Probing freeze-in dark matter using Bose-Einstein condensate in neutron star

arXiv:2605.22245 · hep-ph, astro-ph.CO, astro-ph.HE · Submitted 2026-05-21 · Read on arXiv

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

Vera: Hi, I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "Probing freeze-in dark matter using Bose-Einstein condensate in neutron star".

Jocelyn: Here's the gist of the paper. The gist Neutron star (NS) is one of the most promising astrophysical targets to probe non-gravitational interaction of dark matter (DM) with visible matter, and if DM particles are bosonic, they may form a Bose-Einstein condensate at the center of the NS, significantly increasing the DM density and enhancing annihilation rates Dark Matter Capture and Condensate Formation The capture rate of a NS is given by an expression involving the local DM density, where it is suppressed by limited phase space due to neutron degeneracy The thermal radius of the core, rth, is given by a formula dependent on the NS temperature TNS and DM mass mχ If DM particles are bosonic in nature, they can form a Bose-Einstein condensate (BEC) if the temperature falls below a critical value Tc The physical extension of the BEC state can be estimated using the Heisenberg uncertainty principle to yield rBEC = 9.2 × 10−6 cm mχ/100 GeV −1/2 Therefore, it is evident from Eq.(2.7) and Eq.(2.3) that rBEC ≪ rth for the DM mass range we are considering As the core of the NS cools below Tc while DM particles are accumulated in the core, the thermal DM core makes a transition to a BEC with a much smaller size Effect of Enhanced Annihilation on Temperature The evolution of TNS is governed by dTNS/dt = −ϵν + ϵγ − ϵχcV The emissivity due to DM annihilation, ϵχ, is given by a formula dependent on the annihilation cross-section and the volume The enhancement factor for the annihilation rate in the BEC state compared to the non-BEC case is Ca(BEC)/Ca(non − BEC) = rth/rBEC 3 This enhancement factor depends on the NS temperature through Eq.(3.5) As a result of this enhanced annihilation rate, the CA equilibrium is achieved with smaller values of ⟨σv⟩ compared to the non-BEC case, implying a substantial relaxation in the minimum value of the s-wave annihilation cross-section Implications for Dark Matter Parameters The study systematically examines the effect of enhanced annihilation in determining CA equilibrium, black hole formation, and late-time heating by solving evolution equations for TNS and Nχ over a 1010 year lifetime In Fig.

Vera: 3, the BEC scenario NS surface temperature remains above the JWST sensitivity while the non-BEC case remains unobservable The minimum ⟨σv⟩ needed to establish CA equilibrium is reduced by a factor of O(1010−1015) compared to the scenario without BEC formation and enhanced annihilation This implies that feebly interacting DM models with very small annihilation cross-sections can be probed using JWST observation Concrete Model Realization A concrete dark sector model is introduced involving a real scalar singlet, χ, and a scalar mediator, ϕ with trilinear interaction and Yukawa interaction terms Sufficient DM-neutron elastic scattering cross-section, i.e., σχn ∼ O(10−35 − 10−55) cm2 and feeble DM annihilation cross-section, i.e., ⟨σv⟩ ∼ O(10−40−10−80) cm3/s − 1 is achievable within this realistic setup The thermal average of the resonant part of the cross-section, ⟨σv⟩ res, can be found analytically This is important for the viability of the phenomenological results discussed in the previous section The BEC formation ensures that NS surface is ‘bright’ enough to be observed with a telescope which would not be possible otherwise Moreover, this is possible with a much smaller DM annihilation and DM-neutron scattering cross-sections that are not ruled out by indirect and direct detection experiments, respectively The JWST sensitivity to ⟨σv⟩ improves down to ⟨σv⟩ as small as 10−72 cm3/s − 1 with enhanced annihilation This opens up the DM parameter space The combination of deep-observation telescopes with wide sky-survey facilities will be able to observe more old NSs and enable us to study their thermal evolution Page 1 --- The gist Neutron star (NS) is one of the most promising astrophysical targets to probe non-gravitational interaction of dark matter (DM) with visible matter, and if DM particles are bosonic, they may form a Bose-Einstein condensate at the center of the NS, significantly increasing the DM density and enhancing annihilation rates Concrete Model Realization A concrete dark sector model is introduced involving a real scalar singlet, χ, and a scalar mediator, ϕ with trilinear interaction and Yukawa interaction terms Sufficient DM-neutron elastic scattering cross-section, i.e., σχn ∼ O(10−35 − 10−55) cm2 and feeble DM annihilation cross-section, i.e.

Paper summary: Vera: Alright team. We're looking at the paper "Probing freeze-in dark matter using Bose-Einstein condensate in neutron star," and it starts by saying neutron stars are a fantastic place to check if dark matter interacts with normal matter in a non-gravitational way.

Jocelyn: Exactly. The paper focuses on the case where the dark matter particles are bosonic, because that's what lets them form a Bose-Einstein condensate at the center of the star if they get packed in there.

Subrahmanyan: They set up this scenario where this condensate formation significantly boosts the local dark matter density, and that boost is what drives the enhanced annihilation rate they see in their model.

Vera: The main point they make is that this enhanced annihilation makes the neutron star surface hotter than it would be in a standard cooling scenario, and they quantify that enhancement factor as O(one thousand fifteen to one thousand twenty) if dark matter forms a condensate without big self-interaction.

Jocelyn: They show that the physical size of this BEC state, r BEC, is much smaller than the thermal radius of the core, r th, which depends on the square root of the neutron star temperature, T NS.

Subrahmanyan: That size difference means that as the star cools and dark matter particles pile up in there, it transitions into this tiny BEC state with a radius that is much smaller than before.

Vera: So what they find is that this leads directly to capture-annihilation equilibrium being reached with much smaller values for the s-wave annihilation cross-section compared to when you don't have the BEC case.

Jocelyn: So if you're listening, the main idea is that a condensate completely changes the thermal picture and lets us probe dark matter even when its annihilation cross-section is quite small.

Subrahmanyan: It means this mechanism provides a way to test those dark matter scattering cross-sections that might be too small for current direct detection experiments to catch easily.

Vera: It shifts the focus away from just looking at things like gamma rays, which are annihilation products, toward looking at thermal signatures like surface temperature constraints from telescopes such as the James Webb Space Telescope.

Jocelyn: The implication is that using deep observation and sensitive telescopes could allow us to study more old neutron stars and see their thermal history in a way that probes this condensate physics.

Subrahmanyan: Ultimately, it shows that even for models with very small annihilation cross-sections, if the dark matter interacts with visible matter in a specific way leading to BEC, we still get observational constraints on those interactions.

Vera: That’s what the paper "Probing freeze-in dark matter using Bose-Einstein condensate in neutron star" is all about. It's really about how we can use the thermal evolution of old, cool neutron stars to look for evidence of dark matter interactions.

Jocelyn: The authors, Deep Ghosh and Anirban Das, are pointing out that if dark matter is a boson, the physics inside a neutron star changes dramatically when it cools enough to form a condensate.

Subrahmanyan: What this means for the bigger picture is that this mechanism provides an avenue to test dark matter scattering cross-sections that might be too small for current direct detection experiments to catch easily.

Vera: It shifts the focus from just looking at annihilation products, like gamma rays, to looking at thermal signatures like surface temperature constraints from telescopes like James Webb Space Telescope.

Jocelyn: The implication is that the combination of deep observation and sensitive telescopes could allow us to study more old neutron stars and see their thermal history in a way that probes this condensate physics.

Subrahmanyan: Ultimately, it shows that even for models with very small annihilation cross-sections, if the dark matter interacts with visible matter in a specific way leading to BEC, we still have observational constraints on those interactions.

Conclusion: Vera: So we've been talking about how these neutron star condensates change the temperature picture, and now we're looking at what this paper actually means for our search for dark matter.

Jocelyn: The authors of "Probing freeze-in dark matter using Bose-Einstein condensate in neutron star" are Deep Ghosh and Anirban Das.

Subrahmanyan: They're essentially showing how this BEC state can change the lower limits on the s-wave annihilation cross-section needed for capture to happen.

Vera: It’s about how we can test dark matter interactions that have very small annihilation cross-sections, which are usually too faint for direct detection experiments to see.

Jocelyn: They introduce a specific scalar dark sector model with a real scalar singlet, chi, and a scalar mediator, phi.

Subrahmanyan: This model allows for the DM-neutron elastic scattering cross-section to be as small as ten-thirty-five to ten-fifty-five square centimeters.

Vera: And they show that even with those tiny scattering cross-sections, if the condensate forms, we still get observable heating effects on the neutron star surface.

Jocelyn: It’s a big deal because it means JWST observations might be able to constrain these interactions that current experiments are missing.

Subrahmanyan: If you listen to this show, think about how this moves the problem from just looking at annihilation signals to thermal evolution of old stars.

Vera: That's right. So next time we talk about a paper, we look at what the authors actually managed to constrain with their measurements and error bars.

Saha Institute of Nuclear Physics · Homi Bhabha National Institute

hep-ph, astro-ph.CO, astro-ph.HE

Submitted: 2026-05-21

Updated: 2026-10-20

Comments: 25 pages, 8 figures, one new figure and a section added, matches published version in JCAP

Journal ref: JCAP10(2026)047

DOI: 10.1088/1475-7516/2026/10/047

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 76/100

The gist: The gist Neutron star (NS) is one of the most promising astrophysical targets to probe non-gravitational interaction of dark matter (DM) with visible matter, and if DM particles are bosonic, they may

Key concepts

Bose-Einstein Condensate (BEC): If dark matter particles are bosonic, they can form a BEC in the neutron star core when the temperature drops below a critical value. This concentrates the DM into a much smaller physical volume compared to the normal thermal core, leading to a significantly higher local DM density.
Enhanced Annihilation Rate: The formation of a BEC enhances the dark matter annihilation rate by a factor proportional to the ratio of the thermal radius to the BEC radius. This increased annihilation makes the neutron star surface hotter than in standard cooling scenarios, which is crucial for detection.
Neutron Star Cooling/Heating: The temperature evolution of a neutron star is determined by various energy loss and gain mechanisms, including neutrino emission and dark matter annihilation. The enhanced DM annihilation rate acts as a heating term, affecting the star's surface temperature over its lifetime.
Freeze-in Dark Matter: This scenario describes how dark matter particles are produced in the early universe through interactions with visible matter rather than thermal freeze-out. This mechanism is relevant for setting the initial conditions for DM accumulation within neutron stars.

Summary

The gist Neutron star (NS) is one of the most promising astrophysical targets to probe non-gravitational interaction of dark matter (DM) with visible matter, and if DM particles are bosonic, they may form a Bose-Einstein condensate at the center of the NS, significantly increasing the DM density and enhancing annihilation rates

Dark Matter Capture and Condensate Formation

The capture rate of a NS is given by an expression involving the local DM density, where it is suppressed by limited phase space due to neutron degeneracy The thermal radius of the core, rth, is given by a formula dependent on the NS temperature TNS and DM mass mχ If DM particles are bosonic in nature, they can form a Bose-Einstein condensate (BEC) if the temperature falls below a critical value Tc The physical extension of the BEC state can be estimated using the Heisenberg uncertainty principle to yield rBEC = 9.2 × 10−6 cm mχ/100 GeV −1/2 Therefore, it is evident from Eq.(2.7) and Eq.(2.3) that rBEC ≪ rth for the DM mass range we are considering As the core of the NS cools below Tc while DM particles are accumulated in the core, the thermal DM core makes a transition to a BEC with a much smaller size

Effect of Enhanced Annihilation on Temperature

The evolution of TNS is governed by dTNS/dt = −ϵν + ϵγ − ϵχcV The emissivity due to DM annihilation, ϵχ, is given by a formula dependent on the annihilation cross-section and the volume The enhancement factor for the annihilation rate in the BEC state compared to the non-BEC case is Ca(BEC)/Ca(non − BEC) = rth/rBEC 3 This enhancement factor depends on the NS temperature through Eq.(3.5) As a result of this enhanced annihilation rate, the CA equilibrium is achieved with smaller values of ⟨σv⟩ compared to the non-BEC case, implying a substantial relaxation in the minimum value of the s-wave annihilation cross-section

Implications for Dark Matter Parameters

The study systematically examines the effect of enhanced annihilation in determining CA equilibrium, black hole formation, and late-time heating by solving evolution equations for TNS and Nχ over a 1010 year lifetime In Fig. 3, the BEC scenario NS surface temperature remains above the JWST sensitivity while the non-BEC case remains unobservable The minimum ⟨σv⟩ needed to establish CA equilibrium is reduced by a factor of O(1010−1015) compared to the scenario without BEC formation and enhanced annihilation This implies that feebly interacting DM models with very small annihilation cross-sections can be probed using JWST observation

Concrete Model Realization

A concrete dark sector model is introduced involving a real scalar singlet, χ, and a scalar mediator, ϕ with trilinear interaction and Yukawa interaction terms Sufficient DM-neutron elastic scattering cross-section, i.e., σχn ∼ O(10−35 − 10−55) cm2 and feeble DM annihilation cross-section, i.e., ⟨σv⟩ ∼ O(10−40−10−80) cm3/s − 1 is achievable within this realistic setup The thermal average of the resonant part of the cross-section, ⟨σv⟩ res, can be found analytically This is important for the viability of the phenomenological results discussed in the previous section

Conclusion and Future Outlook

The BEC formation ensures that NS surface is ‘bright’ enough to be observed with a telescope which would not be possible otherwise Moreover, this is possible with a much smaller DM annihilation and DM-neutron scattering cross-sections that are not ruled out by indirect and direct detection experiments, respectively The JWST sensitivity to ⟨σv⟩ improves down to ⟨σv⟩ as small as 10−72 cm3/s − 1 with enhanced annihilation This opens up the DM parameter space The combination of deep-observation telescopes with wide sky-survey facilities will be able to observe more old NSs and enable us to study their thermal evolution

--- Page 1 ---

Concrete Model Realization

Improvements for AI systems

Bold Header: Enhanced Dark Matter Capture Rate Modeling

This allows AI systems to accurately model DM capture rates in neutron stars by incorporating detailed scattering cross-sections derived from a scalar portal model, specifically using Eq.(2.1) and Eq.(2.2), leading to more precise predictions for the DM capture rate of a NS of mass MNS and radius RNS.

Bold Header: Bose-Einstein Condensate (BEC) Dynamics Simulation

AI systems can simulate the transition from a thermal dark core to a BEC state by solving the evolution equations for temperature TNS and DM number Nχ, as described in Eq.(3.1) and Eq.(3.4), enabling the prediction of when "the core temperature falls below the critical temperature, i.e., TNS < Tc."

Bold Header: Enhanced Annihilation Rate Calculation

The system can calculate the enhanced annihilation rate by applying the enhancement factor derived from BEC formation, specifically using Eq.(3.5), which states that The enhancement factor is given by Ca(BEC) / Ca(non-BEC) = rth / rBEC3.

Bold Header: Constraining Dark Matter Parameters via JWST Observability

AI systems can determine the constraints on DM annihilation cross-section and scattering cross-section by solving the coupled equations for a 1010-year-old NS lifetime, specifically to find the lower limit on ⟨σv⟩ for achieving the CA equilibrium and BH formation in a 1010 year-old NS.

Bold Header: Probing Low Cross-Section DM Scenarios

The system can probe feebly interacting DM models by using the derived constraints that the minimum value of the s-wave annihilation cross-section to reach the detection threshold of JWST becomes much smaller compared to the canonical WIMP scenario.

Abstract

Neutron star (NS) is one of the most promising astrophysical targets to probe non-gravitational interaction of dark matter (DM) with visible matter. Their compactness makes them an ideal object which can capture particle DM efficiently over its lifetime using the DM-nucleon scattering cross-section. If DM particles are bosonic, then the captured DM population may form a Bose-Einstein condensate at the center of the NS, increasing the DM density significantly. In this work, we study the phenomenology of such scenario with enhanced DM annihilation rate due to the increased density in a condensate. The enhanced DM annihilation makes the NS surface `hotter' than in the standard cooling scenario. We show that the annihilation rate is enhanced by a factor of O(10 15-10 20) if DM forms a condensate without large self-interaction, and DM with freeze-in value annihilation cross-section can heat up the NS to higher temperatures, bringing it within the reach of James Webb Space Telescope. It also allows us to probe DM-nucleon scattering cross section within the neutrino fog regime which will complement the terrestrial direct detection searches. Moreover, the enhanced annihilation from the condensate changes the lower limits on s-wave DM annihilation cross-section for capture-annihilation equilibrium and the formation of a black hole inside the NS. Finally, we show an example of a scalar DM model where such small annihilation and DM-nucleon scattering cross sections can generically arise.