New electron scattering experiment designed for the excitation of the helium nucleus raises fundamental questions about our current understanding of nuclear forces

Theoretical predictions and new experimental data on 4He, measured with great accuracy, diverge significantly from each other

24 April 2023

At the Mainz electron accelerator MAMI, the A1 Collaboration, in connection with the dissertation work of Dr. Simon Kegel, has systematically measured the excitation of an α particle ‒ the nucleus of a 4He atom ‒ from its ground state to its first excited state with unprecedented accuracy. Comparing the experimental results and recent calculations using the corresponding low-energy theory, it becomes evident that the excitation of α particles is not correctly described based on the current understanding of nuclear forces ‒ and this raises a wealth of challenging questions. The related scientific article has been published as an Editors' suggestion in the eminent journal Physical Review Letters.

The properties of an atomic nucleus, such as its size and its binding energy, are primarily determined by the nuclear interactions between the protons and neutrons within the nucleus. These interactions can be described in phenomenological terms but can also be systematically calculated using state-of-the-art concepts, whereby in particular chiral effective field theory provides a promising framework on the basis of which these can be studied. However, the larger the nucleus, the more complex the computations become. It is thus obvious that the analysis of smaller nuclei is the best way to work out various theoretical aspects and test these using experimental data. The nucleus of a 4He atom consists of just two protons and two neutrons. Given the small number of constituents, it is ideally suited for systematic investigations of this kind and is thus one of the most extensively studied atomic nuclei.

Using the MAMI accelerator, the excitation of the α particle from its ground state to the first excited state has been measured with an accuracy that has not previously been achieved. For this purpose, the team measured the so-called monopole transition form factor in an electron scattering experiment at small momentum transfers with the aim of subsequently comparing their results to the current best theoretical prediction. Their new results have much smaller uncertainties compared to previous measurements; moreover, the older data sets each covered only a portion of the momentum transfer range now measured.

Major discrepancy between experimental data and theoretical predictions

Although the form factors now extracted from experiment and theory exhibit a similar shape as function of the momentum transfer, they differ substantially by a factor of roughly 2. The results of previous measurements had already indicated that there may well be an inconsistency with theory but the experimental uncertainties were too great to allow conclusions to be drawn. Thanks to the enhanced precision of the results obtained by the Mainz team, it can now be concluded that the excitation of the α particle cannot be accurately reproduced using the currently available description of nuclear forces.

"Our experiment was performed with very good control of systematic uncertainties. The disagreement with the best theoretical calculations is thus a serious indication that either an important aspect of the nuclear interactions is overlooked which is particularly evident in this monopole transition, or that the properties of the first excited state of the α particle depend very strongly on minute details of the nuclear forces. Both possibilities are of considerable interest and inspire us to further studies," explains Professor Concettina Sfienti, corresponding author of the article.

Follow-up investigations in Mainz in the hope of finding the answer to the puzzle

Indeed, the MESA accelerator which is currently under construction on the Mainz campus will offer excellent opportunities for follow-up experiments. In the energy recovery mode, MESA will provide an electron beam of remarkable intensity, which is brought to collision with a gas jet target at the MAGIX experiment. The resultant scattered particles will be detected by magnetic spectrometers optimized for low energies. This will allow measurements at even lower momentum transfers than achieved with the A1 setup.

From the theoretical side, too, it is planned to shed light on this low-energy puzzle for the nuclear forces. Thus, the calculations of the transition form factor performed in the group of Professor Sonia Bacca, also from the Johannes Gutenberg University Mainz (JGU), are to be systematically improved and studied in detail in the framework of chiral effective field theory.