Exploring the mass radius of ⁴He and implications for nuclear structure

In this study, we determine the mass radius of He-4, a very light nucleus, by examining the near-threshold phi-meson photoproduction data of the LEPS Collaboration. To assess the gravitational form factor of He-4, we employ multiple models for the mass distribution, including Yukawa-type, exponential, Gaussian, and uniform functions. The mass radius of He-4 is measured to be 1.70 +/- 0.14 fm, which is approximately equal to the charge radius of He-4. Surprisingly, in contrast to the findings of the proton, no noticeable discrepancy between the charge radius and the mass radius is noted for the He-4 nucleus. The proton and neutron distributions within He-4 are likely to be identical, confirming its regular tetrahedral structure in a new way. We propose exploring the difference between charge and mass radii as a new approach to examine the nuclear structure.