Introduction

Abstract

The standard model of particle physics, put forward almost forty years ago, has been so successful at explaining observations that there are few experimental hints, at least from traditional sources such as accelerator centers, that anything more is needed. Perhaps this situation will change as new data emerges from the Large Hadron Collider at CERN; it is, after all, unlikely that the standard model is the end of the story. The Higgs sector seems a little unnatural, among other things. There is also a strong CP problem, there are many free parameters, there is no explanation of dark matter (let alone dark energy) or three fermion generations, and the theory does not accommodate gravity. A minor adjustment is also required to allow for neutrino masses. On the other hand, given this adjustment, there is little reason to doubt the validity of the standard model as a description of all the known elementary particles, and their interactions, down to a distance scale of at least 10⁻¹⁵ cm, and there is good reason to admire a theory which can describe such a wide range of phenomena so economically.

Because of the great importance of the standard model, and the central role it plays in our understanding of particle physics, it is unfortunate that, in one very important respect, we don’t really understand how it works. The problem lies in the sector dealing with the interactions of quarks and gluons, the sector known as Quantum Chromodynamics or QCD. We simply do not know for sure why quarks and gluons, which are the fundamental fields of the theory, don’t show up in the actual spectrum of the theory, as asymptotic particle states. There is wide agreement about what must be happening in high energy particle collisions: the formation of color electric flux tubes among quarks and antiquarks, and the eventual fragmentation of those flux tubes into mesons and baryons, rather than free quarks and gluons. But there is no general agreement about why this is happening, and that limitation exposes our general ignorance about the workings of non-abelian gauge theories in general, and QCD in particular, at large distance scales.