System size and shape dependences of collective flow fluctuations in relativistic nuclear collisions

Quantum fluctuations plays an essential role in forming the collective flow of hadrons observed in relativistic heavy-ion collisions. Event-by-event fluctuations of the collective flow can arise from various sources, such as the fluctuations in the initial geometry, hydrodynamic expansion, hadronization and hadronic evolution of the nuclear matter, while the exact contribution from each source is still an open question. Using a (3+1)-dimensional relativistic hydrodynamic model coupled to a Monte-Carlo Glauber initial condition, Cooper-Frye particlization and a hadronic transport model, we explore the system size and shape dependences of the collective flow fluctuations in Au+Au, Cu+Au and O+O collisions at √(s_NN)=200 GeV. The particle yields, mean transverse momenta, 2-particle and 4-particle cumulant elliptic flows (v_2{2} and v_2{4}) from our calculation agree with the currently existing data from RHIC. Different centrality dependences of the flow fluctuations, quantified by the v_2{4}/v_2{2} ratio, are found for different collision systems due to their different sizes and shapes. By comparing v_2{4}/v_2{2} between different hadron species, and comparing v_2{4}/v_2{2} to the initial state geometric fluctuations quantified by the cumulant eccentricity ratio ε_2{4}/ε_2{2}, we find that while the initial state fluctuations are the main source of the v_2 fluctuations in large collision systems, other sources like hadronization and hadronic afterburner can significantly affect the v_2 fluctuations in small systems.