star focus: Phi Meson and Strangeness Enhancement at RHIC
Highlights from the STAR paper: Energy and system size dependence of phi-meson production in Cu+Cu and Au+Au collisions. Submitted for publication to Physics Letters B.

Posted: Oct 29, 2008

In a Quark-Gluon Plasma, thermal s and sbar quarks can be produced by gluon-gluon interactions. These interactions could occur very rapidly and the s-quark abundance would equilibriate. During hadronisation, the s and sbar quarks from the plasma coalesce to form phi-mesons. Production by this process is not suppressed as per the OZI (Okubo-Zweig-Izuka) rule. This, coupled with large abundances of strange quarks in the plasma, may lead to a dramatic increase in the production of phi-mesons and other strange hadrons relative to non-QGP p+p collisions.

Alternative ideas of canonical suppression of strangeness in small systems as a source of strangeness enhancement in high energy heavy-ion collisions have been proposed for other strange hadrons (e.g Kaon, Lambda, Cascade, Omega). The strangeness conservation laws require the production of an sbar-quark for each s-quark in the strong interaction. The main argument in such canonical models is that the energy and space time extensions in smaller systems may not be sufficiently large. This leads to a suppression of strange hadron production in small collision systems. These statistical models fit the data reasonably well. According to these models, strangeness enhancement in nucleus-nucleus collisions, relative to p+p collisions, should increase with the strange quark content of the hadrons. This enhancement is predicted to decrease with increasing beam energy.

So far, discriminating between the two scenarios : strange hadron enhancement being due to dense partonic medium formed in heavy-ion collisions or due to canonical suppression of their production in p+p collisions using the available experimental data has been, to some extent, ambiguous. Enhancement of phi-meson (s,sbar) production (zero net strangeness, hence not subjected to canonical suppression effects) in Cu+Cu and Au+Au relative to p+p collisions would clearly indicate the formation of a dense partonic medium in these collisions. This would then rule out canonical suppression effects being the most likely cause for the observed enhancement in other strange hadrons in high energy heavy-ion collisions.

Figure shows the ratio of strange hadron production normalized to average number of participating nucleons (calculated using a Monte Carlo Glauber model) in nucleus-nucleus collisions relative to corresponding results from p+p collisions at 200 GeV (upper panel). The results are plotted as a function of average number of participating nucleons for K, Lambdabar and Cascades are seen to show an enhancement (value > 1) that increases with the number of strange valence quarks. However, the enhancement of phi-meson production from Cu+Cu and Au+Au collisions shows a deviation in ordering in terms of the number of strange constituent quarks. More explicitly, this enhancement is larger than for K and Lambdabar, at the same time being smaller than in case of Cascades. Despite being different particle types (meson-baryon) and having different masses, the results for K and Lambdabar are very similar in the entire centrality region studied. This rules out a produced baryon-meson effect as being the reason for the deviation of phi-mesons from the number of strange quark ordering seen in the figure (upper panel). The observed deviation is also not a mass effect as the enhancement in phi-meson production is larger than that in Lambdabar (which has mass close to that of the phi-meson).

In heavy-ion collisions, the production of phi-mesons is not canonically suppressed due to its ssbar structure and experimental indication that they are not produced via KK coalescence at RHIC energies. Further the p+p collisions at RHIC are at an energy which is 25 times higher than energies where violations of the OZI rule were reported. So the observed enhancement of phi-meson production in heavy-ion collisions may not be due to OZI suppression of phi-meson production in p+p collisions.

The observed enhancement of phi-meson production then is a clear indication for the formation of a dense partonic medium being responsible for the strangeness enhancement in Au+Au collisions at 200 GeV. Furthermore, phi-mesons do not follow the strange quark ordering as expected in the canonical picture for the production of other strange hadrons. The observed enhancement in phi-meson production being related to medium density is further supported by the energy dependence shown in the lower panel of the figure. The phi-meson production relative to p+p collisions is larger at higher beam energy, a trend opposite to that predicted in canonical models for other strange hadrons. This in turn suggests that the observed centrality dependence of the enhancement for other strange hadrons (shown in Fig) is likely to be related to the same reasons as in the case of the phi-meson, that it is due to the formation of a dense partonic medium in the collisions. These experimental data rule out the possibility of canonical suppression being the only source of the observed strangeness enhancement at beam energies of 200 GeV.

Further details can be found in the following STAR paper -
Phi Meson and Strangeness Enhancement at RHIC - arXiv:0810.4979.

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