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

Understand

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 model describes rather well the ALICE and ATLAS data for Pb+Pb collisions at center-of-mass energy $\sqrt{s_{NN}} = 5.02$~TeV per nucleon pair, assuming that the second, $v_2$, and third, $v_3$, harmonics of the anisotropic flow are simply linearly proportional to the eccentricities $\varepsilon_2$ and $\varepsilon_3$, respectively.
• We show that the eccentricity $\varepsilon_3$ has a pure fluctuation origin and is substantially dependent on the size of the overlap area only, while the eccentricity $\varepsilon_2$ is mainly related to the average collision geometry.
• Elliptic flow, therefore, is weakly dependent on the event-by-event fluctuations everywhere except of the very central collisions 0--2%, whereas triangular flow is mostly determined by the fluctuations.