star focus: Beam-Energy and System-Size Dependence of Dynamical Net Charge at RHIC
Highlights from the STAR paper Beam-Energy and System-Size Dependence of Dynamical Net Charge submitted recently to Physical Review C.
Posted: Jul 28, 2008

Anomalous transverse momentum and net charge event-by-event fluctuations have been proposed as indicators of the formation of a quark gluon plasma (QGP) in high-energy heavy ion collisions. In particular, Koch et al. [1] have estimated that entropy conserving hadronization of a plasma of quarks and gluons should produce a final state characterized by a dramatic reduction of the net charge fluctuations relative to those observed in a hadron gas. Published STAR measurements indicate the fluctuations observed in Au + Au collision at center of mass energy of 130 GeV are little suppressed relative to those observed in p + p collisions [2]. STAR found the measured fluctuations in this collision system and energy are in qualitative accord with expectations based on hadron gas models, and significantly larger than those predicted for a QGP. However, given the finite size of colliding nuclei, it is possible that the QGP volume produced in any given collision is small relative to the total size of the system. It is thus interesting to study the magnitude of the fluctuations as a function of the colliding system size by varying both collision centrality and colliding nuclei. There is also a possibility that final state interactions may partly wash out the expected suppression through collision and diffusion processes [3]. Best conditions to observe the predicted suppression may not be at 130 GeV. It is, therefore, of great interest to carry out a study of the system size, and beam energy dependence of the net charge fluctuations.

In this new work submitted to PRC, the STAR experiment reports measurements of net charge fluctuations in Au + Au collisions at center of mass energies of 19.6, 62.4, 130 and 200 GeV, Cu + Cu collisions at center of mass energies of 62.4 and 200 GeV, and p + p collisions at 200 GeV using the dynamical net charge fluctuations measure nu_dyn(+/-) . The Figure shows a plot of the magnitude of nu_dyn(+/-) scaled by dN/deta for the systems and energies mentioned above. One finds that all distributions exhibit the same qualitative behavior: scaled dynamical net charge fluctuations in peripheral Cu + Cu and Au + Au collisions are essentially equal to those observed in p + p collision and grow in magnitude by approximately 40% from peripheral to central collisions. Note as a reference that Poisson fluctuations would produce a vanishing nu_dyn(+/-) signal. The dashed line corresponds to charge conservation effects and the solid line represents the prediction for a resonance gas [2]. The QGP prediction is off scale. One finds that for all measured systems and energies, measured fluctuations are more closer to those predicted for a hadron gas, and much larger than those anticipated for an entropy conserving hadronization of a QGP. While the magnitude of scaled nu_dyn(+/-) is largest for Au + Au collisions at 200 GeV, it is not obvious that this larger value reflects an increased role of QPG hadronization given the charged particle multiplicity increases sizably from 130 GeV to 200 GeV. Given the reported observations of a strongly interacting medium in A+A collisions at RHIC, it is possible the entropy conservation hadronization hypothesis used by Koch et al. to make their predictions is invalid. However, it is also conceivable that in all measured systems and energies, some final state interaction process washes out, or masks, the predicted signal. Consider as an example that the observed centrality dependence may in part be due to increasingly large radial flow building up from peripheral to central collisions. Other dynamical phenomena could of course also contribute.

The current measurements are thus insufficient to put the suppression hypothesis to rest. Clearly, one needs to understand the role of the collision dynamics in greater detail. This may perhaps be accomplished by ongoing correlation studies of produced particles in all measured systems. Stay tuned for further analysis results from STAR.


[1] S. Jeon and V. Koch, Phys. Rev. Lett. 85, 2076 (2000).

[2] J. Adams et al. (STAR Collaboration), Phys. Rev. C68, 044905 (2003).

[3] M. A. Aziz and S. Gavin, Phys. Rev. C70, 034905 (2004).

Further details can be found in the following STAR paper -
Beam-Energy and System-Size Dependence of Dynamical Net Charge at RHIC - arXiv:0807.3269.

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