Eliminating experimental bias in anisotropic-flow measurements of high-energy nuclear collisions

Abstract

We argue that the traditional event-plane method, which is still widely used to analyze anisotropic flow in ultrarelativistic heavy-ion collisions, should be abandoned because flow fluctuations introduce an uncontrolled bias in the measurement. Instead, one should use an alternative, such as the scalar-product method or cumulant method, which always measures an unambiguous property of the underlying anisotropic flow and therefore eliminates this bias, and does so without any disadvantages. It is known that this correction is important for precision comparisons of traditional v_n measurements requiring better than a few percent accuracy. However, we show that it is absolutely essential for correlations between different harmonics, such as those that have been recently measured by the ATLAS Collaboration, which can differ from the nominally-measured quantity by a factor two or more. We also describe how, using the corrected analysis method, the information from different subevents can be combined in order to optimize the precision of analyses.

9 pages, 2 figures v3: significantly revised and expanded explanation and derivations, especially in Section II, including new notation. Results and conclusions unchanged. Published version; v2: added some clarifications and corrected a mistake in the description of the resolution correction used by ATLAS, which affects the results in Fig. 2. Conclusions are unchanged