There is new 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. Rather than being related to an absence of particles and communications in an unfilled space, the physical science hypothesis views this state as loaded up with the purported condensates, which have similar quantum numbers as the vacuum and can’t be straightforwardly noticed.

Even though many theoretical physicists have talked about the QCD vacuum’s properties, it has been hard to test these theoretical predictions in the real world because condensates in this state are hard to find. 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 supposed “medium impacts.” The presence of high matter densities, such as nuclear matter, causes these effects, which alter the QCD vacuum’s structure.

“Our spectroscopy thus provides complementary information that can be analyzed in conjunction with previous experimental findings focusing on head-on collisions,”

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 theory of the “non-trivial” structure of the vacuum has been around for a long time. 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 essential goal of the new work by Itahashi and his associates was to additionally explain the secret construction of the QCD vacuum and its development over the historical backdrop of the universe. 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. By using high-precision spectroscopy to measure pionic atoms at higher densities and lower temperatures, the team wanted to determine the expected value of quark–antiquark pairs in the QCD vacuum.

Itahashi explained, “We measured pion-nuclear bound systems in a spectroscopic way.” 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. It might be said, Atomic matter acts as a pollutant stacked into the 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.

“Supposedly, there is at present no data on the request boundary at a high-matter thickness that is pretty much not entirely set in stone as our own,” Itahashi said. “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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