Velocity of sound beyond the high-density relativistic limit from lattice simulation of dense two-color QCD

We obtain the equation of state (EoS) for two-color quantum chromodynamics (QCD) at low temperature and high density from the lattice Monte Carlo simulation. We find that the velocity of sound exceeds the relativistic limit ($c_s^2/c^2=1/3$) after BEC-BCS (Bose-Einstein condensation-Bardeen-Cooper-Schrieffer) crossover in the superfluid phase. Such an excess of the sound velocity is previously unknown from any lattice calculations for QCD-like theories. This finding might have a possible relevance to the EoS of neutron star matter revealed by recent measurements of neutron star masses and radii.