star focus: K/pi Fluctuations at Relativistic Energies
Highlights from the STAR paper: K/pi Fluctuations at Relativistic Energies. Submitted for publication to Physical Review Letters.

Posted: Feb 19, 2009

Strangeness enhancement has been predicted to be one of the important signatures of the formation of the quark gluon plasma (QGP). The study of dynamic fluctuations in event-by-event K/pi ratio may produce information concerning QCD phase transitions and may lead to the observation of the critical point in the QCD phase diagram. Studying fluctuations of particle ratios has few advantages, it has been argued that considering fluctuations of the multiplicity ratio eliminates the effect of volume fluctuations, further fluctuation of the particle ratio like K/pi could be sensitive to the particle numbers at chemical freeze-out and not at kinetic freeze-out. STAR experiment has recently reported the results on beam energy dependence of event-by-event K/pi ratio fluctuations at RHIC.

The dynamical fluctuations in the event-by-event variation of K/pi ratio is obtained by subtracting the the relative width (standard deviation divided by the mean) of the event-by-event K/pi distribution for the data and those of the relative width of the K/pi distribution for mixed events. This quantity is plotted in the figure shown for central collisions as a function of collision beam energy. The blue rectangles are from the measurements at the CERN SPS NA49 experiment, the red star symbols are measurements from the STAR experiment at RHIC. The NA49 results show a strong incident energy dependence while the STAR results show little dependence on the incident energy. The STAR results are consistent with the highest energy NA49 result, although the statistical error bar for the 19.6 GeV case is large. In the figure, we compare the statistical hadronization model results to the experimental data. We see that when the light quark phase space occupancy, gamma_q, is one, corresponding to equilibrium scenario, the calculations underestimate the experimental results at all energies. When gamma_q is varied to reproduce the excitation function of K+/pi+ yield ratios and the excitation function of temperature versus chemical equilibrium over the SPS and RHIC energy ranges, the statistical hadronization model correctly predicts the dynamical fluctuations at the higher energies but underpredicts the NA49 data at the lower energies, supporting the conclusion that the lower energy fluctuation data are anomalous. In these fits, gamma_q > 1 (chemically oversaturated) for beam energies < 9 GeV and gamma_q < 1 (chemically under-saturated) for beam energies > 9 GeV.

Further details can be found in the following STAR paper -
K/pi Fluctuations at Relativistic Energies - arXiv:0901.1795.

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