Do current astronomical observations exclude the existence of nonstrange quark stars?

Abstract

As is pointed out in a recent work (Phys. Rev. Lett. 120, 222001), quark matter may not be strange. Inspired by this argument, we use a new self-consistent mean field approximation method to study the equation of state of cold dense matter within the framework of the two-flavor NJL model. Then the mass-radius relationship of twoflavor pure quark stars is studied. In the new self-consistent mean field approximation method we introduced, there is a free parameter α, which reflects the weight of different interaction channels. In principle, α should be determined by experiments rather than the self-consistent mean field approximation itself. In this paper, thus, the influence of the variation of α on the critical chemical potential of chiral phase transition and the stiffness of the EOS are thoroughly explored. The stiffness of the EOS can be increased greatly to support a two-solar-mass pure quark star when α is greater than 0.9, because the contribution from the vector term is retained by the Fierz transformation. Our result is also within the constraints on the radius from the recent data analysis of the tidal deformability. This means that the current theoretical calculations and astronomical observations cannot rule out the possibility of a two-flavor pure quark star.

QCD phase diagram in chiral imbalance with self-consistent mean field approximation

“…If the temperature is lower than T CEP max , the crossover will become a first-order phase transition. The previous works show that when α > 0.71 [37,38], there is no CEP on the QCD phase diagram. But as we have shown in this paper that, in the chiral imbalanced system, the CEP will appear again with increasing chiral chemical potential µ 5 .…”

“…Considering the L NJL and L F are mathematically equivalent, we employ the most general Lagrangian introduced by Refs. [37,38] in this paper. The Lagrangian of the new selfconsistent mean field approximation is…”

“…As the L NJL and L F are equivalent, the Lagrangian doesn't change with α. But in the case of finite chemical potential, which can be regarded as the background vector field, we will get very different results from different α [37,38].…”

“…In fact, as the Refs. [37,38] show, there is no physical requirement under the mean field approximation to ensure that Hartree and Fock contributions should be equal.…”

“…As shown below, the above mean field approximation approach for adding with finite chemical potential µ and chiral chemical potential is not self-consistent. In this paper, we employ a new self-consistent mean field approximation [37,38] of NJL model to study the phase transition at nonzero chiral chemical potential and chemical potential. This model introduces a new free parameter α to reflect the proportion of the different channel contributions from the Fierz-transformed term and will lead to some new results.…”