Flavor Symmetry Anomaly Detected in High-Energy Nuclear Collisions

Flavor Symmetry Anomaly Detected in High-Energy Nuclear Collisions
by Robert Schreiber
Berlin, Germany (SPX) May 12, 2025

An international team of physicists working on the NA61/SHINE experiment has discovered a striking violation of a fundamental quark-level symmetry known as flavor symmetry during high-energy nuclear collisions. This unexpected observation, made in collisions of argon and scandium atomic nuclei, challenges long-held assumptions about the behavior of quarks under extreme conditions and may hint at new physics beyond the Standard Model.

The research, led in part by scientists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow, was recently published in the journal Nature Communications. The findings reveal a significant deviation from theoretical predictions, suggesting that current models of high-energy nuclear interactions might need substantial revisions.

The concept of flavor symmetry arises from the similarity in mass between the two lightest quarks - up and down quarks - which are the primary building blocks of protons and neutrons. In principle, collisions involving particles composed of these quarks should produce secondary particles with roughly equal probability, reflecting this approximate symmetry. However, the latest results from the NA61/SHINE experiment indicate a substantial imbalance.

"Our measurements show a statistically significant overproduction of charged kaons, reaching as high as 18%, compared to previous theoretical expectations, which usually assume discrepancies no greater than 3% in this energy range," said Prof. Andrzej Rybicki of IFJ PAN. "This finding suggests that something fundamental is missing in our understanding of these high-energy processes."

The study focused on the production of kaons - particles made of quark-antiquark pairs - in collisions between argon and scandium nuclei. Initially, the team expected that measurements of neutral kaons would simply confirm the flavor symmetry assumption. Instead, they found a pronounced imbalance, with far more up quarks emerging than anticipated, despite the initial neutron-heavy makeup of the colliding nuclei.

"Given that argon has 18 protons and 22 neutrons, while scandium has three more neutrons than protons, the pre-collision environment contained more down quarks than up quarks," explained Prof. Katarzyna Grebieszkow from the Warsaw University of Technology. "Yet, our data clearly indicate an excess of up quarks in the aftermath of these collisions - a direct violation of flavor symmetry."

The implications of this discovery are potentially far-reaching. It could signify that current quantum chromodynamics (QCD) models, which describe the strong nuclear force binding quarks, have overlooked a critical aspect of these interactions, or it might point to a fundamentally new aspect of particle physics.

"This anomaly may even extend beyond the Standard Model, hinting at new physics," added Prof. Rybicki. "It forces us to reconsider many long-standing assumptions in nuclear physics and could reshape our understanding of the strong force."

Looking ahead, the NA61/SHINE collaboration plans to expand their studies to include collisions where the initial conditions are more symmetrical, such as those involving pi+ and pi- mesons with carbon nuclei. Future experiments with heavier systems, including oxygen-oxygen and magnesium-magnesium collisions, are also planned, though the latter must wait for the completion of a major upgrade to the Large Hadron Collider (LHC) over the next three years.

Research Report:Evidence of isospin-symmetry violation in high-energy collisions of atomic nuclei