Publication : t17/136

Abstract:
In ultrarelativistic heavy-ion experiments, one estimates the centrality of a collision using a single observable, say n, typically given by the transverse energy or the number of tracks observed in a dedicated detector. The correlation between n and the impact parameter, b, of the collision is then inferred by fitting a specific model of the collision dynamics, such as the Glauber model, to experimental data. The goal of this paper is to assess precisely which information about b can be extracted from data without any specific model of the collision. Under the sole assumption that the probability distribution of n is Gaussian for a fixed b, we show that the probability distribution of impact parameter in a narrow centrality bin can be accurately reconstructed up to 5% centrality. We apply our methodology to Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) data. We propose a simple measure of the precision of the centrality determination, which can be used to compare different experiments.

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