In this study, we determine the mass radius of ${}^4\mathrm{He}$, a very light nucleus, by examining the near-threshold $\varphi$-meson photoproduction data of the LEPS Collaboration. To assess the gravitational form factor of ${}^4\mathrm{He}$, we employ multiple models for the mass distribution, including Yukawa-type, exponential, Gaussian, and uniform functions. The mass radius of ${}^4\mathrm{He}$ is measured to be $1.70\pm 0.14$ fm, which is approximately equal to the charge radius of ${}^4\mathrm{He}$. Surprisingly, in contrast to the findings of the proton, no noticeable discrepancy between the charge radius and the mass radius is noted for the ${}^4\mathrm{He}$ nucleus. The proton and neutron distributions within ${}^4\mathrm{He}$ 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.

• Received 3 November 2023
• Accepted 8 December 2023

DOI:https://doi.org/10.1103/PhysRevC.109.L012201

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Published by the American Physical Society