Isospin symmetry broken more than expected

In the autumn of 2023, Wojciech Brylinski was analysing data from the NA61/SHINE collaboration at CERN for his thesis, when he noticed an unexpected anomaly – a strikingly large imbalance between charged and neutral kaons in argon–scandium collisions. Instead of producing roughly equal numbers, he found that charged kaons were produced 18.4% more often. This suggested that the “isospin symmetry” between up (u) and down (d) quarks might be broken by more than expected due to the differences in their electric charges and masses – a discrepancy that existing theoretical models would struggle to explain. Known sources of isospin asymmetry only predict deviations of a few percent.

“When Wojciech got started, we thought it would be a trivial verification of the symmetry,” says Marek Gaździcki of Jan Kochanowski University of Kielce, spokesperson of NA61/SHINE at the time of the discovery. “We expected it to be closely obeyed – though we had previously measured discrepancies at NA49, they had large uncertainties and were not significant.”

Isospin symmetry is one facet of flavour symmetry, whereby the strong interaction treats all quark flavours identically, except for kinematic differences arising from their different masses. Strong interactions should therefore generate nearly equal yields of charged K⁺ (us) and K^– (us), and neutral K⁰ (ds) and K⁰ (ds), given the similar masses of the two lightest quarks. NA61/SHINE’s data contradict the hypothesis of equal yields with 4.7σ significance.

“I see two options to interpret the results,” says Francesco Giacosa, a theo­retical physicist at Jan Kochanowski University working with NA61/SHINE. “First, we substantially underestimate the role of electromagnetic interactions in creating quark–antiquark pairs. Second, strong interactions do not obey flavour symmetry – if so, this would falsify QCD.” Isospin is not a symmetry of the electromagnetic interaction as up and down quarks have different electric charges.

While the experiment routinely measures particle yields in nuclear collisions, finding a discrepancy in isospin symmetry was not something researchers were actively looking for. NA61/SHINE’s primary focus is studying the phase diagram of high-energy nuclear collisions using a range of ion beams. This includes looking at the onset of deconfinement, the formation of a quark-gluon plasma fireball, and the search for the hypothesised QCD critical point where the transition between hadronic matter and quark–gluon plasma changes from a smooth crossover to a first-order phase transition. Data is also shared with neutrino and cosmic-ray experiments to help refine their models.

The collaboration is now planning additional studies using different projectiles, targets and collision energies to determine whether this effect is unique to certain heavy-ion collisions or a more general feature of high-energy interactions. They have also put out a call to theorists to help explain what might have caused such an unexpectedly large asymmetry.

“The observation of the rather large isospin violation stands in sharp contrast to its validity in a wide range of physical systems,” says Rob Pisarski, a theoretical physicist from Brookhaven National Laboratory. “Any explanation must be special to heavy-ion systems at moderate energy. NA61/SHINE’s discrepancy is clearly significant, and shows that QCD still has the power to surprise our naive expectations.”