In collisions of argon and scandium atomic nuclei, scientists from the international NA61/SHINE experiment have observed a clear anomaly indicative of a violation of one of the most important symmetries of the quark world: the approximate flavor symmetry between up and down quarks.
The presence of this anomaly might be attributed to previously unrecognized flaws in our present nuclear collision models; however, we can’t dismiss the possibility that it could be linked to the much sought-after “new physics.”
If we were to assemble a structure using the same number of wooden and plastic blocks, we would expect the proportions between the blocks of the two types not to alter after it has been dismantled. Physicists have so far lived in the belief that a similar symmetry of the initial and final states, called flavor symmetry, occurs in collisions between particles containing up and down quarks.
However, a different picture of reality emerges from a paper
published
in
Nature Communications
.
A fascinating discovery with substantial implications was reported by the NA61/SHINE collaboration, where many contributors are Polish scientists, including members affiliated with the Institute of Nuclear Physics at the Polish Academy of Sciences (IFJ PAN) located in Kraków.
The team studied collisions between argon and scandium nuclei accelerated by the Super Proton Synchrotron (SPS)—the same accelerator that is also responsible for the final phase of accelerating protons before injecting them inside the Large Hadron Collider (LHC) at CERN near Geneva.
“According to the current state of knowledge, the world of matter we perceive is mainly made up of elementary particles called quarks. They come in six types, each having its antimatter counterpart. Protons and neutrons, the basic constituents of atomic nuclei, are composed of triplets of—always mixed—up and down quarks, while quark-antiquark pairs are called mesons,” Prof. Andrzej Rybicki (IFJ PAN) explains.
The force that binds quarks together to form protons, neutrons, or mesons is known as the strong interaction, which is explained through a theoretical framework called quantum chromodynamics. According to the equations within this theory, were all types of quarks to have identical masses, the strong interaction wouldn’t differentiate between them. However, various flavors of quarks exhibit considerable differences in mass, disrupting this symmetry.
What becomes crucial, however, is that the two lightest types of quarks—the previously mentioned up and down quarks—differ little in their masses. Strong interactions therefore do not treat them in exactly the same manner, but similarly enough to speak of the existence of an approximate flavor symmetry.
In nuclear research, the importance of this symmetry is significant. It is what makes it known that if a high-energy collision involving up quarks produces some secondary particles with a given probability, then with almost the same probability other corresponding secondary particles would be produced in a collision in which down quarks would be present (and vice versa).
The NA61/SHINE collaboration focused on investigating K mesons (kaons) that emerge in different forms during high-energy collisions involving argon and scandium atomic nuclei. Initially, the team intended to measure solely the electrically charged kaons. Although it was understood that short-lived neutral kaons, lacking an electrical charge, were generated as well in these collisions, recording their presence seemed unnecessary at first.
After all, it was clear from the flavor symmetry that, when negative kaons and positive kaons were added, the result should correspond with the number of neutral kaons to a good approximation. In the end, however, the group decided to carry out measurements of kaons of all types—and this was a great success.
The findings released by our group have turned out to be statistically notably distinct from earlier theoretical forecasts. Typically, it’s believed that inconsistencies in experimental data, attributed to the approximative character of flavor symmetry, remain below 3% within this particular energy spectrum. However, we document a substantial excess of charged kaons amounting up to 18%, according to Professor Rybicki.
Upon closer inspection, the observed phenomenon becomes even more fascinating. An atom of argon with a stable isotope contains 18 protons and 22 neutrons. In contrast, for scandium, a stable nucleus possesses three additional neutrons compared to the number of protons.
Protons consist of two up quarks and one down quark, whereas neutrons have the opposite composition. Therefore, basic calculations demonstrate that there were marginally more down quarks present in the examined systems prior to the collisions.
As we initially had an excess of down quarks over up quarks, one might assume that breaking this flavor symmetry would lead to observing even more down quarks post-collision. However, contrary to expectations, our analysis clearly indicates that the symmetry was broken in the opposite way, resulting in a higher abundance of up quarks at the conclusion,” explains Professor Katarzyna Grebieszkow from the Warsaw University of Technology, who initiated the measurement of neutral kaons.
The causes of the detected symmetry breaking during collisions between argon and scandium atomic nuclei remain unidentified at present.
It could be that the theoretical predictions derived from quantum chromodynamics failed to consider certain crucial aspects of these collisions. Alternatively, there’s an even more thrilling prospect: the phenomenon we’re observing might transcend our current understanding of strong interactions as well as the Standard Model constructed upon this knowledge, suggesting that it may represent evidence of the elusive “new physics” everyone has been searching for.
Despite any future advancements, this finding holds substantial consequences for researchers focusing on high-energy particle and atomic nucleus collision experiments. In fact, the presumption regarding the presence of the particular symmetry has been extensively utilized over several decades in simulating numerous nuclear experiment processes and analyzing their outcomes.
The key observation is that we’ve identified flavor symmetry violation during collisions involving atomic nuclei. However, at present, we cannot determine if this is a general occurrence across all scenarios where quarks are involved, or if it specifically happens only under certain conditions—such as when colliding particular masses of nuclei or operating within limited ranges of collision energy,” emphasizes Professor Rybicki.
This essentially means that we need to carefully reassess almost all models related to particle creation in high-energy collisions as well as various experimental outcomes.
Over the next few months, researchers from the NA61/SHINE collaboration will start their efforts to verify flavor symmetry breaking in collisions where the initial number of up and down quarks is identical.
Dr. Seweryn Kowalski, a professor from the University of Silesia, explains that their initial emphasis will be on analyzing the numerous recordings of interactions between pi+ and pi- mesons colliding with carbon atoms, noting that these particles exhibit complete flavor symmetry before impact. He leads this research alongside Professor Eric Zimmerman from the University of Colorado Boulder as part of the NA61/SHINE project.
The following step involves examining the behavior of oxygen-oxygen and magnesium-magnesium interactions, with the magnesium-magnesium system appearing especially promising because of the intricate nature of atomic nuclei akin to those found in elements like argon and scandium. These particular interactions facilitated the discovery of the phenomenon under investigation.
The researchers indicate that we must await more intriguing outcomes: the collisions involving magnesium nuclei can only occur following the upcoming three-yearupgrade of the LHC, which is set to commence shortly.
More information:
Giacosa, F., et al., Indications of isospin symmetry breaking in high-energy collisions between atomic nuclei,
Nature Communications
(2025).
DOI: 10.1038/s41467-025-57234-6
Supplied by the Polish Academy of Sciences
The tale was initially released on
Dailyexe
. Subscribe to our
newsletter
For the most recent science and technology news updates.
