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Understanding the negative binomial multiplicity fluctuations in relativistic heavy ion collisions

By deriving a general expression for multiplicity distribution (a conditional probability distribution) in statistical model, we demonstrate the mismatches between experimental measurements and previous theoretical calculations on multiplicity fluctuations. From the corrected formula, we develop an improved baseline measure for multiplicity distribution under Poisson approximation in statistical model to replace the traditional Poisson expectations. We find that the ratio of the mean multiplicity to the corresponding reference multiplicity are crucial to systemically explaining the measured scale variances of total charge distributions in different experiments, as well as understanding the centrality resolution effect observed in experiment. The improved statistical expectations, albeit simple, work well in describing the negative binomial multiplicity distribution measured in experiments, e.g. the cumulants (cumulant products) of total (net) electric charge distributions.