Derivation of transition density from the observed 4He(e, e′)4He(02+) form factor raising the α-particle monopole puzzle

Abstract

Abstract Recently, the monopole transition form factor of the electron-scattering excitation of the $0^+_2$ state (Ex = 20.21 MeV) of the 4He nucleus was observed over a broad momentum transfer range ($0.5 \le q^2 \le 5.0 \, {\rm fm}^{-2}$) with dramatically improved precision compared with older sets of data; modern nuclear forces, including those derived from the chiral effective field theory, failed to reproduce the form factor, which is called the α-particle monopole puzzle. To resolve this puzzle by improving the study of the spatial structure of the $0^+_2$ state, we derive in this letter a possible $0^+_1\!\rightarrow \! 0^+_2$ transition density ρtr(r) for r ≳ 1 fm from the observed form factor. The shape of the transition density is significantly different from that obtained theoretically in the literature.