Relative flow fluctuations as a probe of initial state fluctuations (1702.01730v2)

Abstract: Elliptic flow, $v_2$, and triangular flow, $v_3$, are to a good approximation linearly proportional to the corresponding spatial anisotropies of the initial density profile, $\varepsilon_2$ and $\varepsilon_3$. 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 $v_n$ fluctuations are equal to relative $\varepsilon_n$ fluctuations, and one can directly probe models of initial conditions using ratios of cumulants, for instance $v_n{4}/v_n{2}$. 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 $v_3{6}$ in noncentral Pb+Pb collisions, and for $v_3{4}$ and $v_3{6}$ in high-multiplicity p+Pb collisions.