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

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