Relative flow fluctuations as a probe of initial state fluctuations

Elliptic flow, v2, and triangular flow, v3, are to a good approximation linearly proportional to the corresponding spatial anisotropies of the initial density profile, 2 and 3. Using event-by-event hydrodynamic simulations, we point out when deviations from this linear scaling are to be expected. When these deviations are negligible, relative vn fluctuations are equal to relative n fluctuations, and one can directly probe models of initial conditions using ratios of cumulants, for instance vn{4}/vn{2}. We argue that existing models of initial conditions tend to overestimate flow fluctuations in central Pb + Pb collisions, and to underestimate them in peripheral collisions. We make predictions for v3{6} in noncentral Pb + Pb collisions, and for v3{4} and v3{6} in high-multiplicity p+Pb collisions.