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

We analysed the $\varphi$ 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 $\varphi$ meson production in heavy-ion collisions. The rise of $\varphi$ 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 $\varphi$ 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 $0.36\pm 0.05$.