| The discovery of the 2M(?) neutron stars requires that the neutron star maximum mass, predicted by the theoretical models, be larger than 2M(?). However, with the increase of nuclear matter density, phase transitions may occur, producing new degrees of freedom, such as hyperons and quarks. This results in the softening of the equation of state (EOS) and the decrease of the neutron star maximum mass with its value well below 2M(?). Thus, the aim of this work is to explore the mechanisms that stiffen the EOS and enable the neutrons star maximum mass not less than 2M(?).The phase transitions considered in this work include the nucleon-hyperon and hadron-quark transitions. We adopt the relativistic mean-field models NL3 and TM1 to describe hadron matter, and the MIT bag model for quark matter. The hadron-quark phase transition is regarded as the first-order transition, and we use the Gibbs construction to describe the mixed phase which meets the mechanical and chemical equilibriums.We have investigated the influence of the transition density from the hadron phase to the quark phase on the EOS of neutron star matter and the properties of neutron stars. With the increase of the transition density, the maximum mass of hybrid neutron stars increases. We have also studied the correction of the perturbative QCD to the MIT bag model, and found that the perturbative QCD correction can enhance the neutron star maximum mass. In addition, we have taken into account the light vector U-boson and considered its weak coupling with the fermions including baryons and quarks. In this case, the neutron star can be as massive as 2M(?). At last, we have investigated the influence of hyperons on the properties of neutron stars. The appearance of hyperons reduces the neutron star maximum mass significantly. For hybrid neutron stars that include hyperons and quark phases, the perturbative QCD correction and the coupling to the U boson can also enhance the neutron star maximum mass explicitly. |
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