Challenging claims of 'elliptic flow' by comparing azimuth quadrupole and jet-related angular correlations from Au-Au collisions at $\sqrt{s_{NN}} = $ 62 and 200 GeV

摘要

Background: A component of azimuth correlations from high-energy heavy ioncollisions varying as $\cos(2\phi)$ and denoted by symbol $v_2$ isconventionally interpreted to represent "elliptic flow," a hydrodynamicmanifestation of the initial-state \aa overlap geometry. Several numericalmethods are used to estimate $v_2$, resulting in various combinations of "flow"and "nonflow" that reveal systematic biases in the $v_2$ estimates. QCD jetscontribute strongly to azimuth correlations and specifically to the$\cos(2\phi)$ component. Purpose: We question the extent of jet-related("nonflow") bias in and hydrodynamic "flow" interpretations of $v_2$measurements. Method: We introduce two-dimensional (2D) model fits to angularcorrelation data that distinguish accurately between jet-related correlationcomponents and a {\em nonjet azimuth quadrupole} that might represent "ellipticflow" if that were relevant. We compare measured jet-related and "flow"-relateddata systematics and determine the jet-related contribution to $v_2$measurements. Results: Jet structure does introduce substantial bias toconventional $v_2$ measurements, making interpretation difficult. The nonjetquadrupole exhibits very simple systematics on centrality and collisionenergy---the two variables factorize. Within a \auau centrality interval wherejets show no indication of rescattering or medium effects the nonjet quadrupoleamplitude rises to 60% of its maximum value. Conclusions: Disagreements betweennonjet quadrupole systematics and hydro theory expectations, the largequadrupole amplitudes observed in more-peripheral \auau collisions and asignificant nonzero value in \nn $\approx$ \pp collisions strongly suggest thatthe nonjet quadrupole does not arise from a hydrodynamic "flow" mechanism.