Sound velocity peak as a signature of quark matter formation

Neutron stars are cosmic laboratories to study the properties of dense QCD matter at almost zero temperature. The neutron star mass-radius relation has the one-to-one correspondence to the QCD equation of state and gives strong constraints on our understanding on the properties of the matter. Recent neutron star observations indicate similar radii for 1.4- and 2.1-solar mass neutron stars, disfavoring the existence of strong first order phase transitions in the density range of 2-5n0 (n0: nuclear saturation density). Instead, it suggests radical stiffening of matter beyond 2n0. I argue how such stiffening is achieved by treating the quark substructure of baryons. The key element is the quark Pauli blocking which occurs even before baryons substantially overlap. The saturation of quark states at low momenta leads to a peak in sound velocity that signals the onset of quark matter formation. I also discuss the hyperon puzzle in light of the quark saturation effects.