Eccentricities, fluctuations and A-dependence of elliptic and triangular flows in heavy-ion collisions

A simple geometrical model with event-by-event fluctuations is suggested to study elliptical and triangular eccentricities in the initial state of relativistic heavy-ion collisions. This describes rather well ALICE ATLAS data for Pb+Pb collisions at center-of-mass energy $\sqrt{s_{NN}} = 5.02$~TeV per nucleon pair, assuming that second, $v_2$, third, $v_3$, harmonics anisotropic flow are simply linearly proportional $\varepsilon_2$ $\varepsilon_3$, respectively. We show eccentricity $\varepsilon_3$ has a pure fluctuation origin substantially dependent on size overlap area only, while mainly related average collision geometry. Elliptic flow, therefore, weakly everywhere except very central 0--2%, whereas mostly determined by fluctuations. The scaling dependence magnitude atomic number, $v_n \propto A^{-1/3}$, predicted this centrality interval.