Production of φ mesons in central Pb+Pb collisions at SPS energies

The NA49 experiment has measured φ production in central Pb+Pb collisions at the CERN SPS at beam energies from 20 to 158 AGeV. We present preliminary results on the energy dependence of tranverse spectra as well as of the mean multiplicities. . Strangeness production in heavy-ion collisions is believed to give insight into the collision dynamics. The φ meson is of special interest due its s-anti-s quark content. Being strangeness-neutral as a hadron, it should be sensitive to whether the strangeness content is determined on a partonic or hadronic level. NA49 [1] is a fixed-target hadron spectrometer operated at the CERN-SPS. Previously, the NA49 experiment has reported on φ production in central Pb+Pb collisions at 158 AGeV beam momentum [2]. This measurement has been extended towards lower beam energies in the context of the NA49 energy scan programme. The φ meson is measured via its decay into charged kaons (branching ratio 49.1 %). The daughter kaons are detected in the MTPCs; kaon candidates are selected based on the energy loss measurement in the TPC gas. The ±1.5 σ selection results in a 75 % identification efficiency for the pair. The φ signal was obtained by calculating the invariant mass of all combinations of oppositely charged kaon candidates. The combinatorial background was reconstructed by event mixing and subtracted. The φ yield is then obtained from a fit of a relativistic Breit-Wigner distribution, folded with the detector resolution, to the backgroundsubtracted spectrum. This procedure is done differentially in p BBtBB, integrated over a large rapidity range, and in rapidity. By extrapolation of the rapidity distribution to full phase space, the φ multiplicity is obtained. The acceptance corrections are calculated by a GEANT3 based simulation of the decay φ→K PPPPK PP PP in the NA49 detector system. For the track selection applied, track density effects are negligible. The acceptance covers forward rapidities and transverse momenta from 0 to 2.1 GeV. Typically, about 20 % of the decays are reconstructed. Additional correction factors take into account the branching ratio and the kaon identification efficiency. The transverse momentum distributions were fitted by thermal distributions. Figure 1 shows the extracted inverse slope parameters as a function of collision energy. They exhibit a plateau-like behaviour in the measured energy range, except for top SPS energy. This resembles the observations reported for pions, kaons and protons, which can be interpreted as a signal of the mixed phase of a first order phase transition at SPS energies [3]. The energy dependence of the integrated φ multiplicity, normalised to the pion multiplicity, is shown in figure 2. The excitation function is similar to that observed for the K PP PP[4]. The full equilibrium hadron gas model [5] overestimates this ratio from low SPS energies on, similar to the K/π ratio. Statistical models allowing for strangeness undersaturation parameter γ BBsBB [6] seem to account better for the data. In contrast, the hadron-string transport model UrQMD strongly underpredicts the φ yield at all energies. In comparison to proton-proton collisions, the φ/π ratio shows an enhancement of a factor of about three at AGS and SPS energies. The fact that the φ meson seems to be sensitive to strangeness enhancement and/or to the strangeness undersaturation parameter γ BBsBB suggests that the strangeness content in the heavy-ion reaction is determined in a partonic stage. This conclusion is strengthened by the failure of hadronic transport models to describe the measured yields.