The determination of QCD matter viscosity from experimental data on ultrarelativistic heavy ion collisions is largely based on understanding the systematics of the azimuthal asymmetries of the hadron spectra, usually characterized by their Fourier harmonics $v_n$. Fluid dynamics hasbecome a standard tool in relating the viscous properties of the matter to the experimental observables, and the ability to accurately compute $v_n$'s is widely regarded as a triumph of fluid dynamical description of the bulk matter evolution.

A key ingredient to fluid dynamical modeling is, however, the initial spatial distribution of matter created in the primary nuclear collisions. A measurement directly probing this initial state geometry is therefore of high value for constraining models of pre-equilibrium dynamics. Recently, it has been shown that such a measurement is indeed possible in terms of the event by event probability distributions of the normalized $v_n$, which are to high accuracy independent on the details of the fluid dynamical evolution and hence directly reflects the primary distribution of spatial eccentricities [1]. We present a study of this observable using a variety of Glauber-based models and argue that theexperimental data place very tight constraints on the initial distribution of matter and rule out all simple Glauber-based models [2].

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