Suppression of High Transverse Momentum $\pi^0$ Spectra in Au+Au Collisions at RHIC

TL;DR

This paper investigates the suppression of high transverse momentum neutral pions in Au+Au collisions at RHIC, proposing a late-stage cascade of pre-hadrons as the main suppression mechanism supported by pQCD estimates.

Contribution

It extends a pre-hadronic cascade model to heavy ion collisions, explaining high-$p_T$ suppression through a two-phase process with a short-lived colored phase and a longer pre-hadron phase.

Findings

Suppression mainly occurs during the late pre-hadron cascade stage.

A pQCD-based estimate supports a brief colored phase duration.

The model accounts for observed suppression patterns at RHIC.

Abstract

Au+Au, $s^{1/2} = 200$ A GeV measurements at RHIC, obtained with the PHENIX, STAR, PHOBOS and BRAHMS detectors, have all indicated a suppression of neutral pion production, relative to an appropriately normalized NN level. For central collisions and vanishing pseudo-rapidity these experiments exhibit suppression in charged meson production, especially at medium to large transverse momenta. In the PHENIX experiment similar behavior has been reported for $\pi^0$ spectra. In a recent work on the simpler D+Au interaction, to be considered perhaps as a tune-up for Au+Au, we reported on a pre-hadronic cascade mechanism which explains the mixed observation of moderately reduced $p_\perp$ suppression at higher pseudo-rapidity as well as the Cronin enhancement at mid-rapidity. Here we present the extension of this work to the more massive ion-ion collisions. Our major thesis is that much of the suppression is generated in a late stage cascade of colourless pre-hadrons produced after an initial short-lived coloured phase. We present a pQCD argument to justify this approach and to estimate the time duration $\tau_p$ of this initial phase. Of essential importance is the brevity in time of the coloured phase existence relative to that of the strongly interacting pre-hadron phase. The split into two phases is of course not sharp in time, but adequate for treating the suppression of moderate and high $p_\perp$ mesons.