Measurement of [phi] meson production in Cu+Cu collisions at 200 GeV using the PHOBOS detector at RHIC

Abstract

Strong enhancement of production of strange particles, and in particular of [phi] mesons, in heavy ion collisions of sufficiently high energies has been predicted to be an indication of a formation of a new state of matter, composed of deconfined quarks and gluons and having a property of chiral symmetry, called Quark Gluon Plasma (QGP). Studying production of [phi] mesons is of special interest due to their small cross-section of interaction with non-strange hadrons and due to their long lifetime, which should allow [phi] mesons to decouple from the strongly interacting medium produced in heavy ion collisions early in time and to escape the medium before decaying, thereby preserving information about the conditions in which the mesons were produced. In addition, the decay properties of [phi] mesons have been predicted to be modified in a hadronic gas medium. The [phi] -> K+K~ decay is of particular interest since the mass of a [phi] meson in vacuum is very close to the mass of two charged kaons, and consequently, even a small change in the mass or the width of [phi] mesons or in the mass of kaons would have a strong effect on the decay properties. Measurement of [phi] meson production using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC) has proven to be especially challenging due to a small acceptance of the PHOBOS spectrometer and due to a much lower than predicted yield of [phi] mesons in heavy ion collisions at the highest RHIC energy. The measurement required a development of a new tracking algorithm, specifically tailored to reconstruct charged kaons with a high efficiency in a high hit density environment, keeping at the same time the necessary computing time within feasible limits. Results of a measurement of [phi] meson invariant yield in the rapidity interval 0 < y <1 as a function of transverse momentum in Cu+Cu collisions at 200 GeV are presented in various ranges of centrality. The results were used to determine the dN/dy values, the inverse slope parameters T (corresponding to mT-scaling fits of the invariant yields), and average transverse momentum of [phi] mesons as a function of collision centrality. The PHOBOS results on [phi]5 meson invariant yield were compared to the corresponding measurements performed by the STAR and the PHENIX collaborations and were found to be compatible with both within the estimated measurement uncertainties. The centrality dependence of the yield of [phi] mesons at mid-rapidity was studied for all of the available data on [phi] meson production in heavy ion collisions at /N,=200 GeV, and it was found that the data are not sufficient to distinguish between the Npan and Ncou scaling laws of the yield, thereby making it impossible to tell apart two mechanisms of q5 meson production: 1) disintegration of a thermalized QGP, and 2) production in the primordial hard scatterings of partons of the collided nuclei. The V/7 dependence of the inverse slope parameter T was studied for various particle species, showing an evidence of a change in the mechanism of particle production at temperatures and energy densities corresponding to , ~4-9 GeV, however no indication of a plateau was found in any of the dependencies in contrast to theoretical predictions. The T versus V dependence of [phi] mesons was found to have a local maximum at f %20 GeV. Invariant yield of [phi] mesons with p, < 130 MeV/c was measured in the 0-60% Cu+Cu collisions at 200 GeV showing a strong suppression of the yield (by about a factor of ~6 in comparison to an extrapolation to pt = 0 of the PHOBOS results at pt > 390 MeV/c), however, since a full analysis of the systematic errors was not performed for the measurement, the results cannot be considered conclusive. The V dependence of the yield of [phi] mesons at mid-rapidity was studied separately and with respect to the charged particle multiplicity, showing no indication of any special interval of js values which could be associated with such a transition (of a QCD matter into a new state) that would produce a noticeable sudden increase in the number of produced [phi] mesons. No evidence of any change in the mean or the width of the [phi] meson invariant mass peak with respect to the vacuum values was observed.