Abstract
An old neutron star (NS) may capture halo dark matter (DM) and get heated up by the deposited kinetic energy, thus behaving like a thermal DM detector with sensitivity to a wide range of DM masses and a variety of DM-quark interactions. Near future infrared telescopes will measure NS temperatures down to a few thousand Kelvin and probe NS heating by DM capture. We focus on GeV-mass Dirac fermion DM (which is beyond the reach of current DM direct detection experiments) in scenarios in which the DM capture rate can saturate the geometric limit. For concreteness, we study (1) a model that invokes dark decays of the neutron to explain the neutron lifetime anomaly, and (2) a framework of DM coupled to quarks through a vector current portal. In the neutron dark decay model a NS can have a substantial DM population, so that the DM capture rate can reach the geometric limit through DM self-interactions even if the DM-neutron scattering cross section is tiny. We find NS heating to have greater sensitivity than multipion signatures in large underground detectors for the neutron dark decay model, and sub-GeV gamma-ray signatures for the quark vector portal model.
Highlights
A dark matter (DM)-neutron cross section of ∼ 2 × 10−45 cm2 is large enough to heat up an old neutron star to ∼ 1000 K for DM masses between GeV and PeV
We focus on GeV-mass Dirac fermion DM in scenarios in which the DM capture rate can saturate the geometric limit
We study scenarios with three aspects: (1) the DM is of GeV mass, which makes direct detection problematic, (2) the DM is a Dirac fermion, so that it matters whether the particle or the antiparticle is the DM, and (3) the DM capture rate can reach the geometric limit
Summary
DM-neutron scattering only occurs when the momentum exchange δp is larger than pF We take this Pauli blocking into account by introducing a factor ξ = min(δp/pF , 1) in the above capture rate Cs. Note that once the sum of cross sections (ξσχelnastic for χ DM, or ξσχelnastic + σχannn for χ DM) is. Larger than critical cross section, σcrit = πR2mn/M , and the sum of the capture rate and annihilation rates cannot be larger than the geometric limit, i.e., Cc + Cann ≤ Cc|geom. This is equivalent to Nn(ξσχelnastic) ≤ πR2 if DM is χ, and Nn(ξσχelnastic + σχannn) ≤ πR2 if DM is χ.
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