The eccentricity in coordinate space at midrapidity of the overlap zone in high-energy heavy-ion collisions predicted by the $k_{\perp }$-factorization formalism is generically larger than expected from scaling with the number of participants. We provide a simple qualitative explanation of the effect which shows that it is not caused predominantly by edge effects. We also show that it is quite insensitive to “details” of the unintegrated gluon distribution functions such as the presence of leading-twist shadowing and of an extended geometric scaling window. The larger eccentricity increases the azimuthal asymmetry of high transverse momentum particles. Finally, we point out that the longitudinal structure of the color glass condensate initial condition for hydrodynamics away from midrapidity is nontrivial but requires understanding of large-$x$ effects.

• Received 18 May 2006

DOI:https://doi.org/10.1103/PhysRevC.74.044905