Hyperon puzzle, hadron-quark crossover and massive neutron stars

Kota Masuda; Tetsuo Hatsuda; Tatsuyuki Takatsuka

doi:10.1140/epja/i2016-16065-6

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Hadrons and Nuclei

Exotic Matter in Neutron Stars

Eur. Phys. J. A (2016) 52: 65
https://doi.org/10.1140/epja/i2016-16065-6

Review

Hyperon puzzle, hadron-quark crossover and massive neutron stars

Kota Masuda¹^,2^*, Tetsuo Hatsuda²^,3 and Tatsuyuki Takatsuka²

¹ Department of Physics, The University of Tokyo, 113-0033, Tokyo, Japan
² Theoretical Research Division, Nishina Center, RIKEN, 351-0198, Wako, Japan
³ Kavli IPMU (WPI), The University of Tokyo, 277-8583, Chiba, Japan

^* e-mail: masuda@nt.phys.s.u-tokyo.ac.jp

Received: 19 August 2015
Revised: 5 March 2016
Accepted: 7 March 2016
Published online: 24 March 2016

Abstract

Bulk properties of cold and hot neutron stars are studied on the basis of the hadron-quark crossover picture where a smooth transition from the hadronic phase to the quark phase takes place at finite baryon density. By using a phenomenological equation of state (EOS) “CRover”, which interpolates the two phases at around 3 times the nuclear matter density ( $\rho_{0}$ , it is found that the cold NSs with the gravitational mass larger than $2M_{\odot}$ can be sustained. This is in sharp contrast to the case of the first-order hadron-quark transition. The radii of the cold NSs with the CRover EOS are in the narrow range $(12.5 \pm 0.5)$ km which is insensitive to the NS masses. Due to the stiffening of the EOS induced by the hadron-quark crossover, the central density of the NSs is at most 4 $\rho_{0}$ and the hyperon-mixing barely occurs inside the NS core. This constitutes a solution of the long-standing hyperon puzzle. The effect of color superconductivity (CSC) on the NS structures is also examined with the hadron-quark crossover. For the typical strength of the diquark attraction, a slight softening of the EOS due to two-flavor CSC (2SC) takes place and the maximum mass is reduced by about $0.2M_{\odot}$ . The CRover EOS is generalized to the supernova matter at finite temperature to describe the hot NSs at birth. The hadron-quark crossover is found to decrease the central temperature of the hot NSs under isentropic condition. The gravitational energy release and the spin-up rate during the contraction from the hot NS to the cold NS are also estimated.