There are four sets of valence quarks that produce kaons, two of which are charged and two of which are electromagnetically neutral:
K⁺ (positive charge: up quark + anti-strange quark)K⁻ (negative charge: anti-up quark + strange quark)K⁰ (neutral: down quark + anti-strange quark)K̄⁰ (neutral antiparticle: anti-down quark + strange quark)
In certain heavier atom collisions, there are more charged kaons than expected relative to the number of neutral kaons observed, which means that there are more up quarks (and anti-up quarks) relative to down quarks (and anti-down quarks) than expected.
Isospin symmetry is the concept that particles with up quarks substituted for down quarks (and vice versa) are identical in strong force interactions (in part, due to their similar mass and the indifference of the strong force to electromagnetic or weak force charge).
For some reason, however, more charged kaons are seen in argon-scandium collisions and also in some other particle collisions. The paper below fails to come up with a good explanation for why this is the case and calls for further study. The introduction to the paper explains that:
This paper focuses on hadron–nucleus and nucleus–nucleus collisions at center-of-mass (cms) energies of the order of 10 GeV per nucleon pair. Such processes are governed by (mostly) non-perturbative strong interactions, and result in the production of a large number– tens, hundreds– of new particles in the final state (multiparticle production).
Isospin symmetry (IS) which can be considered as a remnant of the QCD flavor symmetry between u and d quarks, remains an essential even if approximate feature of the production process.
At energies equal (or higher) to those specified above, the bulk features of multiparticle production (inclusive spectra, total multiplicities, etc) are widely believed to obey IS up to a very good accuracy, of the order of 3−4%. Nevertheless, the simultaneous measurement of inclusive spectra and yields of charged (K+, K−) and neutral (K0 S) mesons reported by the NA61/SHINE experiment in central Ar+Sc collisions at √ sNN = 11.9 GeV [1] reveals an unexpectedly large IS violation (18 ±6% at kaon mid-rapidity).
As will be discussed below, measurements from other experiments over a wide range of collision energy appear consistent with this observation. At the present moment (Oct., 2026), the origin of this effect remains unclear.
The paper and its abstract are as follows:
The NA61/SHINE experiment at the CERN SPS reports an unexpectedly large excess in the inclusive production of charged over neutral kaons in 0-10% central Ar+Sc collisions at sNN‾‾‾‾√=8.8 and 11.9 GeV. This effect corresponds to an unexpectedly large violation of isospin symmetry, that cannot be explained by known mechanisms contained in the present theoretical models.
The comparison of the NA61/SHINE results to data from other experiments reveals, within sizeable uncertainties, a consistent isospin symmetry-violating excess of charged kaon production in the energy regime 2.6<sNN‾‾‾‾√<200 GeV. At LHC energies (0.9<sNN‾‾‾‾√<13 TeV) the charged-over-neutral kaon ratio appears consistent with unity, but the uncertainties remain sizeable.
A similar excess of charged over neutral kaon yields is observed by NA61/SHINE in minimum bias π−+C reactions at sNN‾‾‾‾√=17.3 and 25.7 GeV, also in disagreement with model predictions.
No clear theoretical explanation of these phenomena has been provided so far. An unexpectedly large violation of isospin symmetry brings potential heavy consequences for our understanding of particle production, and calls for a concentrated experimental and theoretical effort.
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