ar X iv : h ep - p h / 05 12 23 5 v 1 1 9 D ec 2 00 5 About earlier history of two – photon physics

The earlier history of two–photon physics is reviewed. The term two–photon processes is used now for the reactions in which some system of particles is produced in collision of two photons, either real or virtual. In the study of these processes the principal goal is to describe main features of proper two–photon process separating it from mechanism which responsible for the production of incident photons. • An early interest in the two–photon physics has arisen after discovery of positron by Anderson (1932). There appeared a necessity to find out the process in which positrons are generated. In 1934 studying e + e − pair production in collision of ultrarelativistic charged particles Landau and Lifshitz [1] have ascertained that the two photon channel of Fig. 1 is dominated here. ¡ p2 p1 p ′ 2 p ′ 1 1 Figure 1: Two photon particle production in collision of two fast particles. They calculated the cross section of the process Z 1 Z 2 → Z 1 Z 2 e + e − in the leading logarithmic approximation. Almost simultaneously Bethe and Heitler [2] considered e + e − pair production by photon in the field of a nuclei. This process contain subprocess γγ → e + e − , like Fig. 1. The leading log result of [1] was improved by Racah [3] who have calculated the corresponding cross section with a high accuracy ∼ (M/E) 2 where E and M are energy and mass of incident nuclei. The process γA → e + e − A was included in the theory of wide atmospheric showers in cosmic rays [4] and in the description of the energy losses of fast muons in matter [5]. The two-photon hadron production was considered for the first time by Primakoff [6] suggested in 1951 to measure the π 0 life–time in the reaction γZ → π 0 Z. The new interest to such processes appeared when the construction of e + e − colliders become close to a reality. In 1960 Low [7] paid attention to the fact that the π 0 life–time can be measured also in in e + e − collisions. Simultaneously the two–photon reaction e + e − → e + e − π + π − (for point–like pions) was considered [8]. However, the rates involved seemed unmeasurable at that time and no further work was done.