Quantification of the low-$p_{\rm T}$ pion excess in heavy-ion collisions at the LHC and top RHIC energy

TL;DR

This paper uses Bayesian inference to quantify discrepancies between hydrodynamic models and experimental data, revealing a pion excess at low transverse momentum in heavy-ion collisions at RHIC and LHC energies.

Contribution

It introduces a Bayesian framework to assess model-data differences in heavy-ion collision particle spectra, highlighting a pion excess below 500 MeV/c.

Findings

Identified a pion excess below 500 MeV/c in heavy-ion collisions.

Provided a method to test the completeness of hydrodynamic models.

Suggested possible explanations for the pion excess.

Abstract

While the abundances of the final state hadrons in relativistic heavy-ion collisions are rather well described by the thermal particle production, the shape of the transverse momentum, $p_{\rm T}$, distribution below $p_{\rm T}\approx500$ MeV$/c$, is still poorly understood. We propose a procedure to quantify the model-to-data differences using Bayesian inference techniques, which allows for consistent treatment of the experimental uncertainties and tests the completeness of the available hydrodynamic frameworks. Using relativistic fluid framework Fluid${\it u}$M with PCE coupled to \trento initial state and FastReso decays, we analyse \pt distribution of identified charged hadrons measured in heavy-ion collisions at top RHIC and the LHC energies, and identify an excess of pions produced below $p_{\rm T}\approx500$ MeV$/c$. Our results provide new input for the interpretation of the pion excess as either missing components in the thermal particle yield description or as an evidence for a different particle production mechanism.