Deep sub-threshold �� production and implications for the K+/K- freeze-out in Au+Au collisions

We present data on charged kaons (K+-) and �� mesons in Au(1.23A GeV)+Au collisions. It is the first simultaneous measurement of K and �� mesons in central heavy-ion collisions below a kinetic beam energy of 10A GeV. The ��/K- multiplicity ratio is found to be surprisingly high with a value of 0.52 +- 0.16 and shows no dependence on the centrality of the collision. Consequently, the different slopes of the K+ and K- transverse-mass spectra can be explained solely by feed- down, which substantially softens the spectra of K- mesons. Hence, in contrast to the commonly adapted argumentation in literature, the different slopes do not necessarily imply diverging freeze- out temperatures of K+ and K- mesons caused by different couplings to baryons.
Corrected and updated version, 28 November 2017