Ramanujan Auditorium

Sound velocity peak as a signature of quark matter formation in neutron stars

Toru Kojo

Tohoku University

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 2-5 times the nuclear saturation density n0. 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.