Centrality dependence of subthreshold φ meson production in Ni + Ni collisions at 1.9A GeV

We analyzed the φ meson production in central Ni + Ni collisions at a beam kinetic energy of 1.93A GeV with the FOPI spectrometer and found a production probability per event of [8.6±1.6(stat)±1.5(syst)]×10-4. This new data point allows us for the first time to inspect the centrality dependence of subthreshold φ meson production in heavy-ion collisions. The rise of φ meson multiplicity per event with mean number of participants can be parametrized by a power function with exponent α=1.8±0.6. The ratio of φ to K- production yields seems not to depend, within the experimental uncertainties, on the collision centrality, and the average of measured values was found to be 0.36±0.05.