Leading twist shadowing, black disk regime and forward hadron production

TL;DR

This paper reviews leading twist nuclear shadowing and the black disk regime, explaining suppression patterns in forward hadron production and highlighting the dominance of certain mechanisms in different collision types.

Contribution

It introduces the post-selection suppression mechanism and clarifies the dominant processes in forward pion production in d-Au and pp collisions.

Findings

Dominance of the 2→2 mechanism in forward pion production in d-Au collisions.

Post-selection suppression explains the magnitude and pattern of forward pion suppression.

Double parton interactions significantly contribute to two-pion production, revealing correlations of leading quarks.

Abstract

We review theory of the leading twist nuclear shadowing, and describe phenomenon of post-selection suppression of leading parton spectrum (effective fractional energy losses) in the proximity of the black disk regime. We argue that $2 \to 2$ mechanism dominates in the inclusive leading pion production in d-Au collisions and explain that the post-selection naturally explains both the magnitude of the suppression of the forward pion production in d-Au collisions and the pattern of the forward - central correlations. At the same time this pattern of correlations rules out $2\to 1$ mechanism as the main source of the inclusive leading pion yield. It is demonstrated that the mechanism of the double parton interactions gives an important contribution to the production of two leading pions in $pp$ scattering opening a new way to study correlations of leading quarks in the nucleon. The same mechanism is enhanced in $dAu \to \pi^0\pi^0 +X$ collisions and explains the dominance of $\Delta\phi$ independent component and suppression of the away side peak.