Do Strange Particles Still Deserve Their Given Name?
by Mark Heinz, Yale University
The enhanced production of strange particles in heavy ion collisions is considered an important signature of the presence of a Quark-Gluon Plasma. Quantitative predictions of the strangeness enhancement with respect to elementary particle collisions as a function of collision volume can be obtained from statistical model calculations. In contrast to these models STAR has observed that the enhancements of strange baryons do not obey the predicted volume dependence and thus question our understanding of their production process.
In 1982, well before the first heavy ion accelerator results became available two theorists by the names of Mueller and Rafelski  predicted that the production of strange particles would be especially interesting to measure in these collisions. The large energy density created in such collisions would “dissolve” the nuclear matter and produce a “soup” of free quarks and gluons, a state called the Quark-Gluon Plasma (QGP).
This in turn would favor the production of strange particles via the gluon-fusion channel, a channel not accessible in “normal” collisions between hadrons. A hadron is a bound state of two (mesons) or three quarks (baryons) as opposed to free quarks or gluons. Therefore the presence of the QGP would enhance strange particle production, which would become a measurable signature of the QGP, still just a hypothetical concept at the time.
Today, over 20 years later, the evidence for the QGP is overwhelming and most physicists do not question its presence in Au-Au collisions at RHIC. In fact, the experimental results from CERN and RHIC both have measured significantly enhanced production of strange baryons, the 3-quark bound state. The enhancement compared to proton-proton (p+p) collisions at RHIC energy is actually very significant, up to 10 times more for Ξ (2 strange quarks, 1 down quark) and Ω (three strange quarks) baryons. These values include accounting for the larger number of incoming particles participating in the heavy ion collision.
To make things slightly more complicated theoretical models describing the particle yields are distinctly different for heavy ion and proton-proton collisions. They include a concept called phase space suppression by which strange particles produced in “small” systems like proton-proton are suppressed due to exact conservation of quantum numbers. As a consequence this creates an alternate explanation of the observed effect as being a suppression in p+p instead of an enhancement in gold+gold (Au+Au).
In order to disentangle the two effects (suppression vs. enhancement) it seems natural to seek an intermediate state and to measure the effect as a function of the “size” of the collision. Luckily, in heavy ion collisions, we produce collisions with a smoothly varying range of volumes, measured by the amount of geometrical overlap when they collide. The first measurements at CERN (WA97 experiment)  agreed with theoretical calculations, based on a statistical model, predicting that the enhancement should scale linearly with the number of participating nucleons (Npart).
However the new results from the STAR experiment at RHIC, which will be submitted to Physics Review Letters shortly, show clearly that the enhancement of strange baryon production does not scale linearly with the number of participants. Figure 1 shows the enhancement values divided by Npart (in this logarithmic representation linear would be a flat line) for Λ, Ξ and Ω from STAR and a more recent CERN (NA57) experiment  at lower energy as a function of Npart. A hierarchy in the scale of enhancements, which grows with increased strangeness is observed and consistent with the theoretical model. The measurement of protons (on the left panel of figure 1), which don’t contain strange quarks, is shown as a reference and exhibits linear Npart scaling. The fact that the enhancements at RHIC and CERN reach similar magnitudes in the most central collisions is also rather puzzling, since the models would favor a decreasing enhancement when going from CERN to RHIC energies.
In addition STAR has also measured strange baryons in the intermediate momentum range and compared heavy ion to proton-proton measurements. Figure 2 shows the ratio of transverse momentum spectra from central Au+Au divided by p+p, this time scaled by the number of binary collisions (Nbin), a different centrality measure. The striking feature in this figure is that the ratio is clearly above unity at transverse momenta pT > 1.5 GeV/c, whereas for non-strange particles a suppression below unity is observed. The proton in fact is consistent with unity for this range. In the absence of any other explanation for a ratio greater than unity, the result suggests that this is caused by phase space suppression effects in p+p. This however implies that phase space suppression is valid over a much wider range in momentum than initially expected by statistical models.
In summary, we see an enhancement of strange baryon production in gold+gold collisions consistent with the formation of a QGP. However, the magnitude and volume dependence do not agree with current models. By measuring heavy ion collisions of different volumes, using other nuclei like copper, and different collision energies, both of which are planned at RHIC, we hope to shed more light on these questions. For now, coming back to the rhetorical question in the title, it appears that strange particles continue to escape our full understanding and therefore fully deserve their name.
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