Neutron Stars Exclude Light Dark Baryons

Exotic particles carrying baryon number and with a mass of the order of the nucleon mass have been proposed for various reasons including baryogenesis, dark matter, mirror worlds, and the neutron lifetime puzzle. We show that the existence of neutron stars with a mass greater than $0.7M_{\odot }$ places severe constraints on such particles, requiring them to be heavier than 1.2 GeV or to have strongly repulsive self-interactions.

Figure 1

Hybrid EOS and underlying nuclear EOS. The standard nuclear matter is shown as dash-dotted curves. The stiff EOS makes a second-order transition to a causal EOS at $n_B=1.5n_s$. This is the stiffest possible EOS and predicts a maximum mass of $\simeq 3.3M_{\odot }$ (Fig. 2). Adding a dark baryon with $m_{\chi }=938\mathrm{MeV}$ results in solid curves, which are dominated by $\chi$’s Fermi gas EOS for $\epsilon \gtrsim 0.1\mathrm{MeV}/{\mathrm{fm}}^3$. Dotted lines show a hybrid EOS with $m_{\chi }=1.2\mathrm{GeV}$. All curves are truncated at maximum central densities inside stable neutron stars.

Figure 2

The mass-radius relationship generated using the EOS in Fig. 1. Even for the extremely stiff EOS, the maximum mass of hybrid stars containing noninteracting dark neutrons does not exceed $0.8M_{\odot }$. The measured masses of the two most massive neutron stars $\mathrm{J}0348+0432$ and $\mathrm{J}1614-2230$ are also shown.