The Pion Form Factor

The pion, and specifically its charge form factor, is of key interest in the study of the quark-gluon structure of hadrons. This is exemplified by the many calculations that treat the pion as one of their prime examples. One of the reasons is that the valence structure of the pion, being 〈qq̄〉, is relatively simple. Hence it is expected that the value of the four-momentum transfer squared Q, down to which a pQCD approach to the pion structure can be applied, is lower than for the nucleon. Whereas, e.g., the proton form factors seem to be completely dominated by constituent quark properties 1 up to at least Q = 10 20 (GeV/c), recent estimates 2 suggest that pQCD contributions start to dominate the pion form factor at Q ≥ 5 (GeV/c). Furthermore, the asymptotic normalization of the pion wave function, in contrast to that of the nucleon, is known from the pion decay. Within perturbative QCD one can then derive 3