Subthreshold Phi-meson yield in central ⁵⁸Ni+⁵⁸Ni collisions at 1.93 AGeV

The Phi-meson production cross section is measured for the first time at a sub-threshold energy of 1.93 AGeV in ⁵⁸Ni+⁵⁸Ni collisions. The Phi data were obtained within the acceptance of the CDC/Barrel subsystem of the FOPI detector. For a sample of 4.7x10⁶ central events, after background subtraction, 23 Phi candidates were observed. Extensive simulation calculations of the detector performance are shown in a detailed comparison to the data, aiming at a good understanding of the apparatus response and at a reliable determination of the efficiencies, production probability and possible systematic errors. A filter procedure is elaborated, which is meant to facilitate the comparison of any theoretical calculation with the data in the detector acceptance. How to extrapolate the result to a Phi-meson cross section in 4\pi is also discussed. This first result on pseudo-vector mesons can now be compared to existing data for the same reaction at the same incident energy for various outgoing channels, K⁺ and K^- included. It suggests that a significant fraction (at least 20%) of the K^--mesons is originating in the decay of the Phi showing that the two channels are strongly correlated. A first comparison to existing model calculations indicates the possible role of the Rho-meson channels in Phi production.