Robustness of principal component analysis of harmonic flow in heavy ion collisions

The principal component analysis (PCA), a mathematical tool commonly used in statistics, has recently been employed to interpret the pT-dependent fluctuations of harmonic flow vn in terms of leading and subleading flow modes in heavy ion collisions. Using simulated data from ampt and hijing models, we show that the PCA modes are not fixed, but depend on the choice of the particle weight and the pT range. Furthermore, the shape of the leading mode is affected by the presence of nonflow correlations, and a fake subleading mode may arise from the mixing of nonflow correlations with leading flow mode with a magnitude that could be larger than the genuine subleading flow mode. Therefore, the meaning of PCA modes and their relations to physical leading and subleading flow modes associated with initial state eccentricities need to be further clarified/validated in realistic model simulations.