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

The Sun may capture asymmetric dark matter (DM), which can subsequently form bound-states through the radiative emission of a sub-GeV scalar. This process enables generation of scalars without requiring DM annihilation. In addition to DM capture on nucleons, the DM-scalar coupling responsible for bound-state formation also induces capture from self-scatterings of ambient DM particles with DM particles already captured, as well as with DM bound-states formed in-situ within the Sun. This scenario is studied in detail by solving Boltzmann equations numerically and analytically. In particular, we take into consideration that the DM self-capture rates require a treatment beyond the conventional Born approximation. We show that, thanks to DM scatterings on bound-states, the number of DM particles captured increases exponentially, leading to enhanced emission of relativistic scalars through bound-state formation, whose final decay products could be observable. We explore phenomenological signatures with the example that the scalar mediator decays to neutrinos. We find that the neutrino flux emitted can be comparable to atmospheric neutrino fluxes within the range of energies below one hundred MeV. Future facilities like Hyper-K, and direct DM detection experiments can further test such scenario.

Italy

Raghuveer Garani

Spain

Camilo García-Cely

CERN

Belgium

Thomas Hambye

Dark matter bound-state formation in the Sun

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

United States

Jui-Lin Kuo

Multipole vector dark matter below the GeV scale

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>

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.

Yanou Cui

Canada

Michael Shamma

Dark freeze-out cogenesis

We revisit the possibility of light-scalar dark matter, in the MeV to GeV mass bracket and coupled to electrons through fermion or vector mediators, in light of significant experimental and observational advances that probe new physics below the GeV scale. We establish new limits from electron colliders and fixed-target beams and derive the strength of loop-induced processes that are probed by precision physics, among other laboratory probes. In addition, we compute the cooling bound from SN1987A, consider self-scattering, structure formation, and cosmological constraints as well as the limits from dark matter–electron scattering in direct detection experiments. We then show that the combination of constraints largely excludes the possibility that the galactic annihilation of these particles may explain the long-standing International Gamma-Ray Astrophysics Laboratory excess of 511 keV photons as observed in the Galactic bulge. As a caveat to these conclusions, we identify the resonant annihilation regime where the vector mediator goes nearly on shell.

Australia

Céline Bœhm

Scalar dark matter candidates revisited

We consider electromagnetically neutral dark states that couple to the photon through higher dimensional effective operators, such as electric and magnetic dipole moment, anapole moment and charge radius operators. We investigate the possibility of probing the existence of such dark states, taking a Dirac fermion <math><mi>χ</mi></math> as an example, at several representative proton-beam experiments. As no positive signal has been reported, we obtain upper limits (or projected sensitivities) on the corresponding electromagnetic form factors for dark states lighter than several GeV. We demonstrate that while the current limits from proton-beam experiments are at most comparable with those from high-energy electron colliders, future experiments, such as DUNE and SHiP, will be able to improve the sensitivities to electric and magnetic dipole moment interactions, owing to their high intensity.

Dark sector-photon interactions in proton-beam experiments

The existence of dark matter particles that carry phenomenologically relevant self-interaction cross sections mediated by light dark sector states is considered to be severely constrained through a combination of experimental and observational data. The conclusion is based on the assumption of specific dark matter production mechanisms such as thermal freeze-out together with an extrapolation of a standard cosmological history beyond the epoch of primordial nucleosynthesis. In this work, we drop these assumptions and examine the scenario from the perspective of the current firm knowledge we have results from direct and indirect dark matter searches and cosmological and astrophysical observations, without additional assumptions on dark matter genesis or the thermal state of the very early universe. We show that even in the minimal setup, where dark matter particles self-interact via a kinetically mixed vector mediator, a significant amount of parameter space remains allowed. Interestingly, however, these parameter regions imply a metastable, light mediator, which in turn calls for modified search strategies.

Colombia

Nicolás Bernal

Suchita Kulkarni

Self-interacting dark matter without prejudice

If dark matter has a finite size that is larger than its Compton wavelength, the corresponding self-interaction cross section decreases with the velocity. We investigate the implications of this puffy dark matter for addressing the small-scale problems of the <math><mi>Λ</mi></math> cold dark matter model and show that the way the nonrelativistic cross section varies with the velocity is largely independent of the dark matter internal structure. Even in the presence of a light particle mediating self-interactions, we find that the finite-size effect may dominate the velocity dependence. We present an explicit example in the context of a QCD-like theory and discuss possible ways to differentiate puffy dark matter from the usual light-mediator scenarios. Particularly relevant for this are low-threshold direct-detection experiments and indirect signatures associated with the internal structure of dark matter.