Energy losses in the black disc regime and correlation effects in the STAR forward pion production in dAu collisions

TL;DR

The paper investigates energy losses of forward partons in the black disk regime (BDR) of small-$x$ QCD in $pA$ and $dA$ collisions, predicting substantial, energy- and $A$-dependent fractional energy loss and a strong centrality dependence that favors peripheral interactions for leading-hadron production. A quantitative framework is developed based on the inelastic interaction probability $P_{inel}(b,s,Q^2)$ and the effective number of collisions $N(b)$, yielding fractional losses $\epsilon_N \approx 0.06$ per collision and total losses $\epsilon_A(b) \approx N(b)\epsilon_N$, which can exceed 10% in central RHIC kinematics and grow with energy toward the LHC. Comparisons with STAR and BRAHMS data show patterns consistent with peripheral-dominated, energy-loss-driven suppression of forward hadrons and no strong suppression of recoil jets, contrasting with central-dominant CGC expectations; the authors propose observables such as $RR$ and $\Delta RR$ and ZDC-based centrality tagging to robustly reveal BDR onset in future RHIC and LHC experiments.

Abstract

We argue that in the small x processes, in the black disc QCD regime (BDR) a very forward parton propagating through the nuclear matter should loose a significant and increasing with energy and atomic number fraction of its initial energy as a result of dominance of inelastic interactions, causality and energy-momentum conservation. We evaluate these energy losses and find them to lead to the significant suppression of the forward jet production in the central NA collisions at collider energies with a moderate suppression of recoiling jet at central rapidities. We confront our expectations with the recent RHIC data of the STAR collaboration on the probability, $P$, for emission of at least one fast hadron at a central rapidity in association with production of a very forward high $p_t$ neutral pion in $pp$ and $dAu$ collisions. We calculate the A-dependence of $P$, and find that the data imply a strong suppression of leading pion production at central impact parameters. We also conclude that production of recoil jets in the hard subprocess is not suppressed providing further evidence for the dominance of peripheral collisions. Both features of the data are consistent with the onset of BDR. We suggest new phenomena and new observables to investigate BDR at RHIC and LHC.