Novel QCD Phenomenology

TL;DR

This paper reviews recent challenges to conventional hadron physics wisdom, highlighting phenomena explained by direct QCD processes, initial/final-state interactions, and non-universal nuclear effects, emphasizing the importance of dynamical structure functions and advanced QCD methods.

Contribution

It introduces novel QCD phenomenological insights, emphasizing the role of direct processes, non-universal nuclear effects, and the distinction between static and dynamical structure functions.

Findings

Direct hadron production explains deviations in inclusive cross sections.

Antishadowing depends on quark flavor, not universal.

Eliminating renormalization scale ambiguity improves QCD predictions.

Abstract

I review a number of topics where conventional wisdom in hadron physics has been challenged. For example, hadrons can be produced at large transverse momentum directly within a hard higher-twist QCD subprocess, rather than from jet fragmentation. Such "direct" processes can explain the deviations from perturbative QCD predictions in measurements of inclusive hadron cross sections at fixed x_T= 2p_T/\sqrt s, as well as the "baryon anomaly", the anomalously large proton-to-pion ratio seen in high centrality heavy ion collisions. Initial-state and final-state interactions of the struck quark lead to Bjorken-scaling single-spin asymmetries, diffractive deep inelastic scattering, the breakdown of the Lam-Tung relation in Drell-Yan reactions, as well as nuclear shadowing and antishadowing. The Gribov-Glauber theory predicts that antishadowing of nuclear structure functions is not universal, but instead depends on the flavor quantum numbers of each quark and antiquark, thus explaining the anomalous nuclear dependence measured in deep-inelastic neutrino scattering. One cannot attribute such phenomena to the structure of the hadron or nucleus itself. It is thus important to distinguish "static" structure functions computed from the square of the target light-front wavefunctions, versus "dynamical" structure functions which include the effects of the final-state rescattering of the struck quark. The importance of the J=0 photon-quark QCD contact interaction in deeply virtual Compton scattering is emphasized. The scheme-independent BLM method for setting the renormalization scale is discussed. Eliminating the renormalization scale ambiguity greatly improves the precision of QCD predictions and increases the sensitivity of searches for new physics at the LHC. Other novel features of QCD are discussed, including the consequences of confinement for quark and gluon condensates.