2015ApJ...800..141Z

Isospin-violating dark matter (IVDM), which couples differently with protons and neutrons, provides a promising mechanism to ameliorate the tension among recent direct detection experiments. Assuming dark matter (DM) is non-interacting bosonic asymmetric IVDM, we investigate how the existence of old neutron stars limits the DM-proton scattering cross-section σ_p, especially the effects of the isospin-violating DM-nucleon interactions and the symmetry energy in the equation of state (EOS) of isospin asymmetric nuclear matter. Our calculations are completely based on general relativity and the structure of neutron stars is obtained by solving the Tolman-Oppenheimer-Volkoff equations with nuclear matter EOS constrained by terrestrial experiments. We find that, by considering the more realistic neutron star model rather than a simple uniform neutron sphere as usual, the σ_p bounds from old neutron stars can be varied by more than an order of magnitude depending on the specific values of the DM neutron-to-proton coupling ratio f_n/f_p, and they can be further varied by more than a factor of two depending on the density dependence of the symmetry energy. In particular, we demonstrate that the observed nearby isolated old neutron star PSR B1257+12 can set a very strong limit on σ_p for low-mass DM particles (≤ 20 GeV) that reaches a sensitivity beyond the current best limits from direct detection experiments and disfavors the DM interpretation of previously reported positive experimental results, including the IVDM.