2 00 2 Sub - threshold φ - meson yield in central 58 Ni + 58 Ni collisions

The φ-meson production cross section is measured for the first time at a subthreshold energy of 1.93 AGeV in Ni+Ni central collisions. The φ data were obtained within the acceptance of the CDC/Barrel subsystem of FOPI. For a sample of 4.7 · 10 central events, after background subtraction, 23 candidates were observed. Extensive GEANT simulations of the detector performance are shown in a thorough comparison to the real response, aiming at a good understanding of the apparatus and at a trustable 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 or new data with the current ones. How to extrapolate the present value to a φ-meson cross section in 4π is also discussed. This result on pseudo-vector mesons can now be compared to existing experimental knowledge for the same reaction at the same incident energy for various outgoing channels, K and K included. A significant fraction (at least 20%) of the K-mesons is originating in the decay of the φ, supporting the statement that the two channels are strongly correlated.