Introduction to QCD

TL;DR

This work provides a compact, graduate-level roadmap to quantum chromodynamics for collider physics, linking perturbative calculations to non-perturbative modeling. It details factorization, PDFs, and fixed-order perturbation theory, then builds to parton showers and various matching schemes that merge matrix elements with resummation, all within a coherent MC framework. It also covers hadronization via string/cluster models and the soft QCD sector (MPI, UE, MB), emphasizing tuning and uncertainties. The overall message is that precise collider predictions require a multi-layered approach that combines first-principles QCD with phenomenological, data-driven modeling and systematic uncertainty assessment.

Abstract

These lectures were originally given at TASI and are directed at a level suitable for graduate students in High Energy Physics. They are intended to give an introduction to the theory and phenomenology of quantum chromodynamics (QCD), focusing on collider physics applications. The aim is to bring the reader to a level where informed decisions can be made concerning different approaches and their uncertainties. The material is divided into five main areas: 1) fundamentals, 2) fixed-order perturbative QCD, 3) Monte Carlo event generators and parton showers, 4) Matching at Leading and Next-to-Leading Order, and 5) Soft QCD physics.

Figures (34)

• Figure 1: The title and part of the abstract of the 1951 paper Brueckner:1952zz (published in 1952) in which the existence of the $\Delta^{++}$ baryon was deduced, based on data from Sachs and Steinberger at Columbia Chedester:1951sc and from Anderson, Fermi, Nagle, et al. at Chicago Fermi:1952zz. Further studies at Chicago were quickly performed in Anderson:1952nwAnderson:1952zza. See also the memoir by Nagle nagle1984delta.
• Figure 2: Illustration of a $qqg$ vertex in QCD, before summing/averaging over colours: a gluon in a state represented by $\lambda^1$ interacts with quarks in the states $\psi_{qR}$ and $\psi_{qG}$.
• Figure 3: Illustration of the three crossings of the interaction of a lepton current (black) with a quark current (red) via an intermediate photon or $Z$ boson, with corresponding colour factors.
• Figure 4: Illustration of a $ggg$ vertex in QCD, before summing/averaging over colours: interaction between gluons in the states $\lambda^2$, $\lambda^4$, and $\lambda^6$ is represented by the structure constant $f^{246}$.
• Figure 5: Illustration of the running of $\alpha_s$ at 1- (open circles) and 2-loop order (filled circles), starting from the same value of $\alpha_s(M_Z)=0.12$.