In spite of fack, that K⁺ (with quark content us) and K^- (us) mesons seems to be completely "symmetrical", their behaviour in presence of other quarks is not the same. Duringa a collision of proton (udd) with another proton to create K⁺ kaon, there is necessary creation of one s antiquark, and for creation of K^- kaon, there is needed creation of s quark and simoultaneously change of u w antykwark u. From this reason during proton - proton collisions (plot on right top) probability of creation K^- kaon, especially with small energy range of collisions, is decisively lower than K⁺ kaon creation probability.

Theoretical model (plot on left) predicts, that K^- meson mass decreases with increase of density of sorrounding it nuclear matter, but for K⁺ meson mass increase (from about 493 MeV, which is value for free mesons).As far as, in free protons collisions probability of creation of K⁺ i K^- mesons is not the same, for nuclei collisions (such experiments are performed in Darmstadt) mass change effect causes, that probability are the same (plot on right low).