Centrality dependence of subthreshold $\phi$ meson production in Ni + Ni collisions at 1.9A GeV

We analyzed the phi 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)] x 10(-4). This new data point allows us for the first time to inspect the centrality dependence of subthreshold phi meson production in heavy-ion collisions. The rise of phi meson multiplicity per event with mean number of participants can be parametrized by a power function with exponent alpha = 1.8 +/- 0.6. The ratio of phi 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.