Subthreshold φ meson production in nucleus-nucleus collisions

Part of emission of the negatively charged kaons from the nucleus-nucleus collisions appears to originate from decays of phi(1020) mesons into the K+K- channel. We report on the measurement of the emission yield of phi mesons in the collisions of Ni+Ni at the beam kinetic energy of 1.91A GeV. The experiment was done with the FOPI spectrometer at the SIS 18 accelerator at GSI, Darmstadt. The total yield of phi was found to be (4.5 +/- 0.7(-1.2)(+1.8)) . 10(-4) per triggered event. Ratio of phi/K- emission yields was estimated at 0.42 +/- 0.13(-0.11)(+0.17), meaning that about 20% of negatively charged kaons emitted from the collision originate from decays of phi mesons. The results are in agreement with similar HADES results done for the collisions of Ar+KCl.