Qcd Physics Opportunities in Low-energy Electron-positron Annihilation∗

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. Some of the tests require e+e− center of mass energy as small as √ s = 2 GeV. Other tests which involve a spectrum of energies can be carried out advantageously at high energy facilities using the radiative return method. These include 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 and thus strongly discriminate between analytic forms of models which fit the form factors in the spacelike region. The role of two-photon exchange amplitudes can be tested using the charge asymmetry of the e+e− → BB processes. These tests can help resolve the discrepancy between the Jefferson laboratory measurements of the ratio of GE and GM proton form factors using the polarization transfer method versus measurements using the traditional Rosenbluth method. 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.