  The ability to accurately compute the series of coefficients $v_n$
characterizing the momentum space anisotropies of particle production in
ultrarelativistic heavy ion collisions as a function of centrality is widely
regarded as a triumph of fluid dynamics as description of the bulk matter
evolution. A key ingredient to fluid dynamical modeling is however the initial
spatial distribution of matter as created by a yet not completely understood
equilibration process. A measurement directly sensitive to 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 distribution of the
normalized $v_n$ distribution as a function of centrality, which is to high
accuracy independent on the details of the subsequent fluid dynamical evolution
and hence directly reflects the primary distribution of spatial eccentricities.
We present a study of this observable using a variety of Glauber-based models
and argue that the experimental data place very tight constraints on the
initial distribution of matter and rule out all simple Glauber-based models.
