Transverse-rapidity $\bf y_t$ dependence of the nonjet azimuth quadrupole from 62 and 200 GeV Au-Au collisions

TL;DR

This paper analyzes the azimuthal quadrupole component in heavy ion collisions, distinguishing it from jet contributions, and finds that its behavior is consistent across various parameters, challenging hydrodynamic interpretations.

Contribution

It introduces a method to isolate the nonjet azimuth quadrupole component and provides a universal parametrization of its dependence on multiple collision variables.

Findings

The nonjet quadrupole component is systematically characterized across different energies and centralities.

The quadrupole source boost exhibits unexpected properties, indicating complex underlying dynamics.

The quadrupole trends are largely independent of jet-related medium changes, questioning hydrodynamic models.

Abstract

Previous measurements of a quadrupole component of azimuth correlations denoted by symbol $v_2$ have been interpreted to represent elliptic flow, a hydrodynamic phenomenon conjectured to play a major role in noncentral nucleus-nucleus collisions. $v_2$ measurements provide the main support for conclusions that a ``perfect liquid'' is formed in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). However, conventional $v_2$ methods based on one-dimensional (1D) azimuth correlations give inconsistent results and may include a jet contribution. In some cases the data trends appear to be inconsistent with hydrodynamic interpretations. In this study we distinguish several components of 2D angular correlations and isolate a nonjet (NJ) azimuth quadrupole denoted by $v_2\{\text{2D}\}$. We establish systematic variations of the NJ quadrupole on $y_t$, centrality and collision energy. We adopt transverse rapidity $y_t$ as both a velocity measure and as a logarithmic alternative to transverse momentum $p_t$. Based on NJ quadrupole trends we derive a completely factorized universal parametrization of quantity $v_2\{\text{2D}\}(y_t,b,\sqrt{s_{NN}})$ which describes the centrality, $y_t$ and energy dependence. From $y_t$-differential $v_2(y_t)$ data we isolate a quadrupole spectrum and infer a quadrupole source boost having unexpected properties. NJ quadrupole $v_2$ trends obtained with 2D model fits are remarkably simple. The centrality trend appear to be uncorrelated with a sharp transition in jet-related structure that may indicate rapid change of Au-Au medium properties. The lack of correspondence suggests that the NJ quadrupole may be insensitive to such a medium. Several quadrupole trends have interesting implications for hydro interpretations.