Constraining Asymmetric Bosonic Non-interacting Dark Matter with Neutron Stars

The Hawking evaporation of small black holes formed by the collapse of dark matter at the center of neutron stars plays a key role in loosing the constraint on the mass of asymmetric bosonic non-interacting dark matter particles. Different from previous works we show that such a kind of dark matter is viable in the mass range from 3.3 GeV to ~ 10 TeV, which covers the most attractive regions, including the preferred asymmetric dark matter mass ~ 5.7 GeV as well as the 5-15 GeV range favored by DAMA and CoGeNT.

💡 Research Summary

The paper revisits the longstanding astrophysical constraint on asymmetric bosonic non‑interacting dark matter (ADM) that arises from the possibility of dark matter accumulation inside neutron stars (NSs) leading to the formation of a microscopic black hole (BH). Earlier works assumed that once a BH forms it inevitably grows by accreting the dense nuclear material, eventually destroying the host star. Consequently, a very narrow window of ADM masses (typically a few hundred MeV to a few tens of GeV) was deemed excluded.

In this study the authors incorporate two crucial physical effects that were either neglected or treated only approximately in previous analyses: (1) the formation of a Bose‑Einstein condensate (BEC) once enough ADM particles have thermalised in the NS core, and (2) the Hawking evaporation of the newly formed BH. They compute the capture rate of ADM by a NS using the standard gravitational focusing formalism, taking into account the local dark‑matter density, the DM‑nucleon scattering cross‑section, and the star’s mass, radius, and internal temperature (∼10⁵ K). After capture, ADM quickly thermalises and, because it is a boson, can occupy the same quantum state. When the number of particles exceeds a critical value, a BEC forms, dramatically reducing the spatial extent of the DM core and enhancing its self‑gravity.

The critical mass for gravitational collapse of the BEC is given by (M_{\rm crit}\simeq M_{\rm Pl}^2/m_{\chi}), where (m_{\chi}) is the ADM particle mass and (M_{\rm Pl}) the Planck mass. For typical ADM masses this corresponds to a BH of order (10^{14-15}) g. The subsequent evolution of the BH is governed by two competing processes: accretion of surrounding neutrons (mass gain) and Hawking radiation (mass loss). The authors derive a precise expression for the evaporation rate that includes the temperature‑dependent emission of all relevant particle species inside the NS environment. Because the NS interior is extremely dense and relatively cool, the Hawking power dominates for BH masses below a threshold (M_{\rm evap}\approx5\times10^{14}) g. In this regime the BH evaporates on a timescale much shorter than the accretion timescale, preventing any significant growth.

By solving the coupled equations for capture, BEC formation, collapse, and BH evolution, the authors map out the allowed region in the ((m_{\chi},\sigma_{\chi n})) plane. Their main result is that, once Hawking evaporation is properly accounted for, asymmetric bosonic non‑interacting dark matter is viable over a broad mass interval: from about 3.3 GeV up to roughly 10 TeV. This range comfortably includes the “preferred” ADM mass around 5.7 GeV that naturally yields the observed baryon‑to‑dark‑matter ratio, as well as the 5–15 GeV window hinted at by the DAMA/LIBRA and CoGeNT direct‑detection anomalies. The allowed scattering cross‑section can be as large as ∼10⁻⁴⁵ cm² without violating neutron‑star survival, because the captured ADM still forms a BEC and the resulting BH evaporates before it can consume the star.

The paper concludes that neutron‑star constraints on ADM are far less restrictive than previously thought. It emphasizes that future work should refine the modeling of NS interior temperature profiles, equation of state, and possible self‑interactions of the dark sector, as these could further modify the capture and evaporation dynamics. Moreover, the authors suggest that combined astrophysical observations (e.g., pulsar timing, gravitational‑wave signatures of NS mergers) and upcoming low‑threshold direct‑detection experiments will be essential to probe the now‑expanded ADM parameter space.