Hadrons and Heavy Quarks
The useful starting point for hadron physics is an observable, not a model. A stable one-particle state is identified through its quantum numbers and pole; an unstable hadron through a pole of a coupled-channel amplitude; a current probe through invariant form factors; and a high-energy probe through a factorized light-ray correlator. Heavy-light and heavy-heavy systems then call for different effective expansions. This chapter keeps those statements separate while showing how each is connected to QCD.
Enter this chapter
Section titled “Enter this chapter”Begin with Hadron Quantum Numbers and the QCD Spectrum to classify color-singlet states before selecting a dynamical approximation.
Topics
Section titled “Topics”- Hadron Quantum Numbers and the QCD Spectrum — Organizes stable hadrons by exact and approximate quantum numbers.
- Hadron Resonances and Coupled-Channel Scattering — Defines resonances through analytically continued scattering poles.
- Hadron Form Factors and Local-Current Structure — Extracts invariant structure from local-current matrix elements.
- Partonic Structure, Spin, and Hadron Tomography — Distinguishes the information retained in PDFs, GPDs, and TMDs.
- Heavy-Quark Symmetry and HQET — Expands a single heavy quark’s residual motion.
- Quarkonium and Nonrelativistic QCD — Separates heavy-pair mass, momentum, and binding scales.
How the ideas fit together
Section titled “How the ideas fit together”HQET and nonrelativistic QCD both exploit a large quark mass, but they organize different infrared dynamics. One heavy quark in a light hadron behaves, at leading order, as a source whose spin decouples from the light cloud. A heavy quark–antiquark pair has relative motion and a binding-energy scale that cannot be absorbed into that same counting. Choosing the expansion therefore requires the state and kinematics, not merely a heavy flavor. Neither approximation turns a finite-volume energy into an infinite-volume resonance pole. Neubert 1994, §§ 2–3.
Where to go next
Section titled “Where to go next”Continue to Nuclear and Few-Body EFT to identify the pion and nucleon variables of a chiral expansion.