Assessing the ultra-central flow puzzle in the Bayesian era

Abstract

An outstanding problem in the field of relativistic heavy-ion collisions is the inability for any simulation model to accurately describe experimental flow data in extremely central collisions -in particular, models always predict either an elliptic flow that is too large or triangular flow that is too small (or both). We reassess the status of this puzzle in light of recent progress in Bayesian parameter estimation, in which a large model parameter space can be efficiently explored to determine what parameters are necessary for a good fit to experimental results, and how well state-of-the-art models are able to describe data. We explore predictions for flow in ultra-central collisions from multiple recent Bayesian models that were tuned to various observables in different collision systems at typical centralities. We find that, while ultra-central data can now be described with better accuracy than in previous calculations, tension with experimental observation remains, and progressively gets worse as one goes to more central collisions. Thus, the physics of ultra-central collisions is still not fully understood. A resolution to this puzzle will be an important step in solidifying our understanding of the physics of the strong interactions in these extreme conditions, as well as increasing our confidence in the results of precision analyses.

Jet cone radius dependence of $R_{AA}$ and $v_2$ at PbPb 5.02 TeV from JEWEL+$\rm T_RENTo$+v-USPhydro

“…The lattice QCD based equation of state PDG16+/2+1[WB] from [66] is used. This framework successfully fits (and has made successful predictions for) data from RHIC and the LHC [56,[66][67][68] but ultracentral collisions are still slightly off [69] (this remains as an unresolved issue in the field [70]).…”

Estudo das flutuações em colisões ultracentrais de íons pesados relativísticos

“…Entretanto, em eventos ultra centrais, o estado da arte atual dos modelos de colisões de íons pesados não consegue calcular valores dos harmônicos de fluxo que se ajustem aos dados experimentais para diferentes harmônicos simultaneamente. Em outras palavras, os harmônicos de fluxo v n calculados pelo estado da arte atual não são próximos aos valores experimentais em regime ultracentral, como podemos ver em Rose et al [42], em Luzum et al [3] e em Giannini et al [56]. Na fig.…”