arXiv subjects

Andrzej Hryczuk

Publications and source records attributed to Andrzej Hryczuk.

At least 19 recordsLinked to original sources

arXiv2026

Light Dark Matter from Self-cooling Dark Sectors

Light thermally produced dark matter is subject to strong bounds stemming from its free-streaming impact on suppressing structure formation. In this paper we show that these limits are significantly alleviated if the dark sector undergoes self-cooling, a new mechanism for lowering the temperature of the dark sector plasma through cannibal self-interactions during freeze-in production. We find that the Lyman-$α$ bounds can be modified by a few orders of magnitude in the sub-MeV region and frozen-in dark matter saturating the observed energy density can be as light as $\approx$ 1.9 (0.7) keV, compared to the $\approx$ 5.7 (1.9) keV warm dark matter limits determined from simulations. Interestingly, a secondary phase of cooling due to simultaneously efficient inverse cannibal- and decay-processes can dominate the modifications of Lyman-$α$ bounds, rather than self-thermalization via cannibal-reactions itself.

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arXiv2026

Exploring non-equilibrium effects in sequential freeze-in

Freeze-in of multi-component dark sectors is governed not only by the interaction with the thermal plasma, but also by their internal dynamics. Full thermalisation within the dark sector is not guaranteed, raising the question of impact of departures from local thermal equilibrium onto the evolution and ultimately relic abundance and momentum distribution of dark matter. In this work we explore this question in a minimal two-scalar model, which can give rise to observable signatures in indirect detection and long-lived particle searches at forward physics experiments. Focusing on the phenomenologically viable regions, we analyse the impact of non-thermal evolution on the dark matter abundance, finding deviations of up to an order of magnitude between the full phase-space treatment and the traditional number-density approach. Our results highlight the importance of phase-space level computation for accurate freeze-in predictions and further motivate dedicated numerical tools for studying the evolution of multi-component dark sectors at the phase space level.

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arXiv2025

Freezing-in Cannibals with Low-reheating Temperature

The freeze-in mechanism provides a compelling framework for dark matter (DM) production, particularly suited to scenarios involving feeble interactions with the Standard Model (SM). In this work, we highlight a possible interplay of a non-instantaneous reheating phase and dark sector self-interactions, specifically $2 \to 3$ and $3 \to 2$ cannibalization processes. As an example we study the freeze-in production of a complex scalar DM candidate stabilized by a $\mathbb{Z}_3$ symmetry permitting cubic self-couplings, enabling number-changing interactions that drive internal thermalization and significantly modify the dark sector number density and temperature evolution. We numerically solve the coupled Boltzmann equations for the DM number density and temperature alongside the evolving SM bath, accurately capturing the dynamics of a prolonged reheating epoch. Our analysis reveals a rich and distinctive phenomenology arising from the interplay between the Universe's thermal history, Higgs portal mediated production, and cannibalistic self-interactions. Compared to scenarios with instantaneous reheating or negligible self-interactions, our framework opens new viable regions in parameter space, particularly for light DM, potentially within reach of future probes.

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arXiv2025

Conversions in two-component dark sectors: a phase space level analysis

Conversions between the states in the dark sector affect not only their number densities but also their momentum distributions. In this work we study a phenomenologically motivated two-component dark matter scenario, based on the Coy Dark Matter model, in order to quantify the effect of conversions on departure from kinetic equilibrium and consequently the relic abundance. We perform a detailed numerical analysis at the level of the phase space distributions of dark sector particles, implementing all the relevant processes, including conversions, elastic scatterings and annihilations. Focusing on the parameter regions that lead to the observed relic abundance and provide a good fit to the Galactic Centre excess, we find that departure from kinetic equilibrium can alter the predictions for the total abundance by more than $100\%$, while in most of the interesting parameter space being in the range from around $-20\%$ to $50\%$. The effect on each dark matter constituent separately can be much larger, even up to an order of magnitude, which can significantly affect the expected present-day gamma ray flux, and consequently phenomenology of the model.

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arXiv2025

Freezing-in Cannibal Dark Sectors

Self-Interacting Dark Matter models can successfully explain dark matter (DM) production through interactions confined within the dark sector. However, they often lack measurable experimental signals due to their secluded nature. Including a feeble interaction with the visible sector through a Higgs portal leads not only to potential detection avenues and richer thermal production dynamics, but also to a possible explanation of the initial dark sector population through the freeze-in mechanism. In this work we study, by solving the full system of coupled Boltzmann equations for the number densities and temperatures of all the involved states, three scenarios of this type where the DM is: a real scalar with broken $\mathbb{Z}_2$, a complex scalar with unbroken $\mathbb{Z}_3$, and a $\mathbb{Z}_3$ scalar with an additional scalar mediator. All of these models have viable dark matter candidates in a cannibal phase while having different detection profiles. We show that cosmological bounds can be either exacerbated or evaded by changing the dark sector interactions, leading to potential signatures in long-lived particle and indirect detection experiments.

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arXiv2024

Phase Transitions and Gravitational Waves in a Model of $\mathbb{Z}_{3}$ Scalar Dark Matter

Theories with more than one scalar field often exhibit phase transitions producing potentially detectable gravitational wave (GW) signal. In this work we study the semi-annihilating $\mathbb{Z}_3$ dark matter model, whose dark sector comprises an inert doublet and a complex singlet, and assess its prospects in future GW detectors. Without imposing limits from requirement of providing a viable dark matter candidate, i.e. taking into account only other experimental and theoretical constraints, we find that the first order phase transition in this model can be strong enough to lead to a detectable signal. However, direct detection and the dark matter thermal relic density constraint calculated with the state-of-the-art method including the impact of early kinetic decoupling, very strongly limit the parameter space of the model explaining all of dark matter and providing observable GW peak amplitude. Extending the analysis to underabundant dark matter thus reveals region with detectable GWs from a single-step or multi-step phase transition.

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arXiv2022

Impact of dark matter self-scattering on its relic abundance

Elastic self-scatterings do not change the number of dark matter particles and as such have been neglected in the calculation of its relic abundance. In this work we highlight the scenarios where the presence of self-scatterings has a significant impact on the effectiveness of annihilation processes through the modification of dark matter momentum distribution. We study a few example freeze-out scenarios involving resonant and sub-threshold annihilations, as well as a model with an additional source of dark matter particles from the decays of a heavier mediator state. Interestingly, when the calculation is performed at the level of dark matter momentum distribution function, we find that the injection of additional energetic dark matter particles onto the thermal population can lead to a $\textit{decrease}$ of its final relic abundance.

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arXiv2021

DRAKE: Dark matter Relic Abundance beyond Kinetic Equilibrium

We introduce DRAKE, a numerical precision tool for predicting the dark matter relic abundance also in situations where the standard assumption of kinetic equilibrium during the freeze-out process may not be satisfied. DRAKE comes with a set of three dedicated Boltzmann equation solvers that implement, respectively, the traditionally adopted equation for the dark matter number density, fluid-like equations that couple the evolution of number density and velocity dispersion, and a full numerical evolution of the phase-space distribution. We review the general motivation for these approaches and, for illustration, highlight three concrete classes of models where kinetic and chemical decoupling are intertwined in a way that quantitatively impacts the relic density: i) dark matter annihilation via a narrow resonance, ii) Sommerfeld-enhanced annihilation and iii) `forbidden' annihilation to final states that are kinematically inaccessible at threshold. We discuss all these cases in some detail, demonstrating that the commonly adopted, traditional treatment can result in an estimate of the relic density that is wrong by up to an order of magnitude. The public release of DRAKE, along with several examples of how to calculate the relic density in concrete models, is provided at drake.hepforge.org

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arXiv2021

Dark matter freeze-in from semi-production

We study a novel dark matter production mechanism based on the freeze-in through semi-production, i.e. the inverse semi-annihilation processes. A peculiar feature of this scenario is that the production rate is suppressed by a small initial abundance of dark matter and consequently creating the observed abundance requires much larger coupling values than for the usual freeze-in. We provide a concrete example model exhibiting such production mechanism and study it in detail, extending the standard formalism to include the evolution of dark matter temperature alongside its number density and discuss the importance of this improved treatment. Finally, we confront the relic density constraint with the limits and prospects for the dark matter indirect detection searches. We show that, even if it was never in full thermal equilibrium in the early Universe, dark matter could, nevertheless, have strong enough present-day annihilation cross section to lead to observable signals.

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arXiv2020

Self-interacting dark matter from late decays and the $H_0$ tension

We study a dark matter production mechanism based on decays of a messenger WIMP-like state into a pair of dark matter particles that are self-interacting via exchange of a light mediator. Its distinctive thermal history allows the mediator to be stable and therefore avoid strong limits from the cosmic microwave background and indirect detection. A natural by-product of this mechanism is a possibility of a late time, i.e., after recombination, transition to subdominant dark radiation component through three-body and one-loop decays to states containing the light mediator. We examine to what extent such a process can help to alleviate the $H_0$ tension. Additionally, the mechanism can provide a natural way of constructing dark matter models with ultra-strong self-interactions that may positively affect the supermassive black hole formation rate. We provide a simple realization of the mechanism in a Higgs portal dark matter model and find a significant region of the parameter space that leads to a mild relaxation of the Hubble tension while simultaneously having the potential of addressing small-scale structure problems of $Λ$CDM.

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arXiv2020

Early kinetic decoupling of dark matter: when the standard way of calculating the thermal relic density fails

Calculating the abundance of thermally produced dark matter particles has become a standard procedure, with sophisticated methods guaranteeing a precision that matches the percent-level accuracy in the observational determination of the dark matter density. Here, we point out that one of the main assumptions in the commonly adopted formalism, namely local thermal equilibrium during the freeze-out of annihilating dark matter particles, does not have to be satisfied in general. We present two methods for how to deal with such situations, in which the kinetic decoupling of dark matter happens so early that it interferes with the chemical decoupling process: i) an approximate treatment in terms of a coupled system of differential equations for the leading momentum moments of the dark matter distribution, and ii) a full numerical solution of the Boltzmann equation in phase-space. For illustration, we apply these methods to the case of Scalar Singlet dark matter. We explicitly show that even in this simple model the prediction for the dark matter abundance can be affected by up to one order of magnitude compared to the traditional treatment.

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arXiv2020

Forbidden Freeze-In

We study the importance of a frozen-in dark matter production regime, where the dark matter particle is produced via kinematically forbidden decays that arise from significant thermal correction to the mass a mediator particle in the plasma.

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arXiv2020

Impact of uncertainties in the halo velocity profile on direct detection of sub-GeV dark matter

We use the state-of-the-art high-resolution cosmological simulations by IllustrisTNG to derive the velocity distribution and local density of dark matter in galaxies like our Milky Way and find a substantial spread in both quantities. Next we use our findings to examine the sensitivity to the dark matter velocity profile of underground searches using electron scattering in germanium and silicon targets. We find that sub-GeV dark matter search is strongly affected by these uncertainties, unlike nuclear recoil searches for heavier dark matter, especially in multiple electron-hole modes, for which the sensitivity to the scattering cross-section is also weaker. Therefore, by improving the sensitivity to lower ionization thresholds not only projected sensitivities will be boosted but also the dependence on the astrophysical uncertainties will become significantly reduced.

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arXiv2019

Forbidden frozen-in dark matter

We examine and point out the importance of a regime of dark matter production through the freeze-in mechanism that results from a large thermal correction to a decaying mediator particle mass from hot plasma in the early Universe. We show that mediator decays to dark matter that are kinematically forbidden at the usually considered ranges of low temperatures can be generically present at higher temperatures and actually dominate the overall dark matter production, thus leading to very distinct solutions from the standard case. We illustrate these features by considering a dark Higgs portal model where dark matter is produced via decays of a scalar field with a large thermal mass. We identify the resulting ranges of parameters that are consistent with the correct dark matter relic abundance and further apply current and expected future collider, cosmological, and astrophysical limits.

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arXiv2019

Testing dark matter with Cherenkov light -- prospects of H.E.S.S. and CTA for exploring minimal supersymmetry

We provide an updated and improved study of the prospects of the H.E.S.S. and Cherenkov Telescope Array (CTA) experiments in testing neutralino dark matter in the Minimal Supersymmetric Standard Model with nine free parameters (p9MSSM). We include all relevant experimental constraints and theoretical developments, in particular the calculation of the Sommerfeld enhancement for both present-day annihilation and the relic abundance. We perform a state-of-the-art analysis of the CTA sensitivity with a log-likelihood test ratio statistics and apply it to a numerical scan of the p9MSSM parameter space focusing on a TeV scale dark matter. We find that, assuming Einasto profile of dark matter halo in the Milky Way, H.E.S.S. has already been able to nearly reach the so-called thermal WIMP value, while CTA will go below it by providing a further improvement of at least an order of magnitude. Both H.E.S.S. and CTA are sensitive to several cases for which direct detection cross section will be below the so-called neutrino floor, with H.E.S.S. being sensitive to most of the wino region, while CTA also covering a large fraction of the ~1 TeV higgsino region. We show that CTA sensitivity will be further improved in the monochromatic photon search mode for both single-component and underabundant dark matter.

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arXiv2019

Improved bounds on $\mathbb{Z}_{3}$ singlet dark matter

We reconsider complex scalar singlet dark matter stabilised by a $\mathbb{Z}_{3}$ symmetry. We refine the stability bounds on the potential and use constraints from unitarity on scattering at finite energy to place a stronger lower limit on the direct detection cross section. In addition, we improve the treatment of the thermal freeze-out by including the evolution of the dark matter temperature and its feedback onto relic abundance. In the regions where the freeze-out is dominated by resonant or semi-annihilation, the dark matter decouples kinetically from the plasma very early, around the onset of the chemical decoupling. This results in a modification of the required coupling to the Higgs, which turns out to be at most few per cent in the semi-annihilation region, thus giving credence to the standard approach to the relic density calculation in this regime. In contrast, for dark matter mass just below the Higgs resonance, the modification of the Higgs invisible width and direct and indirect detection signals can be up to a factor $6.7$. The model is then currently allowed at $56.8$ GeV to $58.4$ GeV (depending on the details of early kinetic decoupling) $\lesssim M_{S} \lesssim 62.8$ GeV and at $M_{S} \gtrsim 122$ GeV if the freeze-out is dominated by semi-annihilation. We show that the whole large semi-annihilation region will be probed by the near-future measurements at the XENONnT experiment.

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arXiv2018

Dark Matter Relic Density Revisited: The Case For Early Kinetic Decoupling

Kinetic decoupling of dark matter typically happens much later than chemical freeze-out. In fact, local thermal equilibrium is an important assumption for the usual relic density calculations based on solving the Boltzmann equation (for its 0-th moment) describing the dark matter number density. But is this assumption always justified? Here we address this question and discuss the consequences of more accurate treatments. The first treatment is relying on the inclusion of higher moments of the Boltzmann equation and the second on solving the evolution of the phase-space distribution function fully numerically. For illustration, these methods are applied to the Scalar Singlet model, often referred to as the simplest WIMP DM possibility from a model-building perspective. It is explicitly shown that even in this simple model the prediction for the dark matter abundance can be affected by as much as one order of magnitude.

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arXiv2017

Novel Spectral Features in MeV Gamma Rays from Dark Matter

Astrophysical searches for gamma rays are one of the main strategies to probe the annihilation or decay of dark matter particles. We present a new class of distinct sub-GeV spectral features that generically appear in kinematical situations where the available center-of-mass energy in such processes is just above threshold to produce excited meson states. Using a Fisher forecast with realistic astrophysical backgrounds, we demonstrate that for upcoming experiments like e-ASTROGAM and ComPair these signals can turn out to be the smoking gun in the search for particle dark matter.

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