Exclusive, Hard Diffraction in QCD

TL;DR

This paper explores hard diffractive scattering in QCD, providing theoretical predictions, computational methods, and experimental implications for processes like DVCS at HERA, advancing understanding of skewed parton distributions.

Contribution

It introduces new algorithms for skewed parton distribution evolution, confirms factorization in DVCS, and proposes a novel method to determine the real-to-imaginary ratio of the DIS amplitude.

Findings

Validated evolution algorithms with high accuracy

Confirmed factorization for DVCS amplitude

Predicted significant DVCS cross sections at HERA

Abstract

In the first chapter we give an introduction to hard diffractive scattering in QCD to introduce basic concepts and terminology. In the second chapter we make predictions for the evolution of skewed parton distributions in a proton in the LLA. We calculate the DGLAP-type evolution kernels in the LLA and solve the skewed GLAP evolution equations with a modified version of the CTEQ-package. In the third chapter, we discuss the algorithms used in the LO evolution program for skewed parton distributions in the DGLAP region, discuss the stability of the code and reproduce the LO diagonal evolution within less than 0.5% of the original CTEQ-code. In chapter 4, we show that factorization holds for the deeply virtual Compton scattering amplitude in QCD, up to power suppressed terms, to all orders in perturbation theory. In chapter 5, we demonstrate that perturbative QCD allows one to calculate the absolute cross section of diffractive, exclusive production of photons (DVCS) at large $Q^2$ at HERA, while the aligned jet model allows one to estimate the cross section for intermediate $Q^2 \sim 2 GeV^2$. We find a significant DVCS counting rate for the current generation of experiments at HERA and a large azimuthal angle asymmetry for HERA kinematics. In the last chapter, we propose a new methodology of gaining shape fits to skewed parton distributions and, for the first time, to determine the ratio of the real to imaginary part of the DIS amplitude. We do this by using several recent fits to $F_2(x,Q^2)$ to compute the asymmetry $A$ for the combined DVCS and Bethe-Heitler cross section. In the appendix, we give an application of distributional methods as discussed abstractly in chapter 4.