QCD Physics Opportunities in Low-Energy Electron-Positron Annihilation

Abstract

I survey a number of interesting tests of quantum chromodynamics at the amplitude level which can be carried out in electron-positron annihilation and in photon-photon collisions at low energy, including measurements of fundamental processes such as timelike form factors and transition amplitudes, timelike Compton scattering, timelike photon to meson transition amplitudes, and two-photon exclusive processes. Many of these reactions test basic principles of QCD such as hadronization at the amplitude level, factorization, and hadron helicity conservation, tools also used in the analysis of exclusive $B$ and $D$ decays. Measurements of the final-state polarization in hadron pair production determine the relative phase of the timelike form factors. The role of two-photon exchange amplitudes can be tested using the charge asymmetry of the $e^+ e^- \to B \bar B$ processes. Precision measurements of the electron-positron annihilation cross section can test the generalized Crewther relation and determine whether the effective couplings defined from physical measurements show infrared fixed-point and near conformal behavior. I also discuss a number of tests of novel QCD phenomena accessible in $e^+ e^-$ annihilation, including near-threshold reactions, the production of baryonium, gluonium states, and pentaquarks.