I. THÉORIE / THEORYCURRENT ALGEBRA, PCAC, ITS ANOMALY, AND THE TWO-PHOTON PROCESS (SOFT-PION PRODUCTION BY TWO PHOTONS)

Various applications of the current algebra, PCAC (partially conserved axialvetor current hypothesis) and its anomaly to the hadron production by two photons γ + γ → hadrons are reviewed. They include theprocesses (1) γ + γ → π0, η, η' (or X0) and ε (2) γ + γ → 3 π0, π+ + π– + π0, and (3) γ + γ → nπ+ + nπ– (n = 1, 2, 3, …), where all produced pions are soft. Asymptotic behavoir of the processus (4) γ* + γ* → π+ + π– and (5) γ* + γ* → π0 for highly virtual photons is also discussed