of the constrained parameter space, and provide an analysis of the zero mass limit of <math><mi>V</mi></math>.

Japan

Junji Hisano

Germany

Alejandro Ibarra

Jui-Lin Kuo

Multipole vector dark matter below the GeV scale

While most searches for cosmic axions so far focused on their cold relics as (a component of) dark matter, various well-motivated cosmological sources can produce “boosted” axions that remain relativistic today. We demonstrate that existing/upcoming neutrino experiments such as Super-Kamiokande, Hyper-Kamiokande, DUNE, JUNO, and IceCube can probe such energetic axion relics. The characteristic signature is the monoenergetic single photon signal from axion absorption induced by the axion-photon coupling. This proposal offers to cover parameter ranges complementary to existing axion searches and provides new opportunities for discovery with neutrino facilities.

Yanou Cui

Yu-Dai Tsai

Shining light on cosmogenic axions with neutrino experiments

Thermal dark matter at the MeV mass scale has its abundance set during the highly nontrivial epochs of neutrino decoupling and electron annihilation. The technical obstacles attached to solving Boltzmann equations of multiple interacting sectors being both relativistic and nonrelativistic have to-date prevented the full treatment of this problem. Here, for the first time, we calculate the freeze-out of light dark matter, taking into account the energy transfer between the dark sector, neutrinos, and the electromagnetically interacting plasma from annihilation and elastic scattering processes alike. We develop a numerically feasible treatment that allows to track photon and neutrino temperatures across freeze-out and to arrive at a precision prediction of <math><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>eff</mi></mrow></msub></mrow></math> for arbitrary branching ratios of the dark matter annihilation channels. In addition, our treatment resolves for the first time the dark matter temperature evolution across freeze-out involving three sectors. It enters in the efficiency of velocity-dependent annihilation channels and for a flavor-blind <math><mi>p</mi></math>-wave annihilation into electron- and neutrino-pairs of all generations, we find the present Planck data exclude a complex scalar dark matter particle of mass of <math><msub><mi>m</mi><mi>ϕ</mi></msub><mo>≤</mo><mn>7</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>.

Toward a full description of MeV dark matter decoupling: A self-consistent determination of relic abundance and <math><msub><mi>N</mi><mi>eff</mi></msub></math>

In this work we analyse the ultimate sensitivity of dark matter direct detection experiments to dark radiation in form of SM or semi-sterile neutrinos. This flux-component is assumed to be produced from dark matter decay. Since dark radiation may mimic dark matter signals, we perform our analysis based on likelihood statistics that allows to test the distinguishability between signals and backgrounds. Given the previous bounds from neutrino experiments, we find that xenon-based dark matter searches will not be able to probe new regions of the dark matter progenitor mass and lifetime parameter space when the decay products are SM neutrinos. In turn, if the decay instead happens to a fourth neutrino species with enhanced interactions to baryons, DR can either constitute the dominant background or a discoverable signal in direct detection experiments. In the former case, this lifts the “neutrino floor” for xenon-based experiments.

Suchita Kulkarni

Sensitivity of direct detection experiments to neutrino dark radiation from dark matter decay and a modified neutrino-floor

We propose a new mechanism where a multi-component dark sector generates the observed dark matter abundance and baryon asymmetry and thus addresses the coincidence between the two. The thermal freeze-out of dark matter annihilating into meta-stable dark partners sets the dark matter relic abundance while providing the out-of-equilibrium condition for baryogenesis. The meta-stable state triggers baryon asymmetry production by its decay well after the freeze-out and potentially induces a period of early matter domination before its decay. The dark matter and baryon abundances are related through number conservation within the dark sector (cogenesis). The “coincidence” is a natural outcome with GeV- to TeV-scale symmetric dark matter and the dark sector’s interactions with the Standard Model quarks. We present a UV-complete model and explore its phenomenological predictions, including dark matter direct detection signals, LHC signatures of new massive particles with color charges and long-lived particles with displaced vertices, dark matter-induced nucleon conversions, (exotic) dark matter indirect detection signals, and effects on the cosmological matter power spectrum. As a side result, we provide a novel analytical treatment for dark sector freeze-out, which may prove useful in the study of related scenarios.

Canada

Michael Shamma

Dark freeze-out cogenesis

The scattering of light dark matter off thermal electrons inside the Sun produces a “fast” subcomponent of the dark matter flux that may be detectable in underground experiments. We update and extend previous work by analyzing the signatures of dark matter candidates which scatter via light mediators. Using numerical simulations of the dark matter-electron interaction in the solar interior, we determine the energy spectrum of the reflected flux, and calculate the expected rates for direct detection experiments. We find that large Xenon-based experiments (such as XENON1T) provide the strongest direct limits for dark matter masses below a few MeV, reaching a sensitivity to the effective dark matter charge of <math><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>9</mn></mrow></msup><mi>e</mi></math>.

Haipeng An

Haoming Nie

Maxim Pospelov

Adam Ritz

Solar reflection of dark matter

We study the possibility that dark radiation, sourced through the decay of dark matter in the late Universe, carries electromagnetic interactions. The relativistic flux of particles induces recoil signals in direct detection and neutrino experiments through its interaction with millicharge, electric/magnetic dipole moments, or anapole moment/charge radius. Taking the DM lifetime as 35 times the age of the Universe, as currently cosmologically allowed, we show that direct detection (neutrino) experiments have complementary sensitivity down to <math><mi>ε</mi><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>11</mn></mrow></msup></math> <math><mo>(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>12</mn></mrow></msup><mo>)</mo></math>, <math><msub><mi>d</mi><mi>χ</mi></msub><mo>/</mo><msub><mi>μ</mi><mi>χ</mi></msub><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>9</mn></mrow></msup><msub><mi>μ</mi><mi>B</mi></msub></math> <math><mo>(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>13</mn></mrow></msup><msub><mi>μ</mi><mi>B</mi></msub><mo>)</mo></math>, and <math><msub><mi>a</mi><mi>χ</mi></msub><mo>/</mo><msub><mi>b</mi><mi>χ</mi></msub><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>2</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></math> <math><mo>(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>8</mn></mrow></msup><mtext> </mtext><mtext> </mtext><msup><mrow><mi>GeV</mi></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup><mo>)</mo></math> on the respective couplings. Finally, we show that such dark radiation can lead to a satisfactory explanation of the recently observed XENON1T excess in the electron recoil signal without being in conflict with other bounds.