Deep sub-threshold K*(892)0 production in collisions of Ar + KCl at 1.76 A GeV

Results on the deep sub-threshold production of the short-lived hadronic resonance K∗(892)0 are reported for collisions of Ar + KCl at 1.76 A GeV beam energy, studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. The K∗(892)0 production probability per central collision of P_K∗0 =(4.4±1.1±0.5)×10^−4 and the K∗(892)0/K0 ratio of P_K∗0/P_K0 =(1.9±0.5±0.3)×10^−2 are determined at the lowest energy so far (i.e. deep below the threshold for the corresponding production in nucleon-nucleon collisions, √s_NN−√s_thr = −340 MeV). The K∗0/K0 ratio is compared with results of other experiments and with the predictions of the UrQMD transport approach and of the statistical hadronization model. The experimental K∗0 yield and the K∗0/K0 ratio are overestimated by the transport model by factors of about five and two, respectively. In a chemically equilibrated medium the ratio corresponds to a temperature of the thermalized system being systematically lower than the value determined by the yields of the stable and long-lived hadrons produced in Ar + KCl collisions. From the present measurement, we conclude that sub-threshold K∗ production either cannot be considered to proceed in a system being in thermal equilibrium or these short-lived resonances appear undersaturated, for example as a result of the rescattering of the decay particles in the ambient hadronic medium.