From Non-Thermal to Thermal: Cosmological Imprints of Self-Interacting Secluded Dark Matter

summary

The gist

The gist Dark matter (DM), secluded from the Standard Model (SM), can attain internal thermalization through self-interactions, and this process has cosmological implications for its mass constraints.

In short

The study investigates how dark matter (DM) produced from an oscillating inflaton field thermalizes through self-interactions. Numerical solutions show that the DM temperature and chemical potential depend on inflaton mass and reheat temperature, leading to specific lower mass limits based on cosmological observations like BBN and CMB. Self-scattering primarily affects the free-streaming length, weakening the mass constraints by about 40% compared to non-thermal scenarios.

Key concepts

Thermalization via Self-Interactions: This process describes how dark matter particles, initially produced non-thermally from an inflaton decay, reach a state where their momentum distribution is thermalized through elastic collisions between the DM particles themselves. This self-interaction is crucial because it changes the DM's final temperature and chemical potential, which in turn affects cosmological constraints.
Chemical Potential ($\mu$): The chemical potential is a measure related to the net number of particles in a system, even when the total number of particles is fixed. In this study, it becomes correlated with the DM temperature and mass, depending on whether the reheat temperature is high or low, influencing how constraints are calculated.
Co-moving Free-Streaming Length: This length describes how far dark matter particles can travel freely in space before being affected by gravitational structure formation. Self-interactions suppress this length compared to a non-thermal scenario without collisions, which is important for determining the minimum allowed mass of the dark matter particle.
Reheat Temperature ($T_R$): The reheat temperature is the temperature of the universe immediately after inflation ends, which sets an initial condition for DM production. The final thermal state and resulting cosmological constraints on DM mass are directly determined by this value, showing different results for high ($5 imes 10^6$ GeV) versus low ($5$ MeV) scenarios.

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This episode discusses

The paper

From Non-Thermal to Thermal: Cosmological Imprints of Self-Interacting Secluded Dark Matter · Read on arXiv

Koushik Dutta, Deep Ghosh, Ahamadullah Khana

Department of Physical Sciences, Indian Institute of Science Education and Research · Theory Division, Saha Institute of Nuclear Physics

Dark matter (DM), secluded from the Standard Model (SM), can attain internal thermalization through self-interactions. We investigate the thermalization of a real scalar DM field with a quartic self-coupling, initially produced from the perturbative decay of the coherently oscillating inflaton field during reheating. The thermalization process has been studied numerically by solving Boltzmann equation of the DM momentum distribution function in presence of 2 to2 DM elastic self-scatterings. In the absence of number-changing processes, the thermalized dark sector is characterized by a temperature and a nonzero chemical potential, determined by the inflaton mass and the reheat temperature. We find that if the inflaton mass is greater (smaller) than the reheat temperature, the DM temperature can be greater (smaller) than the SM temperature during Big Bang Nucleosynthesis, without violating any cosmological constraints. This is realized with a negative (positive) chemical potential for the DM temperature greater (smaller) than the SM one. Depending on DM production history, we can achieve the lower limit on the DM mass much below than in warm DM scenarios. The thermalization via self-scatterings tends to suppress the DM co-moving free-streaming length, compared to the non-thermal scenario without any collisions. Consequently, the lower limit on the DM mass weakens approximately by 40% for a suitable choice of quartic self-coupling than in the non-thermal scenario. We obtain the lower limits on the thermal DM mass to be 2.2 times10-10 GeV and 0.47 GeV for reheat temperatures, 5 times 10 6 GeV and 5 MeV respectively, while the inflaton mass is set at 10 cubed GeV.

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: We're looking at the paper: "From Non-Thermal to Thermal".

Jocelyn: The gist Dark matter (DM), secluded from the Standard Model (SM), can attain internal thermalization through self-interactions, and this process has cosmological implications for its mass constraints.

Vera: First, who's behind it and why it matters.

Paper summary: Vera: We're looking at the paper "From Non-Thermal to Thermal: Cosmological Imprints of Self-Interacting Secluded Dark Matter" again, focusing on how dark matter thermalizes through self-interactions #pg4. The core idea is taking dark matter that starts out non-thermal and seeing what happens when it gets these self-interactions #pg3.

Jocelyn: The authors investigate a real scalar dark matter field with a quartic self-coupling #pg4, and they set up this thermalization process by solving the Boltzmann equation for the momentum distribution function in presence of two to two DM elastic self-scatterings #pg4.

Subrahmanyan: The big takeaway is that the resulting thermalized dark sector has a temperature and a chemical potential determined by the inflaton mass and reheat temperature #pg4. They also find that if the inflaton mass is greater than or smaller than its reheat temperature, this dark matter temperature can be higher or lower than the standard model temperature during Big Bang Nucleosynthesis without breaking any of our cosmological constraints #pg4.

Vera: That effect isn't just about the temperature; it’s tied to how they calculate the chemical potential and how that relates to the production history #pg3. The paper says that in a high reheat scenario with TR equal to five times ten to the sixth GeV, the dark matter chemical potential is always positive #pg4.

Jocelyn: And for a low reheat scenario with TR equal to five MeV, it takes a negative value in the relativistic regime #pg3. This correlation between temperature and chemical potential is driven by how the total dark matter number is fixed at production #pg3.

Subrahmanyan: But when you look at the constraints from BBN and CMB radiation, they find those limits on dark matter mass are actually independent of how strong these self-interactions are #pg4. This independence holds true for the basic bounds because the initial temperature is set by the inflaton mass and reheat temperature rather than by all those collision details #pg3.

Vera: That's what makes it interesting, because that independence breaks down when you look at the co-moving free-streaming length #pg4. The effect of elastic scatterings becomes apparent there, weakening that lower bound on the dark matter mass by about forty percent #pg4.

Conclusion: Jocelyn: So we've been looking at "From Non-Thermal to Thermal: Cosmological Imprints of Self-Interacting Secluded Dark Matter" and how it shows that the post-inflationary history and how those dark matter particles interact are what really decide what we can constrain about this secluded dark sector with a nonzero chemical potential #pg3.

Vera: The authors looked at a real scalar DM field with a quartic self-coupling, setting up this thermalization process by solving the Boltzmann equation for the momentum distribution function in presence of two to two DM elastic self-scatterings #pg4. They found that the resulting thermalized dark sector has a temperature and a chemical potential determined by the inflaton mass and reheat temperature #pg4.

Subrahmanyan: The paper shows that depending on whether the inflaton mass is bigger or smaller than its reheat temperature, this dark matter temperature can be higher or lower than the standard model temperature during Big Bang Nucleosynthesis without breaking any of our cosmological constraints #pg4. But that effect isn't just about the temperature; it’s tied to how they calculate the chemical potential and how that relates to the production history #pg3.

Jocelyn: And when you look at the constraints, they find that for BBN and CMB radiation, those limits on dark matter mass are actually independent of how strong these self-interactions are #pg4. That independence holds true for the basic bounds because the initial temperature is set by the inflaton mass and reheat temperature rather than by all those collision details #pg3.

Vera: But that's where things get interesting, because that independence breaks down when you look at the co-moving free-streaming length #pg4. The effect of elastic scatterings becomes apparent there, weakening that lower bound on the dark matter mass by about forty percent #pg4.

Subrahmanyan: So to summarize this paper, we're looking at how self-interactions shift the thermalized constraints on dark matter mass from BBN and CMB observations #pg4. We need to keep looking at these constraints because understanding how this thermalization happens is crucial for designing experiments that look for these kinds of exotic dark matter properties in the future #pg3.

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