Status on φ mesons from Ni+Ni collisions at 1.91A GeV∗

It has been suggested that a considerable part of negatively charged kaons produced in the Heavy Ion collisions stems from the decays of φ mesons [1,2]. As the φ meson lifetime is about cτ = 50 fm, the contribution of this source to K− yield is mostly not affected by the in-medium interactions. The φ/K− yield ratio also plays a role in estimating the correlation length RC within the canonical approach of the Statistical Model. Therefore it is important to extend the experimental knowledge on this ratio. Details of the S325 experiment were described in [3]. The present paper reports the analysis status from all the acquired data on Ni+Ni collisions at 1.91A GeV. 7.8·108 events were registered after the trigger selection of about 50% most central collisions corresponding to σ = 1.35 b. Two Time-of-Flight detection systems were installed: Multi-strip Multi-gap Resistive Plate Chamber (MMRPC) and Plastic Barrel (PLB) covering polar angles 30◦<θlab<100◦ with respect to the target shifted 40 cm upstream from its nominal position. Charged kaons were identified in MMRPC up to momenta 1.15 GeV/c (K) and 0.8 GeV/c (K−) and in Plastic Barrel up to 0.6 GeV/c (K) and 0.5 GeV/c (K−). Positively charged kaons were found on an average signal to background level of 4.4. The efficiency was determined in a full GEANT simulation where kaons were sampled from a homogeneous pt-y distribution. For a series of narrow slices of rapidity the pt distributions were fitted with Boltzmann functions and extrapolated outside the region of detector acceptance. The rapidity distribution obtained by integrating the pt spectra for each slice is shown in Fig. 1. An integration of this distribution delivers the K yield of 0.041 per central collision. φ mesons were investigated in the K+K− decay channel (BR = 49.1%). The maximum momenta for kaon products identified in the MMRPC were slightly lower: 0.9 (0.7) GeV/c for K (K−). The invariant mass spectrum of the φ meson candidates was compared to the background distribution determined using the event mixing technique and normalized in the range 1.05 <minv < 1.18 GeV/c. After background subtraction about 125 φ mesons were found within 2σ integration range. The efficiency was determined in the GEANT simulation, where φ mesons were sampled from an isotropic thermal source using the SiemensRasmussen formula assuming three options: T = 90 MeV, βflow = 0, T = 90 MeV, βflow = 0.17 and T = 100 MeV, βflow = 0.25. The efficiency was found to be within 0.68% and 0.97%. The φ meson yield was found to be (5.5±0.5±1.5)·10−4 per central collision.