Influence of the Multiplicity Fluctuations Distribution on Initial Conditions

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One of the greatest uncertainties in heavy-ion collisions is the description of the initial state. Different models predict a wide range of initial energy density distributions based on their underlying assumptions. Final flow harmonics are sensitive to these differences in the initial state due to the nearly linear mapping between eccentricities and anisotropic flow harmonics. The Trento code uses a model-agnostic approach by phenomenologically parameterizing the initial state and constraining those parameters from a Bayesian analysis. In this framework, multiplicity fluctuations are determined by a one parameter gamma distribution. While this approach covers a broad class of initial-state models, it is not universal. Notably, initial-state models arising from the Color-Glass Condensate (CGC) framework lead to an initial energy density which is outside the functional form considered in Trento and its later Bayesian analyses. Instead, in the CGC approach the multiplicity fluctuations normally use a log-normal distribution. In this work we compare $T_{A}T_{B}$ scaling (CGC-like) to $\sqrt {T_{A}T_{B}}$ scaling (preferred from a Trento Bayesian analysis) and determine that the choice in a gamma distribution may inadvertently exclude $T_{A}T_{B}$ scaling from a Bayesian analysis..

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