Systematic Azimuth Quadrupole and Minijet Trends from Two-Particle Correlations in Heavy-Ion Collisions

Abstract

Heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) produce a tremendous amount of data but new techniques are necessary for a comprehensive understanding of the physics behind these collisions. We present measurements from the STAR detector of both $p_t$-integral and $p_t$-differential azimuth 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 correlations can be fit to extract a quadrupole component---related to conventional $v_2$ measures---and a same-side peak. The azimuth quadrupole component is distinguished from $\eta$-localized same-side correlations by taking advantage of the full 2D $\eta$ and $\phi$ dependence. 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. Angular correlations contain jet-like structure with most-probable hadron momentum \mbox{$\sim 1$ GeV/c}. For better comparison to RHIC data we analyze the energy scale dependence of fragmentation functions from $e^+$-$e^-$ collisions using conventional momentum measures $x_p$ and $\xi_p$ and rapidity $y$. We find that replotting fragmentation functions on a normalized rapidity variable results in a compact form precisely represented by the beta distribution, its two parameters varying slowly and simply with parton energy scale $Q$. The resulting parameterization enables extrapolation of fragmentation functions to low $Q$ in order to describe fragment distributions at low transverse momentum $p_t$ in heavy ion collisions at RHIC. We convert minimum-bias jet-like angular correlations to single-particle hadron yields and compare them with parton fragment yields inferred from differential spectrum analysis (spectrum hard components). We find that jet-like correlations in central 200 GeV Au-Au collisions correspond quantitatively to pQCD predictions, and the jet-correlated hadron yield comprises one third of the Au-Au final state in central collisions.