v4:: A small, but sensitive observable for heavy ion collisions -: art. no. 031902

Higher order Fourier coefficients of the azimuthally dependent single particle spectra resulting from noncentral heavy ion collisions are investigated. For intermediate to large transverse momenta, these anisotropies are expected to become as large as 5%, and should be clearly measurable. The physics content of these observables is discussed from two different extreme but complementary viewpoints, hydrodynamics and the geometric limit with extreme energy loss.