Strangeness production in STAR
We present a summary of strangeness enhancement results comparing data from Cu+Cu and Au+Au collisions at measured by the STAR experiment. Relative yields in central Cu+Cu data seem to be higher than the equivalent sized peripheral Au+Au collision. In addition, strange particle production from these two systems is compared in terms of a statistical model, applying a Grand-Canonical ensemble and also applying a canonical correlation volume for the strange particles. Thermal fit results from the Grand-Canonical formalism shows little dependence on the system size but, when considering a strange canonical ensemble, strangeness enhancement shows a strong dependency on the correlation volume.
J. Takahashi and R. Derradi de Souza
For the STAR collaboration
Instituto de Fisica Gleb Wataghin, University of Campinas - UNICAMP
13083-970 Campinas, SP, Brazil
\keywordSTAR data, strangeness enhancement, thermal model \PACS25.75.-q, 25.75.Ag
Strangeness production is expected to be enhanced in heavy ion collision as one of the possible signatures of the Quark-Gluon Plasma . An increase of the strange quark density should result in an increase of the strange hyperon production thus, the measured integrated yield per participant should be sensitive to the density of quarks in the system. Experimentally, strangeness enhancement has been observed in A+A collisions with respect to p+p collisions when comparing the normalized strange particle yields scaled by factors such as the number of participants in the collision, or the number of wounded nucleons [2, 3]. Thermal models with a canonical approach show that this observed relative strangeness enhancement can also be explained by considering a suppression due to limitations on the phase space available for strange particles production in small systems [4, 5]. These models have been quite successful in describing the level of enhancement and the observed hierarchy with the number of strange valence quarks, but up to now, no model has been able to describe simultaneously all features of the measured strangeness enhancement. In this paper, we present the latest results from the analysis of the STAR experiment with respect to the strangeness enhancement including data from Au+Au and Cu+Cu collisions at RHIC maximum energy. In addition, we apply a statistical thermal model fit to study the system size dependence of thermal parameters.
2 Strangeness Enhancement
Figure 1 left side shows the system size dependence of the strangeness enhancement ratio for , and measured in STAR for Au+Au and Cu+Cu at collision energy of 200 GeV per nucleon pairs. Figure 1 right shows the same plot for , and . Particle yields were measured for different event centrality classes and normalized by the equivalent mean number of participant nucleons and then divided by the equivalent ratio measured in p+p collisions at the same energy. The and yields were corrected for feed-down from decays. In both Au+Au and Cu+Cu data, a large strangeness enhancement is observed even in the most peripheral centrality bin. The data also shows a strong dependency with the system size and do not seem to saturate as expected from a Grand-Canonical Thermal model. Strangeness enhancement is higher for strange particles than for anti-particles, which can be a result of the non zero net baryon density. The striking result is that the strangeness enhancement observed in central Cu+Cu data is higher than the peripheral Au+Au collision with equivalent . This is an indication that the production mechanism of these strange particles does not scale with the pure geometrical parameterization of the system size. Normalized proton yields were included in the plots of figure 1 as a reference to show that the relative discrepancy between the two systems is unique to the strangeness sector. The measured proton yields in STAR are inclusive , and in this plot these proton yields were subtracted by the yield factorized by the decay branching ratio to correct for the feed-down into the proton yield. The normalized proton yields do not show any dependence with the system size and also no difference between Au+Au and Cu+Cu data.
Figure 2 shows the collision energy dependence of the strangeness enhancement factor for the most central events for , , , , . SPS data for the low energy points were extracted from Pb+Pb data of experiment NA57 . The observed strangeness enhancement is decreasing with collision energy for the and , as predicted by Grand-Canonical statistical model, but, an opposite trend is observed for the anti-particles that yields higher enhancement in RHIC data compared to SPS data. Also clear in this plot is the strange quark content hierarchy observed in the enhancement as predicted by statistical thermal models.
3 Statistical thermal model results
Data was analyzed using the THERMUS code  and considering particle ratios that includes (), , , , , , and . Protons were corrected for the feed-down of the decay that resulted in a reduction of the inclusive proton yield of approximately . Since STAR does not yet have measurements, the feed-down contribution from this particle was estimated using the ratio of . Final reduction of the inclusive proton yields was on the order of to . To estimate the error due to this feed-down correction, the relative yield to was varied over . The overall effect on the final thermal parameters was less than .
Figure 3 shows the chemical freeze-out temperature and the strangeness chemical potential as a function of the system size, . is around 155 MeV and seems to show no dependence on the system size and also no sensitivity to the colliding systems, yielding the same results for Au+Au and Cu+Cu data. The baryon chemical potential and strangeness chemical potential also showed no difference between the fits to Au+Au and Cu+Cu data. To study the validity of the statistical thermal fit considering a Grand Canonical formulation, we have applied the same fit to the p+p data. The chemical freeze-out temperature that results from this fit is slightly lower, around 150 MeV.
Figure 4 shows the dependence on the system size of the strangeness under-saturation factor obtained from the statistical thermal fit with a Grand-Canonical ensemble for the Au+Au and Cu+Cu data. Cu+Cu data shows the same results and behavior of the peripheral Au+Au data. The reaches unity only above greater than approximately 100. Most of the Cu+Cu data is below that limit. The factor can be interpreted as a measure of the validity of the Grand-Canonical formalism to describe the data in the strange sector. Under this assumption, from the results shown in figure 4, it is clear that only the most central bins of Cu+Cu collision can be well described with this approach. from the fit to the p+p data is also shown in figure 4, and shows a much lower value, around 0.6, indicating that the strange particles ratios in p+p collisions cannot be well described with this model.
As an alternative to the Grand Canonical formulation (GC) where the conservation of the quantum numbers are ensured on average, the THERMUS code allows for a strangeness canonical (SC) ensemble where only the quantum numbers of the strange particles are required to conserve exactly. In this formalism, a correlation volume of strange particle production is defined as the sub-volume where the strangeness chemical equilibrium is restricted. We applied the fit to the Au+Au 200 GeV data using this approach and obtained a strangeness correlation volume radius of approximately , for the most central event centrality class. This value is much higher than the value presented for a similar analysis on SPS data  that was around . To understand the effect of this correlation volume on the final strange particle yields, we studied the variation of the strange particle yields relative to the pions for different correlation volumes. Figure 5 shows the results of the model prediction in solid lines and the symbols represent the experimental values of the most central Au+Au 200 GeV data. It is interesting to note that the ratio does not depend on the correlation volume. The value of the correlation volume obtained from the best fit to the data indicates that the system is already in the region where the strange particle production is saturated, and thus, consistent with a system where the strangeness is already equilibrated.
Strange particle production is enhanced in heavy ion collisions compared to p+p. This enhancement increases with the system size and does not seem to saturate as expected by models considering Grand Canonical Statistical models. When using as a scaling factor, Cu+Cu central collision show higher strangeness enhancement than peripheral Au+Au collision. Application of thermal model fit to Au+Au and Cu+Cu data seem to show almost no difference in the thermal parameters. In addition, the thermal fit parameters considering a Grand-Canonical approach seem to show no dependency on the system size except for the parameter. However, when considering a strangeness Canonical formalism in the statistical thermal model, strangeness production shows strong dependence on the correlation volume. Fits to the data indicate that the correlation volume is large, around , above the saturation point.
We wish to thank RHIC and STAR group and the U.S. DOE Office of Science for the support in the participation of this conference.
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