The hadron spectrum from QCD sum-rules

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Particle physics is the study of matter and forces at the smallest known distance scales. As we move to smaller and smaller distances, a certain hierarchy emerges: matter is made of molecules, molecules are made of atoms, atoms are made of electrons and nuclei, and nuclei are made of protons and neutrons. It was realized in the 1970's that protons and neutrons themselves are made of even smaller particles called quarks. The force responsible for holding the quarks together is called the strong nuclear force and the corresponding physical theory is called quantum chromodynamics (QCD). Quark combinations aside from protons and neutrons are also possible, and the resulting bound states are collectively referred to as hadrons. An important problem currently being researched in particle physics is that of trying to predict the properties of these hadrons based on known properties of the quarks that each contains. Such properties include mass, lifetime, and decay modes. In particular, I am investigating an as yet undiscovered class of particles called hybrids whose existence is predicted by theory. However, such particles have yet to be conclusively identified in experiment.