The phenomena of dark matter and the baryon asymmetry pose two of the most pressing questions in today’s fundamental physics. Conversion-driven freeze-out has emerged as a successful mechanism to generate the observed dark matter relic density. It supports thermalization of dark matter despite its very weak couplings, aligning with the null results from direct and indirect detection experiments. In this Letter, we demonstrate that the departure from equilibrium of dark matter, induced by semi-efficient conversions, satisfies Sakharov’s conditions, providing a novel explanation for both dark matter and the baryon asymmetry. Specifically, for a leptophilic model, we establish the mechanism of conversion-driven leptogenesis. The scenario predicts dark matter masses close to the electroweak scale offering testable predictions, such as soft displaced leptons at the LHC and future colliders.

Germany

Jan Heisig

Conversion-Driven Leptogenesis: A Testable Theory of Dark Matter and Baryogenesis at the Electroweak Scale

Leptogenesis typically requires the introduction of heavy particles whose out-of-equilibrium decays are essential for generating a matter-antimatter asymmetry, according to one of Sakharov’s conditions. We demonstrate that in Dirac leptogenesis, scatterings between the light degrees of freedom—Standard Model particles plus Dirac neutrinos—are sufficient to generate the asymmetry. The generation requires at least two effective charges conserved by the fast Standard Model interactions. Due to its vanishing source term in the Boltzmann equations, the asymmetry of right-handed neutrinos solely arises through wash-in processes. Sakharov’s conditions are satisfied because the right-handed neutrino partners are out of equilibrium. Consequently, heavy degrees of freedom never needed to be produced in the early universe, allowing for a reheating temperature well below their mass scale. Considering a minimal leptoquark model, we discuss the viable parameter space along with the potential observational signature of an increased number of effective neutrinos in the early universe.

Slovakia

Tomáš Blažek

United States

Julian Heeck

Peter Maták

Viktor Zaujec

Dirac leptogenesis from asymmetry wash-in via scatterings

Conversion-driven freeze-out is an appealing mechanism to explain the observed relic density while naturally accommodating the null results from direct and indirect detection due to a very weak dark matter coupling. Interestingly, the scenario predicts long-lived particles decaying into dark matter with lifetimes favorably coinciding with the range that can be resolved at the LHC. However, the small mass splitting between the long-lived particle and dark matter renders the visible decay products soft, thus challenging current search strategies. We consider four different classes of searches covering the entire range of lifetimes: heavy stable charge particles, disappearing tracks, displaced vertices, and missing energy searches. We discuss the applicability of these searches to conversion-driven freeze-out and derive current constraints highlighting their complementarity. For the displaced vertex search, we demonstrate how a slight modification of the current analysis significantly improves its sensitivity to the scenario.

Brazil

Andre Lessa

Lucas Magno D. Ramos

Probing conversion-driven freeze-out at the LHC

We study a simplified Dark Matter model in the Dark Minimal Flavour Violation framework. Our model complements the Standard Model with a flavoured Dark Matter Majorana triplet and a coloured scalar mediator that share a Yukawa coupling with the right-handed up-type quarks with the coupling matrix λ. We extend previous work on this topic by exploring a large range of cosmologically viable parameter space, including the coannihilation region and, in particular, the region of conversion-driven freeze-out, while considering constraints from   <math> <msup> <mi>D</mi> <mn>0</mn> </msup> <mo>–</mo> <msup> <mover> <mi>D</mi> <mo>¯</mo> </mover> <mn>0</mn> </msup> </math>  $$ {D}^0\hbox{--} {\overline{D}}^0 $$  mixing as well as constraints from direct and indirect Dark Matter searches. We find various realisations of conversion-driven freeze-out within the model, that open up allowed windows of parameter space towards small mass splittings and very weak Dark Matter couplings. Finally, we probe the model by reinterpreting current LHC searches for missing energy and long-lived particles. We point out gaps in the coverage of current constraints as well as new opportunities to search for the model at the LHC, in particular, the charge asymmetry in single-top production associated with jets and missing energy.

Harun Acaroğlu

Monika Blanke

Michael Krämer

Lena Rathmann

Flavoured Majorana Dark Matter then and now: from freeze-out scenarios to LHC signatures

We analyze six classes of <math><mi>t</mi></math>-channel dark matter simplified models in which the Standard Model field content is extended by a colored mediator and a dark matter state. The two new states are enforced to be odd under a new parity, while all Standard Model fields are taken even so that dark matter stability is guaranteed. We study several possibilities for the spin of the new particles and the self-conjugate property of the dark matter, and we focus on model configurations in which the dark matter couples to the right-handed up quark for simplicity. We investigate how the parameter spaces of the six models can be constrained by current and future cosmological, astrophysical and collider searches, and we highlight the strong complementary between those probes. Our results demonstrate that scenarios featuring a complex (non self-conjugate) dark matter field are excluded by cosmology and astrophysics alone, the only possibility to avoid these bounds being to invoke very weak couplings and mechanisms such as conversion-driven freeze-out. For models with self-conjugate dark matter, mediator and dark matter masses are pushed deep into the TeV regime, with the lower limits on the mediator mass reaching 3 to 4 TeV and those on the dark matter mass 1 to 2 TeV. In large parts of the parameter space these strong bounds are driven by same-sign mediator pair production, a channel so far not considered in the experimental analyses embedding <math><mi>t</mi></math>-channel dark matter model interpretations.

Belgium

Chiara Arina

France

Benjamin Fuks

United Kingdom

Luca Mantani

Italy

Luca Panizzi

Comprehensive exploration of <math><mi>t</mi></math>-channel simplified models of dark matter

We explore the impact of highly excited bound states on the evolution of number densities of new physics particles, specifically dark matter, in the early Universe. Focusing on dipole transitions within perturbative, unbroken gauge theories, we develop an efficient method for including around a million bound state formation and bound-to-bound transition processes. This enables us to examine partial-wave unitarity and accurately describe the freeze-out dynamics down to very low temperatures. In the non-Abelian case, we find that highly excited states can prevent the particles from freezing out, supporting a continuous depletion in the regime consistent with perturbativity and unitarity. We apply our formalism to a simplified dark matter model featuring a colored and electrically charged <math><mrow><mi>t</mi></mrow></math>-channel mediator. Our focus is on the regime of superWIMP production which is commonly characterized by a mediator freeze-out followed by its late decay into dark matter. In contrast, we find that excited states render mediator depletion efficient all the way until its decay, introducing a dependence of the dark matter density on the mediator lifetime as a novel feature. The impact of bound states on the viable dark matter mass can amount to an order of magnitude, relaxing constraints from Lyman-<math><mi>α</mi></math> observations.

Tobias Binder

Mathias Garny

Stefan Lederer

Kai Urban

Excited bound states and their role in dark matter production

The singlet scalar Higgs-portal model provides one of the simplest explanations of dark matter in our Universe. Its Higgs resonant region, <math><msub><mi>m</mi><mi>DM</mi></msub><mo>≈</mo><msub><mi>m</mi><mi>h</mi></msub><mo>/</mo><mn>2</mn></math>, has gained particular attention, being able to reconcile the tension between the relic density measurement and direct detection constraints. Interestingly, this region is also preferred as an explanation of the Fermi-LAT <math><mi>γ</mi></math>-ray Galactic center excess. We perform a detailed study of this model using <math><mi>γ</mi></math>-ray data from the Galactic center and from dwarf spheroidal galaxies and combine them with cosmic-ray antiproton data from the AMS-02 experiment that shows a compatible excess. In the calculation of the relic density, we take into account effects of early kinetic decoupling relevant for resonant annihilation. The model provides excellent fits to the astrophysical data either in the case the dark matter candidate constitutes all or a subdominant fraction of the observed relic density. We show projections for future direct detection and collider experiments to probe these scenarios.

Mattia Di Mauro

Nicolao Fornengo

Daniele Massaro

Dark matter in the Higgs resonance region

Leptogenesis typically requires the introduction of heavy particles whose out-of-equilibrium decays are essential for generating a matter-antimatter asymmetry, according to one of Sakharov’s conditions. We show that, in Dirac leptogenesis, scatterings between the light degrees of freedom—Standard Model particles plus Dirac neutrinos—suffice to generate the asymmetry. Sakharov’s conditions are satisfied because the right-handed neutrino partners are out of equilibrium. Consequently, heavy degrees of freedom never needed to be produced in the early Universe, allowing for a reheating temperature below their mass scale. We solve the Boltzmann equations and discuss the viable parameter space together with observational signatures such as an increased number of effective neutrinos in the early Universe as well as proton decay for some realizations.

Anil Thapa

RETRACTED: Dirac leptogenesis via scatterings

The nature of neutrino masses and the matter-antimatter asymmetry of our universe are two of the most important open problems in particle physics today and are notoriously difficult to test with current technology. Dirac neutrinos offer a solution through a leptogenesis mechanism that hinges on the smallness of neutrino masses and resultant nonthermalization of the right-handed neutrino partners in the early universe. We thoroughly explore possible realizations of this Dirac leptogenesis idea, revealing new windows for highly efficient asymmetry generation. In many of them, the number of relativistic degrees of freedom, <math><msub><mi>N</mi><mi>eff</mi></msub></math>, is severely enhanced compared to standard cosmology and offers a novel handle to constrain Dirac leptogenesis with upcoming measurements of the cosmic microwave background. Realizations involving leptoquarks even allow for low-scale postsphaleron baryogenesis and predict proton decay. These novel aspects render Dirac leptogenesis surprisingly testable.

Testing Dirac leptogenesis with the cosmic microwave background and proton decay

Automated tools for the computation of particle physics’ processes have become the backbone of phenomenological studies beyond the standard model. Here, we present MadDM v3.2. This release enables the fully automated computation of loop-induced dark-matter annihilation processes, relevant for indirect detection observables. Special emphasis lies on the annihilation into  $$\gamma X$$  <math> <mrow> <mi>γ</mi> <mi>X</mi> </mrow> </math>  , where  $$X=\gamma , Z, h$$  <math> <mrow> <mi>X</mi> <mo>=</mo> <mi>γ</mi> <mo>,</mo> <mi>Z</mi> <mo>,</mo> <mi>h</mi> </mrow> </math>   or any new particle even under the dark symmetry. These processes lead to the sharp spectral feature of monochromatic gamma lines – a smoking-gun signature of dark matter in our Galaxy. MadDM provides the predictions for the respective fluxes near-Earth and derives constraints from the gamma-ray line searches by Fermi-LAT and HESS. As an application, we discuss the implications for the viable parameter space of a top-philic t-channel mediator model and the inert doublet model.

Fabio Maltoni

Olivier Mattelaer

Indirect dark-matter detection with MadDM v3.2 – Lines and Loops