As recently proposed, a non-vanishing topological angle may play a central role in QCD-like theories of dark matter (DM). In this work, we introduce a dark photon portal to the Standard Model in order to establish thermal equilibrium in the early Universe, and discuss the ensuing phenomenological constraints, including the stability of DM. accounts for the observed DM relic abundance and yields velocity-dependent DM self-interactions in astrophysical halos. Due to the sharp velocity dependence arising from a Breit-Wigner resonance, dedicated studies are required to assess the gravothermal evolution in detail, especially in the core-collapse regime. This is particularly timely in light of self-interacting DM interpretations of strong-lensing systems such as SDSS J0946+1006, which can be naturally explained within our framework.

Spain

Camilo García-Cely

Giacomo Landini

Luca Marsili

Colombia

Óscar Zapata

Pion dark matter in a $\textit{θ}$ vacuum: a thermal relic with sharp velocity-dependent self-interactions

<p>Quantum chromodynamics (QCD)-like theories in which the dark matter (DM) of the Universe is hypothesized to be a thermal relic in the form of a dark pion has been extensively investigated, with most studies neglecting the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math>-violating <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>θ</mi></mrow></math>-angle associated with the topological vacuum. We point out that a nonvanishing <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>θ</mi></math> could potentially trigger resonant number-changing processes giving rise to the observed relic density in agreement with perturbative unitarity as well as observations of clusters of galaxies. This constitutes a novel production mechanism of MeV DM and an alternative to those relying on the Wess-Zumino-Witten term. Moreover, for specific meson mass spectra, similar resonant scatterings serve as a realization of velocity-dependent self-interacting DM without a light mediator. Explicit benchmark models are presented together with a discussion of possible signals, including gravitational waves from the chiral phase transition associated with the dark pions.</p>

Dark matter in QCD-like theories with a theta vacuum: Cosmological and astrophysical implications

The parameter space of freeze-in dark matter (DM) with mass $\textit{m}$ through light dark photon (“minimal freeze-in DM”) is currently being probed by direct detection experiments through electron and nuclear recoil. Exploring the DM production in the mass range 10 MeV < $\textit{m}$ < 10 TeV, we quantify the impact of quantum statistics and the reheating dynamics (beyond the instantaneous reheating approximation) on the DM production in the early universe, in particular, the dependence on the cosmic equation of state and the scaling of the temperature of the Standard Model bath during reheating. Special cases corresponding to matter-domination and kination are carefully studied. To fit the entire observed DM relic abundance, low-temperature reheating scenarios require an increase in the coupling between dark and visible sectors which, in turn, enhances the regions of the parameter space that are already tested and will be probed by next-generation direct detection experiments for diverse reheating scenarios.

United Arab Emirates

Nicolás Bernal

Brazil

Chee Fong

Probing low-reheating scenarios with minimal freeze-in dark matter

Abstract We build an extended two Higgs doublet model theory with spontaneously broken  global symmetry, where the tree-level universal seesaw mechanism generates the mass hierarchy of the Standard Model charged fermions and the Zee–Babu mechanism produces tiny active neutrino masses. The third family of SM charged fermions gets tree-level masses from Yukawa interactions involving the Higgs doublets  (for the top quark) and  (for the bottom quark and tau lepton). The model under consideration is consistent with SM fermion masses and mixings, with the muon and electron  anomalies, and successfully accommodates the constraints arising from charged-lepton-flavor violation and meson oscillations. The proposed model predicts rates for charged-lepton-flavor-violating decays within the reach of forthcoming experiments.

Chile

A E Cárcamo Hernández

Diego Restrepo

Ivan Schmidt

Effective Interactions for the SM Fermion Mass Hierarchy and their Possible UV Realization

In one-component dark matter (DM) scenarios is commonly assumed that a scalar weakly interacting massive particle must either be part of an <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>L</mi></msub></math> multiplet with zero hypercharge or have suppressed vector interactions with the <math><mi>Z</mi></math>-gauge boson to circumvent stringent direct detection (DD) bounds. In this work, we demonstrate that multicomponent scenarios with a dark scalar doublet exhibiting vectorlike interactions with the <math><mi>Z</mi></math> boson are also compatible with bounds arising from DD searches. Specifically, we consider a simple extension of the Standard Model wherein the dark sector comprises a doublet and a complex singlet <math><mi>ϕ</mi></math>, both charged under a <math><msub><mi>Z</mi><mn>6</mn></msub></math> symmetry. We find that semi-annihilation processes drastically reduce the relic abundance of the neutral component of the doublet, <math><msup><mi>H</mi><mn>0</mn></msup></math>, sufficiently attenuating the effects of its large <math><mi>Z</mi></math>-mediated elastic scattering cross-section with nucleons to satisfy the DD constraints. Although the contribution of <math><msup><mi>H</mi><mn>0</mn></msup></math> to the total relic abundance is nearly negligible, with <math><mi>ϕ</mi></math> dominating, both dark matter components are expected to be detectable in ongoing and future DD experiments. The viability of the model is tested against several theoretical and experimental constraints, resulting in a parameter space featuring a nondegenerate mass spectrum at the electroweak scale.

María J. Domínguez

Oscar Rodríguez

Reopening the <math><mi>Z</mi></math> portal with semi-annihilations

We investigate a simple extension of the standard model in which the dark matter consists of a feebly interacting fermion, charged under a new <math><msub><mi>Z</mi><mn>4</mn></msub></math> symmetry, that is produced in the early Universe by the freeze-in mechanism. The only other new particle included in the model is a singlet scalar, also charged under the <math><msub><mi>Z</mi><mn>4</mn></msub></math>, which couples to the fermion via Yukawa interactions and to the standard model Higgs. The model is truly minimal, as it admits just five free parameters: two masses and three dimensionless couplings. Depending on their values, the freeze-in mechanism can be realized in different ways, each characterized by its own production processes. For all of them, we numerically study the relic density as a function of the free parameters of the model and determine the regions consistent with the dark matter constraint. Our results show that this scenario is viable over a wide range of couplings and dark matter masses. This model, therefore, not only offers a novel solution to the dark matter problem, but it also provides a minimal realization of freeze-in for fermion dark matter.

Carlos E. Yaguna

Minimal model of fermion FIMP dark matter

We discuss the production of primordial gravitational waves (GW) from radiative inflaton decay during the period of reheating, assuming perturbative decay of the inflaton either into a pair of bosons or fermions, leading to successful reheating satisfying constraint from big bang nucleosynthesis. Assuming that the inflaton <math><mi>ϕ</mi></math> oscillates in a general monomial potential <math><mi>V</mi><mo>(</mo><mi>ϕ</mi><mo>)</mo><mo>∝</mo><msup><mi>ϕ</mi><mi>n</mi></msup></math>, which results in a time-dependent inflaton decay width, we show that the resulting stochastic GW background can have optimistic detection prospects, especially in detectors that search for a high-frequency GW spectrum, depending on the choice of <math><mi>n</mi></math> that determines the shape of the potential during reheating. We also discuss how this GW energy density may affect the measurement of <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub></math> for bosonic and fermionic reheating scenarios.

Poland

Basabendu Barman

Germany

Yong Xu

Bremsstrahlung-induced gravitational waves in monomial potentials during reheating

We investigate, for the first time, a scenario where the dark matter consists of three complex scalar fields that are stabilized by a single Z 7 symmetry. As an extension of the well-known scalar Higgs-portal, this Z 7 model is also subject to important restrictions arising from the relic density constraint and from direct detection experiments. Our goal in this paper is to find and characterize the viable regions of this model, and to analyze its detection prospects in future experiments. First, the processes that affect the relic densities are identified (they include semiannihilations and conversions) and then incorporated into the Boltzmann equations for the dark matter abundances, which are numerically solved with micrOMEGAs. By means of random scans of the parameter space, the regions consistent with current data, including the recent direct detection limit from the LZ experiment, are selected. Our results reveal that the Z 7 model is indeed viable over a wide range of dark matter masses and that both conversions and semiannihilations play an important role in determining the relic densities. Remarkably, we find that in many cases all three of the dark matter particles give rise to observable signals in future direct detection experiments, providing a suitable way to test this scenario.

France

Geneviève Bélanger

Russia

Alexander Pukhov

Carlos Yaguna

The Z 7 model of three-component scalar dark matter

We revisit a two-component dark matter model in which the dark matter particles are a singlet fermion (<math><mi>ψ</mi></math>) and a singlet scalar (<math><mi>S</mi></math>), both stabilized by a single <math><msub><mi>Z</mi><mn>4</mn></msub></math> symmetry. The model—proposed by Cai and Spray—is remarkably simple, with its phenomenology determined by just five parameters: the two dark matter masses and three dimensionless couplings. In fact, <math><mi>S</mi></math> interacts with the Standard Model particles via the usual Higgs portal, whereas <math><mi>ψ</mi></math> only interacts directly with <math><mi>S</mi></math>, via the Yukawa terms <math><mrow><mover><mrow><msup><mrow><mi>ψ</mi></mrow><mrow><mi>c</mi></mrow></msup></mrow><mrow><mo>¯</mo></mrow></mover><mo>(</mo><msub><mrow><mi>y</mi></mrow><mrow><mi>s</mi></mrow></msub><mo>+</mo><msub><mrow><mi>y</mi></mrow><mrow><mi>p</mi></mrow></msub><msup><mrow><mi>γ</mi></mrow><mrow><mn>5</mn></mrow></msup><mo>)</mo><mi>ψ</mi><mi>S</mi></mrow></math>. We consider the two possible mass hierarchies among the dark matter particles, <math><msub><mi>M</mi><mi>S</mi></msub><mo>&lt;</mo><msub><mi>M</mi><mi>ψ</mi></msub></math> and <math><msub><mi>M</mi><mi>ψ</mi></msub><mo>&lt;</mo><msub><mi>M</mi><mi>S</mi></msub></math>, and numerically investigate the consistency of the model with current bounds. The main novelties of our analysis are the inclusion of the <math><msub><mi>y</mi><mi>p</mi></msub></math> coupling, the update of the direct-detection limits, and a more detailed characterization of the viable parameter space. For dark matter masses below 1.3 TeV or so, we find that not only is the model compatible with all known constraints, but it also gives rise to observable signals in future dark matter experiments. Our results show that both dark matter particles may be observed in direct-detection experiments and that the most relevant indirect-detection channel is due to the annihilation of <math><mi>ψ</mi></math>. We also argue that this setup can be extended to other <math><msub><mi>Z</mi><mi>N</mi></msub></math> symmetries and additional dark matter particles.

Fermion and scalar two-component dark matter from a <math><msub><mi>Z</mi><mn>4</mn></msub></math> symmetry

The Peccei-Quinn (PQ) mechanism not only acts as an explanation for the absence of strong charge parity (<math><mrow><mi>C</mi><mi>P</mi></mrow></math>) violation but also can play a main role in the solution to other open questions in particle physics and cosmology. Here we consider a model that identifies the PQ symmetry as a common thread in the solution to the strong <math><mrow><mi>C</mi><mi>P</mi></mrow></math> problem, the generation of radiative Dirac neutrino masses and the origin of a multicomponent dark sector. Specifically, scotogenic neutrino masses arise at one-loop level with the lightest fermionic mediator field acting as the second dark matter (DM) candidate thanks to the residual <math><msub><mi>Z</mi><mn>2</mn></msub></math> symmetry resulting from the PQ symmetry breaking. We perform a phenomenological analysis addressing the constraints coming from the direct searches of DM, neutrino oscillation data and charged lepton flavor violating (LFV) processes. We find that the model can be partially probed in future facilities searching for weakly interacting massive particles and axions, and accommodates rates for rare leptonic decays that are within the expected sensitivity of upcoming LFV experiments.