Exclusive resonance production in γγ collisions

This paper reports on recent work in the field of resonance production in γγ collisions. It has concentrated on the following exclusive reactions: ε+ε- → ε+ε- f0(975)/f2(1270)/f4(2030)/a0(980)/a2(1320), f0, f2, f4 → π0 π0, a0, a2 → π0 η; ε+ ε- → ε+ ε- η'(958), → π+ π- η; ε+ ε- → ε+ ε- (f1(1285)/f1(1420)), f1(1285) → π+π- η, f1(1420) → Κ±Κ0S πminus-plus. In the third reaction one of the scattered electrons was also observed (single tag events), in the other reactions only the resonance decay products were observed, i.e. the final states 4γ and π+π-γγ. In order to detect the latter low energy final states, a special trigger configuration was used: The central lead glass barrel system was divided into 7 radial sections (septants), each of which contributed an energy sum in the trigger logic. Opposite (1-4, 1-5, 2-5, ...) septants with energy above threshold (∼180 MeV) gave the trigger. In one mode TOF counters were not allowed, i.e. no charged track reached the TOF, in another mode a TOF counter was demanded, together with a low momentum (≥70 MeV/c) track in the inner detector track logic