FOPI

Production of $\phi$ mesons in nucleus-nucleus collisions around 2A GeV, below 2.6 GeV threshold in nucleon-nucleon system, is a rare process to which non-trivial multi-step channels contribute apart from the Fermi motion effects. The measurements carried out by the FOPI Collaboration at 1.9A GeV for symmetric Al+Al and Ni+Ni systems allow for the first time to investigate the important aspects of the $\phi$ meson production, namely:

(i) the feeding of the $\phi$ meson decays to the K$^-$ spectral yield, and

(ii) the centrality dependence of the $\phi$ meson yield and the $\phi$/K$^-$ and $\phi$/$\pi^+$ ratios.

The results are important for the studies of modifications of the effective masses of kaons inside the collision zone.

A simple two-source model of K$^-$ emission allows to extract the spectra of kaons originating directly from the collision zone, which are ready to be compared to the transport model predictions.

[1] K. Piasecki et al. (FOPI Collaboration), Phys. Rev. C 91, 054904 (2015)

[2] P. Gasik et al. (FOPI Collaboration), Submitted to Eur. Phys. Jour., arXiv:1512.06988

[3] K. Piasecki et al. (FOPI Collaboration), Submitted to Phys. Rev. C, arXiv:1602.04378