Measurement of the {\alpha}-particle monopole transition form factor challenges theory: a low-energy puzzle for nuclear forces?

TL;DR

This study measures the alpha-particle monopole transition form factor across a broad momentum range, revealing discrepancies with modern nuclear force theories and highlighting a low-energy puzzle in nuclear physics.

Contribution

It provides high-precision experimental data that challenge current theoretical models of nuclear forces, especially in describing alpha-particle excitations.

Findings

New high-precision electron scattering data covering broad Q^2 range.

Modern nuclear force models fail to reproduce the measured excitation form factor.

Identifies a low-energy discrepancy challenging existing nuclear theories.

Abstract

We perform a systematic study of the $\alpha$-particle excitation from its ground state $0_1^+$ to the $0_2^+$ resonance. The so-called monopole transition form factor is investigated via an electron scattering experiment in a broad $Q^2$-range (from $0.5$ to $5.0$ fm$^{-2}$). The precision of the new data dramatically superseeds that of older sets of data, each covering only a portion of the $Q^2$-range. The new data allow the determination of two coefficients in a low-momentum expansion leading to a new puzzle. By confronting experiment to state-of-the-art theoretical calculations we observe that modern nuclear forces, including those derived within chiral effective field theory which are well tested on a variety of observables, fail to reproduce the excitation of the $\alpha$-particle.