Editorial: Neutron Skin Thickness in Atomic Nuclei: Current Status and Recent Theoretical, Experimental and Observational Developments

Masayuki Matsuzaki^1*Tomotsugu Wakasa²Mitko Gaidarov³Oscar Moreno⁴

• ¹Fukuoka University of Education, Munakata, Japan
• ²Kyushu University, Fukuoka, Japan
• ³Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Sofia, Bulgaria
• ⁴Complutense University of Madrid, Madrid, Spain

are highly intricate, finite quantum many-body systems ruled by strong, electro-magnetic, and weak 5 interactions. The neutron skin thickness measures how nucleons compose the nucleus and therefore it 6 is one of the most basic quantities of nuclear structure. Naturally, it correlates with the coefficients of the 7 equation of state (EOS) of nuclear matter at the limit of large nucleon number, which rule astrophysical 8 problems such as the birth, evolution, structure, and death of neutron stars.The most fundamental problem is to obtain the neutron distribution experimentally, in contrast to that 10 of proton, which can be precisely determined by electron scattering. The strong interaction probes both 11 neutrons and protons, although their contributions are hard to separate. Another approach that is expected 12 to be promising is to probe the weak charge, which is much larger for neutrons than for protons, although 13 the weak interaction is literally weak and accurate measurements are therefore challenging. It may be a consensus that the thick skin, if confirmed, of the heavy nucleus 208 Pb observed in PREX-2 32 implies large values of the slope parameter L in the EOS. If 48 Ca can be regarded as a small droplet 33 of bulk nuclear matter, its thin skin observed in CREX indicates smaller L. However, 48 Ca may not be 34 heavy enough for the bulk features to dominate. Rather, shell effects may be important. Another point that 35 should be kept in mind is that it is not the L around the saturation density, but the stiffness of the EOS at 36 higher densities, that determines the observed properties of neutron stars. Furthermore, it is pointed out 37 that there is still room for improvement in parity-violating electron scattering experiments. Due to all of 38 the above, there may be several directions to approach the problem.The first direction is to elucidate to what extent the tension can be relaxed within the present methods.