Deep sub-threshold K*(892)0 production in collisions of Ar + KCl at 1.76A 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 1.76A 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 \( P_{K^{*0}}=(4.4\pm 1.1 \pm 0.5)\times 10^{-4}\) and the K*(892)0/K0 ratio of \( P_{K^{*0}}/P_{K^0}=(1.9\pm 0.5\pm 0.3)\times 10^{-2}\) are determined at the lowest energy so far (i.e. deep below the threshold for the corresponding production in nucleon-nucleon collisions, \( \sqrt{s_{NN}}-\sqrt{s_{thr}}=-340\) 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.