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 $0.36 \pm 0.05$. 
K. Piasecki, Z. Tymi\'nski, N. Herrmann, R. Averbeck, A. Andronic, V.   Barret, Z. Basrak, N. Bastid, M.L. Benabderrahmane, ...
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