Mirror cores show severe symmetry breaking

Katrin Wimmer et al. / Physical Review Letters, 2021

Mirror nuclei of krypton and selenium showed strong symmetry breaking. Physicists have long noticed the deviation of the energy states of these nuclei from the expected ones and have now measured the probabilities of gamma transitions in krypton, selenium and bromine. The discovered discrepancies with theoretical predictions cannot be explained within the framework of existing models. Scientists have suggested that they are due to differences in the shape of the nuclei. Article published in the journal Physical Review Letters.

Mirror nuclei are nuclei with the same mass number, which transform into each other when replacing protons in them with neutrons, and neutrons with protons. They have similar spectra of excited states, the same spin and parity values, and their masses differ by a small amount.

The existence of these nuclei can be explained by the similarity of protons and neutrons in everything except charge. For a strong interaction that binds quarks inside nucleons and other hadrons, they are absolutely identical, and differ only in one quantum characteristic – the isospin projection. Therefore, they are considered as different states of one particle with isospin ½: the proton has a projection of ½, and the neutron has ½. The slight difference in their masses – only 0.1 percent – is associated with electromagnetic interaction, considering that the symmetry violations of the strong interaction are small and do not lead to visible effects.

Physicists are studying mirror nuclei for information on isospin symmetry breaking. Until now, the main way to do this has been to analyze the energy differences between the excited states of mirror nuclei. Symmetry breaking was recently discovered mirrored pair with mass number 73: 73Sr и 73Br. These nuclei differ in the spins of the ground states, but the difference in their excitation energies is only 27 keV. This violation is very small and can be correlated with other cases.

However, mirror nuclei should not only have close energy spectra. Such nuclei are members of one isobaric multiplet – a set of nuclei with the same mass number. The charge independence of the strong interaction implies that all members of the multiplet will have the same wave functions and pure isospin states without mixing. The excitation energies do not provide information about the symmetry of the wave functions and the purity of isospin states.

Katrin Wimmer of the University of Tokyo and her colleagues used a different, more accurate way to test for isospin symmetry breaking. It is based on a matrix of electromagnetic transitions, each element of which is associated with an isoscalar or isovector transition between different states of nuclei. Knowing the elements of this matrix, you can calculate the transition probabilities. According to theoretical predictions, the probability should change linearly.

For the experiment, we chose a multiplet of nuclei with a mass number of 70: it includes the nucleus 70Br as well as mirror kernels 70Kr и 70Se. Previous studies are already identified unusual behavior of the nuclei of this multiplet, associated with differences in their Coulomb energy.

In a new experiment, physicists used an ion accelerator to produce a beam of nuclei. Then they were sent to a layer of gold foil. When colliding with target nuclei, gamma quanta were emitted, the spectrum of which was recorded and analyzed for the presence of the necessary transitions – from an excited state with spin parity 2+ to the ground state 0+… Excited state 2+ caused by a combination of electromagnetic and nuclear interactions between the nuclear flux and the target. Using a computer model, scientists separated the contributions of nuclear and electromagnetic forces, calculated the amount of deformation of the nuclei and the probability of the desired gamma transition.

These probabilities are directly related to the wave functions of nuclei. Compared to the excitation energies, they depend more strongly on how the number of neutrons and protons affect the wave functions.

To ensure that the analysis was correct, physicists compared the new values ​​with the values ​​from previous studies for 70Se и 70Br, as well as for several close nuclei. For 70Kr such measurements were carried out for the first time.

A linear decrease in the values ​​of isovector matrix elements from 70Se к 70Kr. But the obtained value for krypton deviated significantly from the linear approximation. The deviation was 3σ, which means that it cannot be explained by random fluctuations.

The mixing of states with isospins of 0 and 1 contributes to the deviation from the linear trend. But mixing alone could not cause the observed changes in the transition probabilities. A possible explanation could be deformation 70Kr relative to its mirror core 70Se. The existing theoretical calculations, even taking into account slight deformation, cannot describe the obtained values ​​of the probabilities and their strong increase for 70Kr. It is necessary to create a new theoretical model.

The study of the breaking of various symmetries is an area of ​​interest for many researchers. For example, recently physicists calculated the probability of the decay of parapositronium with charge parity violation and developed a method for registering spatial parity violation in molecules.

Ekaterina Nazarova

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