DarkTurns Turning the tables on dark matter nightmare scenarios

• Flores, Marcos & Petraki, Kalliopi (2026). Unitarizing non-relativistic scattering. Journal of High Energy Physics (JHEP). ISSN 1126-6708. 2026(6). doi: 10.1007/jhep06(2026)148. Full text in Research Archive Show summary

  A bstract Unitarity imposes coupled constraints on elastic and inelastic amplitudes. Satisfying them requires resummation of the self-energy contributions from both elastic and inelastic channels. Inelastic channels generate anti-Hermitian contributions that can be consistently deduced from the unitarity relation underlying the optical theorem, leading to non-local separable potentials and a compact, unique and complete unitarization scheme in the non-relativistic regime. We present two alternative derivations of the anti-Hermitian kernel, from the continuity equation combined with LSZ reduction, and by integrating out inelastic channels. We further extend the unitarization framework to treat non-analytic and non-convergent behavior of inelastic amplitudes in the complex momentum plane and to incorporate bound states. For non-convergent amplitudes, we demonstrate two renormalization procedures in which anti-Hermitian separable potentials necessarily induce Hermitian separable counterterms, yielding finite cross-sections consistent with unitarity. These results provide a general tool for non-relativistic scattering, with clear applications to dark-matter phenomenology.

• Flores, Marcos & Petraki, Kalliopi (2026). Unitarity violation and restoration in radiative bound-state formation. Journal of High Energy Physics (JHEP). ISSN 1126-6708. 2026(6). doi: 10.1007/jhep06(2026)231. Full text in Research Archive Show summary

  A bstract State-of-the-art calculations motivated by dark matter exhibit severe violation of partial-wave unitarity in the non-relativistic regime in radiative bound-state-formation processes. It has been recently shown, in a model-independent fashion, that unitarity is restored by the proper resummation of the inelastic contributions to the self-energy of the incoming state. In this work, we first derive Kramers-like formulae for individual partial waves, demonstrating that existing calculations of bound-state formation severely violate unitarity. We then discuss how unitarity is restored through the resummation of the absorptive contributions to the incoming-state self-energy, generated by bound-state formation processes, taking into account their analytic structure in the complex momentum plane. Our results can be generalized to a variety of theories and employed in phenomenological studies, such as dark-matter freeze-out, indirect detection and self-interactions.

• Bringmann, Torsten; Croon, Djuna & Muñoz, Sergio Sevillano (2026). Updated Constraints on the Primordial Power Spectrum at Sub-Mpc Scales. Physical Review Letters. ISSN 0031-9007. 137(6). doi: 10.1103/2jhb-n2rk. Full text in Research Archive
• Balan, Sowmiya; Bringmann, Torsten; Kahlhoefer, Felix; Matuszak, Jonas & Tasillo, Carlo (2025). Sub-GeV dark matter and nano-Hertz gravitational waves from a classically conformal dark sector. Journal of Cosmology and Astroparticle Physics (JCAP). 2025(08), p. 062–062. doi: 10.1088/1475-7516/2025/08/062. Full text in Research Archive Show summary

  Abstract Strong first-order phase transitions in a dark sector offer a compelling explanation for the stochastic gravitational wave background in the nano-Hertz range recently detected by pulsar timing arrays (PTAs). We explore the possibility that such a phase transition at the same time gives mass to a stable fermion that accounts for the observed dark matter abundance and leads to testable effects in laboratory experiments. Concretely, we consider a classically conformal dark sector with a hidden U(1)^' gauge symmetry that couples to the Standard Model via kinetic mixing. Since the PTA signal requires a phase transition in the MeV temperature range, spontaneous symmetry breaking gives rise to a sub-GeV dark matter candidate that couples to the Standard Model via a dark photon mediator and obtains its relic abundance via annihilations into electrons and dark Higgs bosons. Such a scenario is tightly constrained by laboratory searches for dark photons and cosmological constraints on the decays of dark Higgs bosons after the phase transition. We show that viable parameter regions can be found both for the case that the dark Higgs bosons remain in equilibrium with the Standard Model and that they decouple and only decay much later. In the latter case, the parameter regions preferred by the PTA signal and the dark matter relic abundance can be fully explored by future beam-dump experiments searching for missing energy.

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• Melkild, Halvor; Bringmann, Torsten & Brekke, Anton Andreas (2026). Charged Sterile Neutrinos as Dark Matter. doi: https:/indico2.uis.no/event/64/contributions/2002/. Full text in Research Archive
• Melkild, Halvor; Bringmann, Torsten & Brekke, Anton Andreas (2026). Charged Sterile Neutrinos as Dark Matter. Full text in Research Archive
• Melkild, Halvor; Krichevskiy, Daniil; Kolesova, Helena & Bringmann, Torsten (2026). Beyond 3→2: Towards Realistic SIMP Dark Matter. Full text in Research Archive
• Melkild, Halvor; Krichevskiy, Daniil; Kolesova, Helena & Bringmann, Torsten (2026). Beyond 3→2: Towards Realistic SIMP Dark Matter. Full text in Research Archive
• Parra-López, Álvaro (2026). Gravitational dark matter production, its uncertainties, and its interplay with freeze-in. doi: https:/indico.capa.unizar.es/event/40/contributions/873/. Full text in Research Archive Show summary

  The persistent absence of non-gravitational dark matter signals has increased interest in candidates that interact extremely weakly with the Standard Model—if at all. (Non-perturbative) gravitational particle production provides an unavoidable mechanism for generating such dark matter via the expansion of spacetime. However, predicting the exact relic abundance is subject to significant theoretical uncertainties arising from the choice of inflationary potential, field spin, and the details of the reheating phase. In this talk, I will present an estimation of these uncertainties based on recent results for scalar and vector spectator fields across different inflationary backgrounds. Furthermore, I explore the conceptual and technical challenges of reconciling this gravitational production—rooted in QFT in curved spacetimes—with the freeze-in mechanism, typical in astroparticle physics

• Parra-López, Álvaro (2026). Towards tackling the inherent uncertainties of gravitational dark matter production. doi: https:/indico2.uis.no/event/64/contributions/1989/. Full text in Research Archive Show summary

  The persistent absence of non-gravitational dark matter signals has increased interest in candidates that interact extremely weakly with the Standard Model—if at all. (Non-perturbative) gravitational particle production provides an unavoidable mechanism for generating such dark matter via the expansion of spacetime. However, predicting the exact relic abundance is subject to significant theoretical ambiguities tied to the framework of QFT in curved spacetimes, as well as uncertainties coming from inflationary and reheating dynamics. In this talk, I will present an overview of these uncertainties based on recent results for scalar and vector spectator fields across different inflationary backgrounds. These ambiguities become especially problematic for masses below the scale of inflation due to tachyonic instabilities and the non-adiabaticity of the field modes. Since this regime is of particular interest for other non-thermal production mechanisms, such as freeze-in, it is crucial to address these conceptual and technical challenges to ensure a robust phenomenological approach.

• Parra-López, Álvaro (2026). Ambiguities and uncertainties in gravitational dark matter production. doi: https:/indico.global/event/15863/contributions/155854/. Full text in Research Archive Show summary

  The persistent absence of non-gravitational dark matter signals has increased interest in candidates that interact extremely weakly with the Standard Model—if at all. (Non-perturbative) gravitational particle production provides an unavoidable mechanism for generating such dark matter via the expansion of spacetime. However, predicting the exact relic abundance is subject to significant theoretical ambiguities tied to the framework of QFT in curved spacetimes, as well as uncertainties coming from inflationary and reheating dynamics. In this talk, I will present an overview of these uncertainties based on recent results for scalar and vector spectator fields across different inflationary backgrounds. These ambiguities become especially problematic for masses below the scale of inflation due to tachyonic instabilities and the non-adiabaticity of the field modes. Since this regime is of particular interest for other non-thermal production mechanisms, such as freeze-in, it is crucial to address these conceptual and technical challenges to ensure a robust phenomenological approach.

• Klocke, Tim-Leon (2026). Constraining Beyond-CDM Models with an Effective Theory of the Lyman-α Forest. doi: https:/indico.global/event/15863/contributions/156017/. Full text in Research Archive Show summary

  In the age of precision cosmology, mapping the large-scale structure of the universe has the potential to reveal pieces of the puzzle that is the nature of dark matter. Most deviations from standard cold DM (CDM) models are confined to cosmologically small scales to not violate existing constraints. Thus, there is a fundamental need to have accurate models of the non-linear formation of structures in beyond-CDM models. Therefore, we present a framework in the spirit of the effective field theory of large-scale structure (EFTofLSS) describing observations of the Lyman-α forest and test its applicability to beyond-CDM models of DM. We focus on mixed Hot/Warm + Cold DM models, and present a systematic study of the statistical power of this effective approach, with the goal of quantifying its predictability regarding beyond-CDM models. Moreover, we discuss its applicability to observational data on the largest Lyman-α scales as measured by BOSS and DESI, and the possible challenges this entails.

• Klocke, Tim-Leon (2026). Constraining Beyond-CDM Models from Large-Scale Structures. doi: https:/indico2.uis.no/event/64/contributions/1998/. Full text in Research Archive Show summary

  In the age of precision cosmology, mapping the large-scale structure (LSS) of the universe has the potential to reveal pieces of the puzzle describing the nature of dark matter. Most deviations from standard cold DM (CDM) models are confined to cosmologically small scales as possible signatures at very large scales are already excluded by precise measurements of the CMB. Thus, there is a fundamental need to have accurate models of the (non-linear) formation of structures in beyond-CDM models. To begin, a classification of the impact of different models on the LSS is needed, which enables us to systematically scan the most interesting parameter space. This can be used to evaluate the (quasi) non-linear evolution of structure formation, which is especially relevant for applications to Lyman-α data. To attain this goal, I will present a version of the effective field theory of large-scale structure (EFTofLSS), which enables faster scans than traditional simulation-based methods. Nevertheless, significant challenges remain in the statistical interpretation of any results obtained, because there are multiple astrophysical, theoretical assumptions and degeneracies, which have to be consistently taken into account. However, application of these methods to data has the potential to improve our current understanding of many existing bounds on beyond-CDM models.

• Flores, Marcos (2026). Unitarity in the non-relativistic regime and implications for dark matter. doi: https:/indico.capa.unizar.es/event/40/contributions/877/. Full text in Research Archive Show summary

  Unitarity imposes strict bounds on elastic and inelastic partial-wave cross sections. However, state-of-the-art calculations, motivated by dark-matter phenomenology, can exhibit substantial violations of partial-wave unitarity, with potentially significant consequences for phenomenological predictions. In this talk, I present a new, model-independent formalism that restores unitarity through the consistent resummation of inelastic contributions to the self-energy of the incoming state. This framework provides a systematic and internally consistent treatment of inelastic effects. I will illustrate its implementation in existing bound-state-formation calculations, demonstrating how unitarity is recovered in regimes where existing calculations break down. The method is broadly applicable and has implications for dark-matter freeze-out, indirect detection, and self-interactions.

• Flores, Marcos (2026). Unitarity in the non-relativistic regime and implications for dark matter. doi: https:/indico2.uis.no/event/64/contributions/2001/. Full text in Research Archive Show summary

  Unitarity imposes strict bounds on elastic and inelastic partial-wave cross sections. However, state-of-the-art calculations, motivated by dark-matter phenomenology, may exhibit substantial violations of partial-wave unitarity, with potentially significant consequences for phenomenological predictions. In this talk, I present a new, model-independent formalism that restores unitarity through the consistent resummation of inelastic contributions to the self-energy of the incoming state. This framework provides a systematic and internally consistent treatment of inelastic effects. I will illustrate its implementation in existing bound-state-formation calculations, demonstrating how unitarity is recovered in regimes where existing calculations break down. The method is broadly applicable and has implications for dark-matter freeze-out, indirect detection, and self-interactions.

• Flores, Marcos (2026). Unitarity in the non-relativistic regime and implications for dark matter. Full text in Research Archive Show summary

  Unitarity imposes strict bounds on elastic and inelastic partial-wave cross sections. However, state- of-the-art calculations, motivated by dark-matter phenomenology, may exhibit substantial viola- tions of partial-wave unitarity, with potentially significant consequences for phenomenological predictions. In this talk, I present a new, model-independent formalism that restores unitarity through the consistent resummation of inelastic contributions to the self-energy of the incoming state. This framework provides a systematic and internally consistent treatment of inelastic effects. I will illustrate its implementation in existing bound-state-formation calculations, demonstrating how unitarity is recovered in regimes where existing calculations break down. The method is broadly applicable and has implications for dark-matter freeze-out, indirect detection, and self-interactions.

• Bringmann, Torsten (2026). Dark matter and gravitational wave signals from a phase transition in the early universe. Full text in Research Archive Show summary

  A strong first-order phase transition offers the intriguing possibility of explaining the stochastic gravitational wave background at nHz frequencies. In order to avoid cosmological constraints, such a phase transition must have occurred in a dark sector with non-negligible couplings to the standard model - opening promising avenues of connecting the gravitational wave signal to the cosmological dark matter abundance. In order to illustrate this, I will consider a classically conformal dark sector with a U(1) gauge symmetry. The spontaneous breaking of this symmetry generates both gravitational waves and sources the mass of a fermionic sub-GeV dark matter candidate. Contact with the standard model is established through kinetic mixing of the U(1) gauge bosons with ordinary photons, a possibility that is actively being searched for at various collider experiments. I will discuss the rich phenomenology of such a scenario, demonstrating that it can successfully generate both the observed nHz gravitational wave background and the cosmological dark matter abundance, while at the same time avoiding all current constraints.

• Bringmann, Torsten (2026). New directions in secluded dark matter searches. Full text in Research Archive Show summary

  Less than 20% of the total matter content of the universe consists of known forms of matter. After decades of experimental efforts, the leading hypothesis for the missing part - namely a new type of weakly interacting massive particle - is starting to become seriously pressured. As a result, the true nature of dark matter is more elusive now than ever. I will start with a pedagogic reminder of the most convincing pieces of evidence that we have for the existence of dark matter. I will then highlight new model building avenues that may explain why traditional searches have, so far, failed to see any signals. In fact, I will argue that it might well be generally impossible to directly detect or produce any dark matter particles in the laboratory - but that cosmological and astrophysical observations still have the potential to conclusively pinpoint some of their microscopic properties.

• Bringmann, Torsten (2026). Dark matter and gravitational wave signals from a phase transition in the early universe. Full text in Research Archive Show summary

  A strong first-order phase transition offers the intriguing possibility of explaining the stochastic gravitational wave background at nHz frequencies. In order to avoid cosmological constraints, such a phase transition must have occurred in a dark sector with non-negligible couplings to the standard model -- opening promising avenues of connecting the gravitational wave signal to the cosmological dark matter abundance. In order to illustrate this, I will consider a classically conformal dark sector with a U(1) gauge symmetry. The spontaneous breaking of this symmetry generates both gravitational waves and sources the mass of a fermionic sub-GeV dark matter candidate. Contact with the standard model is established through kinetic mixing of the U(1) gauge bosons with ordinary photons, a possibility that is actively being searched for at various collider experiments. I will discuss the rich phenomenology of such a scenario, demonstrating that it can successfully generate both the observed nHz gravitational wave background and the cosmological dark matter abundance, while at the same time avoiding all current constraints. If time allows, I will further contemplate about what a future gravitational wave signal observed by LISA may tell us about dark matter production in the early universe.

• Klocke, Tim-Leon (2025). Constraining the Dark Sector with Cosmological Large-Scale Structure Observations. doi: https:/indico2.uis.no/event/59/contributions/1907/. Full text in Research Archive Show summary

  Understanding the intersection of particle physics and cosmology – dark matter – is a crucial open question in physics and cosmology. Here the goal is to construct a pipeline in which the implications of different dark matter models – from their particle nature to cosmological observations of large-scale structure (LSS) – can be systematically studied. Specifically, studying the (late-time) evolution of structure using Lyman-α data and, moreover, derive constraints on the particle nature of dark matter and extended dark sectors. This requires the usage of large simulations, novel methods in the realm of effective field theory of large scale structure (EFTofLSS), as well as understanding observational limitations and constraints. I will provide an overview over the most important open questions and how we plan to tackle each of them.

• Bringmann, Torsten (2025). First-order phase transitions and stochastic gravitational wave background. Full text in Research Archive
• Bringmann, Torsten (2025). The (un)reasonable elusiveness of dark matter. Full text in Research Archive Show summary

  Only about 15% of the total matter content of the universe consists of known forms of matter. After decades of experimental efforts, the leading hypothesis for the missing part - namely a new type of weakly interacting massive particle - is starting to become seriously pressured. As a result, the true nature of dark matter is more elusive now than ever. In this talk, I will start with a pedagogic review of some of the most convincing pieces of evidence that we currently have for the existence of dark matter. I will then highlight new model building avenues that may explain why traditional searches have, so far, failed to see any signals. In fact, I will argue that it might well be generally impossible to directly detect or produce any dark matter particles in the laboratory - but that cosmological observations still have the potential to conclusively pinpoint some of their properties.

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