D: es from +·ce ody . cs Quantum chromodynamics (QCD) is the modern theory of the strong force . In this theory, the main objects are quarks and gluons. which are bound by the strong force into protons, neutrons, and other particles called hadrons. In the framework: of QCD, the strong nuclear force binding protons and neutrons together into nuclei is actually only a residue of the much stronger forces acting between quarks and gluons. In fact, inside the proton, even the concept of force is not

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... very useful. Within all hadrons we have a swirl of gluons being exchanged back and forth as a manifestation of the strong force. To make matters worse, gluons can split into two, and then rejoin, or they can split into a quark-antiquark pair. Even the simplest hadron is a complex system hosting constantly interacting components. Despite this complexity, QCD is well established experimentally. This is because at short distances (or high energies), the coupling between the particles is effectively small and particles move around with relative freedom. This is called asymptotic freedom and QCD is amenable to the traditional methods of quantum field theory in this regime. High-energy experiments have tested and confirmed QCD in this realm, which led to the 2004 Nobel Prize in Physics for Drs. David Gross, David Politzer, and Frank Wilczek, the theorists who provided the theory for short-range QCD and asymptotic freedom. On the other hand, the traditional methods break down when gluons (and all the complexities that they entail) travel over "long" distances (to particle physicists, the nuclear size 10-1 5 mis a long distance). There is a mathematically rigorous way to handle the gluons in such cases, which is to introduce a grid in space-time -the "lattice" of lattice QCD -and use a computer to keep track of the quarks and gluons on this grid. Of course, the computer does not do everything; the person running the computer has to make sure that the results do not depend on how fine or coarse the grid is. Fortunately, the grid spacing is considered a "short" distance, so on this scale we can use the wen-tested methods of perturbative (short distance) QCD.