The azimuth quadrupole in nuclear collisions

Abstract

We present measurements of both $p_t$-integral and $p_t$-differential azimuth quadrupole components of two-particle correlations on azimuth ($\phi$) and pseudorapidity ($\eta$) for unidentified hadrons in Au-Au collisions at $\sqrt{s_{NN}}=62$ and 200 GeV. The azimuth quadrupole component is distinguished from $\eta$-localized same-side correlations by taking advantage of the $\eta$ dependence. The quadrupole component is related to conventional $v_2$ measures. Both $p_t$-integral and $p_t$-differential results are presented as functions of Au-Au centrality. We observe simple universal energy and centrality trends for the $p_t$-integral quadrupole component. $p_t$-differential results are constructed using $v_{2}^{2}$ marginal distributions on $p_t$. These results can be transformed to reveal quadrupole $p_t$ spectra that are nearly independent of centrality. A parametrization of the $p_t$-differential quadrupole shows a simple $p_t$ dependence that can be factorized from the centrality and collision energy dependence above 0.75 GeV/c.