Novel features of asymmetric nuclear matter from terrestrial experiments and astrophysical observations of neutron stars

The accurate measurement of neutron skin thickness of 208 Pb by the PREX Collaboration suggests a large value of the nuclear symmetry energy slope parameter, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m1"><mml:mrow><mml:mi>L</mml:mi></mml:mrow></mml:math> , whereas the smaller <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m2"><mml:mrow><mml:mi>L</mml:mi></mml:mrow></mml:math> is preferred to account for the small neutron-star radii from NICER observations. To resolve this discrepancy between nuclear experiments and astrophysical observations, new effective interactions have been developed using relativistic mean-field models with the isoscalar- and isovector-meson mixing. We investigate the effects of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m3"><mml:mrow><mml:mi>δ</mml:mi></mml:mrow></mml:math> -nucleon coupling and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m4"><mml:mrow><mml:mi>σ</mml:mi></mml:mrow></mml:math> – <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m5"><mml:mrow><mml:mi>δ</mml:mi></mml:mrow></mml:math> mixing on the ground-state properties of finite nuclei, as well as the characteristics of isospin-asymmetric nuclear matter and neutron stars. Additionally, we explore the role of the quartic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m6"><mml:mrow><mml:mi>ρ</mml:mi></mml:mrow></mml:math> -meson self-interaction in dense nuclear matter to mitigate the stiff equation of state for neutron stars resulting from the large <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m7"><mml:mrow><mml:mi>δ</mml:mi></mml:mrow></mml:math> -nucleon coupling. It is found that the nuclear symmetry energy undergoes a sudden softening at approximately twice the saturation density of nuclear matter, taking into account the PREX-2 result, the recent NICER observation of PSR J0437 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m8"><mml:mrow><mml:mo>−</mml:mo></mml:mrow></mml:math> 4715, and the binary neutron star merger, GW170817.