Why Believe in QCD?
I will divide the evidence for QCD into a priori evidence and a posteriori evidence, obtained by comparing specific QCD predictions with data.1 The former consists of the very strong evidence, reviewed below, that hadrons are made of colored quarks which interact through the exchange of vector (spin one) gluons. Evidently the force must be color dependent, since we only see a single π meson rather than the nine different color combinations which can be made from a tricolored quark and antiquark. The only sensible (renormal-izable, unitary etc.) theory of vector gluons coupled to color is QCD [or a color gauge theory based on SO(3) but this turns out to fail the a posteriori tests and will not be discussed further].
KeywordsTotal Cross Section Chiral Symmetry Deep Inelastic Scattering Renormalization Group Equation Chiral Limit
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