aa r X i v : . [ nu c l - e x ] A p r System size and energy dependence of φ mesonproduction at RHIC J. H. Chen , for the STAR Collaboration Nuclear Science division, Shanghai Institute of Applied Physics, CAS, Shanghai,201800, China Department of Physics and Astronomy, University of California at Los Angeles, LosAngeles, CA, 90095, USA
Abstract.
We present a system size and energy dependence of φ meson productionin Cu+Cu and Au+Au collisions at √ s NN = 62 . φ mesonyields in heavy ion collisions over that of p+p collisions are larger than 1 and increasewith collision energy. We compare the results with those of open-strange particles anddiscuss the physics implication.PACS numbers: 25.75.Dw, 24.85.+p
1. Introduction
The primary aim of ultrarelativistic heavy-ion collisions is to produce and study a stateof high-density nuclear matter called the quark-gluon plasma (QGP), the existence ofwhich is supported by lattice QCD calculations [1, 2]. One of the predicted signalsfor QGP formation is the enhancement of strange particle production relative to thatfrom light collision volume system, p+p or p+A system. In particularly, it’s arguedthat due to the high production rate of gg → s ¯ s in a QGP, strangeness production willbe increased compared to that from a hadron gas [3]. The subsequent hadronization ofthese (anti)strange quarks results in a significant increase in strange particle production.On the other hand, there is an additional effect in the small systems, e.g. p+pcollisions, where a lack of available phase space caused a suppression of strangenessproduction [4, 5]. The so-called canonical suppression. However, the canonicalsuppression, by definition, doesn’t apply to hidden-strange particles like φ meson.Therefore, by comparing the normalized yields of φ mesons with those of open-strangehadrons, one would be able to extract the information of production dynamics.In this paper, we will present the preliminary transverse momentum ( p T )distributions of φ meson in Cu+Cu collisions at √ s NN = 62 . φ yield in each p T bin was extracted from the invariant mass distributions of K + + K − candidates after subtraction of combinatorial background estimated using event mixingtechnique. The charged Kaons were identified through their dE/dx energy loss in the ystem size and energy dependence of φ meson production at RHIC φ meson production in Au+Au collisions at the same energy will be compared tothese measurements as well.
2. Results φ meson production in Cu+Cu collisions - ) ( G e V / c ) T d y dp T p p N / ( d -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 -1 -2 -3 -4 -5 Levy function
Cu+Cu 200 GeV -1 · -12-11-10-9-8-7-6-5-4-3-2-1 -1 -2 -3 -4 -5 Cu+Cu 62.4 GeV -2 · (GeV/c) T Transverse Momentum p
STAR Preliminary
Figure 1. (color online) Preliminary midrapidity ( | y | < . φ meson transversemomentum spectra for various centrality classes for Cu+Cu collisions at √ s NN =62.4 and 200 GeV. For comparison, results of φ meson spectra from 40-50% Au+Aucollisions at 200 GeV and 40-60% Au+Au spectra at 62.4 GeV are also shown. Theerrors shown are statistical and systematic erros added in quadrature. They are foundto be within the symbol size. The spectra are fitted to a levy function. Figure 1 shows the φ meson spectra in 62.4 GeV and 200 GeV Cu+Cu collisions atvarious centralities. The distributions are well described by a levy function from centralto peripheral collisions. For comparison, results of φ meson p T distributions from 40-50% Au+Au collisions at 200 GeV and 40-60% Au+Au collisions at 62.4 GeV are shownin the figure as well. These centralities are chosen for similar number of participants( h N part i ) at corresponding collisions energy. From the comparison, we find that, at a ystem size and energy dependence of φ meson production at RHIC φ meson are scaled by the number of participating nucleon. / d y / ) pp i ne l ) / ( d N pa r t / d y / N AA ( d N
200 GeV - K L f f X + X Au+Au Cu+Cu part
Number of Participating Nucleons N
Au+AuCu+Cu
200 GeV f f Au+AuCu+Cu part N STAR Preliminary
Figure 2. (color online) Upper panel: Ratio of yield of K − , φ , ¯Λ and Ξ+ ¯Ξ normalizedto N part in nucleus-nucleus collisions to corresponding yield in inelastic p+p collisionsas a function of N part at 200 GeV. Lower panel: Same as above for φ meson inCu+Cu collisions at 200 and 62.4 GeV. The p+p collisions data from 200 GeV arefrom STAR [7] and 62.4 GeV from ISR [10]. The error bars shown are both statisticaland systematic errors added in quadrature. Figure 2 shows the strangeness enhancement factor, the N part normalized φ yieldsfrom nucleus-nucleus collisions relative to that from p+p collisions, in Cu+Cu andAu+Au collisions. The results for published open-strange hadrons [8, 9] are also shownin the figure. As one can see, the enhancement factor for K − , ¯Λ and Ξ + ¯Ξ increaseswith the number of strange valence quarks. The enhancement in these open strangehadrons increases with collision centrality and reaches its maximum at the most centralcollisions. The enhancement of the φ meson, however, deviates from the number ofstrange quark ordering. They are enhanced more than K − and ¯Λ but less than theΞ + ¯Ξ. In spite of being different particle types (meson-baryon) and having differentmasses, the results for K − and ¯Λ are very similar in the entire centrality region studied.This rules out a baryon-meson effect as being the reason for the deviation of φ meson ystem size and energy dependence of φ meson production at RHIC φ meson production is larger than ¯Λ.The φ meson production is unlikely to be canonically suppressed due to its s ¯ s structure. The observed enhancement of φ meson production then is a clearindication of dynamical effect associated with medium density being responsible forstrangeness enhancement in Au+Au collisions at 200 GeV. The observed enhancementin φ meson production being related to medium density is further supported by theenergy dependence shown in the lower panel of Figure 2. The enhancement factor for φ mesons is larger at higher collision energy, a trend opposite to that predicted in canonicalmodels for other strange hadrons.
3. Summary
In summary, we report the preliminary results of φ meson p T spectra in Cu+Cu collisionsat √ s NN =62.4 and 200 GeV. Earlier published φ results from Au+Au collisions areused for the purpose of discussions. At fixed energy, the φ mesons production seemsto scaled with the N part . The centrality and energy dependence of the enhancement inthe φ meson production clearly reflects the enhanced production of s-quarks in a densemedium formed in high energy heavy-ion collisions. It then indicates that the source ofenhancement of strange particle is related to the formation of a dense medium in highenergy nucleus-nucleus collisions and cannot be solely due to canonical suppression oftheir production in smaller systems. References [1] F. Karsch,
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