First evidence for different freeze-out conditions for kaons and antikaons observed in heavy-ion collisions

Differential production cross sections of K sup - and K sup + mesons have been measured in Ni+Ni and Au+Au collisions at a beam energy of 1.5 A.GeV. The K sup - /K sup + ratio is found to be nearly constant as a function of the collision centrality and system size. The spectral slopes and the polar emission pattern differ for K sup - and K sup + mesons. These observations indicate that K sup + mesons decouple earlier from the fireball than K sup - mesons