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Origin of the strong centrality dependence of correlation exponents in ultracentral collisions

Ascertain the physical origin of the pronounced variation with centrality (in the 0–10% window) of the exponents α and α′n that parametrize, at fixed impact parameter, the correlations between initial-state eccentricity εn (and its variance) and multiplicity N observed in TRENTO simulations, and determine why these variables approach the ultracentral limit in a singular manner.

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Background

Using TRENTO initial-state simulations at fixed impact parameter, the authors find negative correlation exponents α (linking mean reaction-plane eccentricity to N) and α′n (linking variance of εn fluctuations to N) and observe that these exponents vary significantly with centrality in the 0–10% range.

They hypothesize that participant selection and the rapid reduction of participant-number fluctuations as c→0 may drive this behavior but explicitly state that the precise origin is not understood and leave a dedicated investigation to future work.

References

Surprisingly, we discover that the exponents α and α′n display a significant variation with c in the 0-10% window. We do not understand the precise origin of this variation. We leave the investigation of this phenomenon to a future study, as here we are only interested in knowing whether the inclusion of non-zero α and α′n values improves the reconstruction of the eccentricities at fixed impact parameter, which we study now.

— Impact parameter dependence of anisotropic flow: Bayesian reconstruction in ultracentral nucleus-nucleus collisions (2407.17308 - Alqahtani et al., 24 Jul 2024) in Subsection: Correlations between anisotropy and multiplicity (Section: Models of initial fluctuations)