A1 collaboration spectrometers at the microtron at Mainz University. Photo : Alexander Sell, JGU
Physicists from the Johannes Gutenberg University of Mainz have shown that theoretical models that satisfactorily describe the ground states of atomic nuclei do not work satisfactorily for excited states. According to the journal Physics Today, for this, scientists conducted high-precision measurements of the scattering of electrons on helium nuclei, identifying the resonance corresponding to the first excited state of the helium nucleus, and comparing its nature with theoretical calculations.
The results of the experiment differed significantly from the theory, which means that the current theoretical models of nuclear interactions must be refined. In particular, this may force scientists to take a different look at the exotic matter in the depths of neutron stars.
Physicists from the A1 collaboration, working at the Mainz microtron , directed an electron beam at a helium sample, which was in an aluminum capsule, and analyzed the energy and direction of scattering of scattered particles with magnetic spectrometers. Only one electron out of 10,000 excited the atomic nucleus, the rest participated in elastic scattering on the nucleus or on the aluminum walls of the capsule and were of no interest to scientists. Physicists have carefully subtracted this "noise" in order to access the signal they need from excited atomic nuclei and leave only the resonance between the ground and excited states they need in their data.
- A microtron is a kind of electron accelerator. The magnetic spectrometers of the microtron can be compared with an electron microscope: at these energies, in comparison with nuclear scales, the electrons in the beam exhibit wave properties, and the properties of scattered electrons can be used to draw conclusions about the structure of the objects under study.
As a result of this experiment, the scattering cross section ( a value that shows how strongly and in which directions a given object deflects particles incident on it ) of electrons on helium nuclei was determined. The scattering cross section near the corresponding resonance was recalculated by physicists into the form factor ( a value that contains information about the structure of the atomic nucleus ) of the transition of helium to the first excited state. These results were then compared with theoretical data.
The theory that allows one to calculate the properties of an atomic nucleus is called chiral effective field theory . It describes the interactions between nucleons – protons and neutrons. It can be used to predict the properties of both the ground state and the excited state. The prediction of the properties of the ground state using this theory was carried out with an accuracy of 1%.
However, in 2013, in their calculations, Sonia Bacca (Sonia Bacca) from the University of Mainz named after Johannes Gutenberg and her colleagues found that, in comparison with experimental measurements of the 1970s, the theory predicts the properties of an excited atomic nucleus poorly (in their calculations, Bakca and colleagues used the nucleon -nucleon and three-nucleon potentials predicted by the chiral effective theory). The old measurements were very inaccurate, so a refinement experiment was needed.
As a result of the current, much more accurate measurements of the form factor of the transition of excited helium nuclei, carried out by the Mainz group, it was shown that the discrepancy with the theory remains significant: the form factor of the transition turned out to be almost half the theoretical expectations. This shows that the theoretical modeling of the atomic nucleus by the chiral effective theory is far from ideal.
The results of the Mainz experiments are important not only for comparing theory and experiment in "terrestrial" nuclear physics. Chiral effective field theory is also applicable to describe the exotic state of matter in the interiors of neutron stars. This stuff, hot and dense, prevents a neutron star from turning into a black hole. Therefore, without a satisfactory theory, it is impossible to understand processes occurring on an astronomical scale, such as the merger of neutron stars.