2 S ep 2 00 7 Fluctuation of the initial condition from Glauber models ∗

We analyze measures of the azimuthal asymmetry, in particular the participant harmonic moments, ε∗, in a variety of Glauber-like models for the early stage of collisions at RHIC. Quantitative comparisons indicate substantial model dependence for ε∗, reflecting different effective number of sources, while the dependence of the scaled standard deviation σ(ε∗)/ε∗ on the particular Glauber model is weak. For all the considered models the values of σ(ε∗)/ε∗ range from ∼ 0.5 for the central collisions to ∼ 0.3-0.4 for peripheral collisions. These values, dominated by statistics, change only by 10-15% from model to model. For central collisions and in the absence of correlations between the location of sources we obtain through the use of the central limit theorem the simple analytic formula σ(ε∗)/ε∗(b = 0) ≃ √