We clarify that chemical and kinetic equilibration in the early Universe are distinct: neither implies the other, and the ordering of their decouplings need not be universal. We illustrate this with Standard Model neutrino decoupling, strong-washout leptogenesis, dark–matter scenarios where kinetic decoupling precedes chemical freeze-out (resonant/forbidden, conversion/coannihilation, coscattering), and dark sectors at with temperatures distinct from the visible-sector temperature, with semiannihilation or <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>3</mn><mo stretchy="false">→</mo><mn>2</mn></math> cannibal dynamics. The moral of the story is simple: Chemical equilibrium governs numbers, kinetic equilibrium governs shapes. In an expanding Universe the operators that control them rarely fade at the same time, and when they do not, the order of decoupling is model dependent. Turning to phase-space evolution whenever momentum selectivity matters is the surest way to obtain robust cosmological predictions.

United States

Stefano Profumo

Chemical and kinetic equilibrium in cosmology: Facts and myths

We investigate benchmark scenarios for Weakly Interacting Massive Particles (WIMPs) that naturally evade current direct detection constraints by featuring suppressed spin-independent cross-sections. Focusing on three representative models — the Singlet-Doublet fermion model, its extension to a Two-Higgs-Doublet plus pseudoscalar sector (2HDM+$\textit{a}$), and a dark SU(3) gauge model, we systematically analyze the interplay between thermal freeze-out, direct detection blind spots, and radiative corrections. In each case, we identify viable regions of parameter space where the predicted dark matter relic abundance is consistent with observations while elastic scattering rates lie below current exclusion limits and, in some cases, but now always, below the neutrino floor. Loop-induced effects are shown to play a critical role, particularly in scenarios with suppressed tree-level interactions. Our findings demonstrate that models with rich electroweak and scalar sectors can populate the experimentally challenging, yet phenomenologically motivated parameter space between existing constraints and the ultimate sensitivity of current-technology direct detection experiments.

Italy

Giorgio Arcadi

WIMPs below the radar: blind spots and benchmarks beyond the neutrino floor

Assuming that (1) the Universe underwent a postinflationary accelerated expansion phase driven by a fluid with equation of state <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>P</mi><mo>=</mo><mi>w</mi><mi>ρ</mi></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo>−</mo><mn>1</mn><mo>&lt;</mo><mi>w</mi><mo>&lt;</mo><mo>−</mo><mn>1</mn><mo>/</mo><mn>3</mn></math>, that (2) the cosmic horizon in an accelerating, quasi–de Sitter Universe has a temperature inversely proportional to the proper size of the horizon, and that (3) we are static observers, we calculate the frozen-in density of a stable particle of mass <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>m</mi></math> produced by the cosmic horizon that does not undergo any number-changing processes in the late Universe. We find that, as a function of the equation of state and the temperature when radiation domination starts and the quasi–de Sitter phase ends, the mass of the dark matter producing the observed cosmological abundance via this mechanism ranges from 10 keV up to close to the Planck scale.

Dark matter from quasi–de Sitter horizons

If a cosmological first-order phase transition occurs sufficiently slowly, delayed vacuum decay may lead to the formation of primordial black holes. Here we consider a simple model as a case study of how the abundance of the produced black holes depends on the model’s input parameters. We demonstrate, using both numerical and analytical arguments and methods, that the black hole abundance is controlled by a double, “super”-exponential dependence on the three-dimensional Euclidean action over temperature at its minimal value. We show that a modified expansion rate during the phase transition, such as one driven by an additional energy density component, leads to a weaker dependence on the underlying model parameters, but maintains the same superexponential structure. We argue that our findings generalize to any framework of black hole production via delayed vacuum decay.

China

Yanda Wu

Superexponential primordial black hole production via delayed vacuum decay

We explore a novel mechanism for dark matter production through the formation of light black holes from the collapse of dark baryons in confining SU(N) gauge theories in the large-<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi></math> limit. While glueballs and mesons cannot form black holes under physically reasonable conditions, we prove that for appropriate ranges of the confinement scale, quark masses, number of colors <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi></math>, and dark sector temperature, dark baryons can produce Planck-scale black hole relics in the early Universe. Assuming the relics are stable, the abundance of both the dark baryon black hole population directly arising at confinement and that frozen in from dark glueball and meson pair annihilation are exponentially suppressed in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi></math>, leading to an upper limit <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi><mo>≲</mo><mn>100</mn></math> and of a few hundred Planck units in mass for models where the black hole relics are the entirety of the dark matter. We present a detailed numerical study of the parameter space where this scenario is realized.

Dark baryon black holes

The final stage of black hole evaporation is a potent probe of physics beyond the Standard Model: Hawking-Bekenstein radiation may be affected by quantum gravity “memory burden effects,” or by the presence of “dark,” beyond-the-Standard-Model degrees of freedom in ways that are testable with high-energy gamma-ray observations. We argue that information on either scenario can best be inferred from measurements of the evaporation’s light curve and by correlating observations at complementary energies. We offer several new analytical insights in how such observations map on the fundamental properties of the evaporating black holes and of the possible exotic particles they can evaporate into.

Kevin Federico

Black hole explosions as probes of new physics

We give a fresh look at the WIMP paradigm by considering updated limits and prospects for direct and indirect dark matter detection and covering realistic dark matter models, beyond the so-called simplified models, which have been the target of experimental scrutiny. In particular, we investigate dark matter scenarios featuring dwindled direct detection signals, due to loop or momentum suppression. Therefore, this review extends previous reviews in different aspects and motivates the search for WIMP dark matter in light of the present and near-future detectors.

Spain

David Cabo-Almeida

Maíra Dutra

India

Pradipta Ghosh

Germany

Manfred Lindner

France

Yann Mambrini

Brazil

Jacinto Neto

Mathias Pierre

Chile

Farinaldo Queiroz

The waning of the WIMP: endgame?

A dark sector with a very large number of massive degrees of freedom is generically constrained by radiative corrections to Newton’s constant. However, there are caveats to this statement, especially if the degrees of freedom are light or massless. Here, we examine in detail and update a number of constraints on the possible number of dark degrees of freedom, including from black hole evaporation, from perturbations to systems including an evaporating black hole, from direct gravitational production at colliders, from high-energy cosmic rays, and from supernovae energy losses.

Chris Ewasiuk

Constraints on the maximal number of dark degrees of freedom from black hole evaporation, cosmic rays, colliders, and supernovae

If the cosmological dark matter (DM) couples to Standard Model (SM) fields, it can decay promptly to SM states in a highly energetic hard process, which subsequently showers and hadronizes to give stable particles including <math><msup><mi>e</mi><mo>±</mo></msup></math>, <math><mi>γ</mi></math>, <math><msup><mi>p</mi><mo>±</mo></msup></math>, and <math><mi>ν</mi><mover><mi>ν</mi><mo>¯</mo></mover></math> at lower energy. If the DM particle is very heavy, the high-energy <math><msup><mi>e</mi><mo>±</mo></msup></math>, due to the Klein-Nishina cross section suppression, preferentially lose energy via synchrotron emission which, in turn, can be of unusually high energies. Here, we present previously unexplored bounds on heavy decaying DM up to the Planck scale, by studying the synchrotron emission from the <math><msup><mi>e</mi><mo>±</mo></msup></math> produced in the ambient Galactic magnetic field. In particular, we explore the sensitivity of the resulting constraints on the DM decay width to (i) different SM decay channels, to (ii) the Galactic magnetic field configurations, and to (iii) various different DM density profiles proposed in the literature. We find that constraints from the synchrotron component complement and improve on constraints from very high-energy cosmic-ray and gamma-ray observatories targeting the prompt emission when the DM is sufficiently massive, most significantly for masses in excess of <math><msup><mn>10</mn><mn>12</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>.

Pankaj Munbodh

Astrophysical constraints from synchrotron emission on very massive decaying dark matter

We show that bottom-strange production at a high-energy muon collider, <math><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mo>→</mo><mi>b</mi><mi>s</mi></math>, is a sensitive probe of new physics. We consider the full set of four-fermion contact interactions that contribute to this process at dimension 6, and discuss the complementarity of a muon collider and of the study of rare <math><mi>B</mi></math> meson decays that also probe said new physics. If a signal were to be found at a muon collider, the forward-backward asymmetry of the <math><mi>b</mi></math>-jet provides diagnostics about the underlying chirality structure of the new physics. In the absence of a signal at a center of mass energy of 10 TeV, <math><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mo>→</mo><mi>b</mi><mi>s</mi></math> can indirectly probe new physics at scales close to 100 TeV. We also discuss the impact that beam polarization has on the muon collider sensitivity performance.

Wolfgang Altmannshofer

Sri Aditya Gadam

Probing new physics with <math><mrow><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>→</mo><mi>b</mi><mi>s</mi></mrow></math> at a muon collider