There is fresh experimental proof that chiral symmetry is restored at high matter densities.

Non-zero expectation values of condensates like gluons and quark–antiquark pairs are what theoretically define the QCD vacuum, which is the ground state of vacuum in the quantum chromodynamics regime. According to physics theory, this state is filled with what are called condensates, which have the same quantum numbers as the vacuum but cannot be directly observed. Instead of being associated with a lack of particles and interactions in an empty space, this state is thought to be filled with them.

While numerous hypothetical physicists have talked about the properties of the QCD vacuum, tentatively approving these hypothetical forecasts has so far demonstrated testing, just on the grounds that the condensates in this state are slippery and can’t be straightforwardly distinguished. The theoretical predictions of the QCD vacuum’s properties hint at experimental “observation.”

According to theories, the partial restoration of the so-called chiral symmetry may result in a decrease in the condensate at high temperatures and/or high matter densities. Some researchers took measurements at particularly high temperatures during ultra-relativistic head-on collisions of heavy ions to support these hypotheses. Different endeavors in this space have attempted to test the properties of the QCD vacuum by estimating purported “medium impacts.” The presence of high matter densities, such as nuclear matter, causes these effects, which alter the QCD vacuum’s structure.

“The ‘non-trivial’ structure of the vacuum has long been the subject of theoretical discussion. Nambu, for instance, described the vacuum’s spontaneous symmetry collapse. Although there are numerous ideas that are connected, there is just a small amount of experimental data.

Kenta Itahashi, one of the researchers who carried out the study,

Recently, researchers at the German Heavy Ion Research Institute, the RIKEN Nishina Center for Accelerator-Based Science, Nara Women’s University, and other institutes around the world set out to collect experimental insights into medium effects in nuclei at lower temperatures. They measured the states of bound systems made up of a pion and an atomic nucleus in (Sn) pionic atoms using spectroscopy techniques in their experiments, which are described in detail in a Nature Physics paper.

According to Kenta Itahashi, one of the study’s authors, “The existence of the hidden structure of the vacuum is one of the most important physics questions of the modern era.” The ‘non-minor’ design of the vacuum has been hypothetically examined for quite a while. For instance, Nambu talked about how the vacuum breaks its symmetry on its own. There are a lot of related theories, but there isn’t much experimental evidence in this area right now.”

The recent research carried out by Itahashi and his colleagues had as its primary objective the clarification of the occult structure of the QCD vacuum and its evolution throughout the universe’s history. The vacuum’s chiral symmetry would be broken by the condensation of quark–antiquark pairs—chiral condensates—in this vacuum state, as predicted by theoretical models.

The chiral symmetry would be partially restored at high temperatures and/or matter densities, which would theoretically lower the expected value of chiral condensates. In their new tests, the group set off to determine the normal worth of quark-antiquark matches in the QCD vacuum by estimating pionic particles at high densities and lower temperatures with high-accuracy spectroscopy procedures.

“We estimated pion-atomic bound frameworks in a spectroscopic manner,” Itahashi made sense of. “As a result, our spectroscopy provides additional data that can be compared to previous experimental results focusing on head-on collisions. We wanted to draw a phase diagram of the vacuum on a plane of temperature and density, just like we did for superconducting materials or water. Nuclear matter behaves in a way like an impurity loaded into a vacuum.”

According to Nambu’s theory, the spontaneous breakdown of the QCD vacuum’s chiral symmetry was confirmed by the researchers’ measurements. The current understanding of the QCD vacuum, the breaking and restoring of chiral symmetry, and how this affects the expected value of chiral condensates at high temperatures and/or high matter densities is advanced in conjunction with the findings of a groundbreaking study they carried out nearly two decades ago.

Itahashi stated, “As far as we know, there is currently no information on the order parameter at a high-matter density that was as precisely determined as ours.” We intend to investigate the density dependence of the chiral symmetry in our subsequent research. The first point of the chiral order parameter has already been plotted on the density axis, and we now plan to study the density derivative by taking systematic measurements. In addition, we intend to develop a new method for pionic atom spectroscopy with the goal of increasing precision and making it possible to study the formation of pionic atoms using radioisotopes.

More information: Takahiro Nishi et al, Chiral symmetry restoration at high matter density observed in pionic atoms, Nature Physics (2023). DOI: 10.1038/s41567-023-02001-x

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