Resonance production in elastic scattering of quasi-real photons

The Crystal Ball detector, running at the DORIS II e+e- storage ring in Hamburg, Germany, has been used to study the production of pseudoscalar mesons in photon-photon collisions. Production of π degree, η, and η' mesons has been observed. For the analysis of π degree events, 46 pb-1 of data have been used; 114 pb-1 have been used to study η and η' production. The mesons, P, are produced by the process e+e- → e+e-P, and observed via their decay to two photons, P → γγ. The outgoing e+e- are undetected, so the observed events contain two low-energy photons in the main detector, and no other tracks or showers. After correcting for background contributions, there are 1182 ± 72 π degree signal events, 1295 ± 44 η events, and 136 ± 14 η' events. These values are used together with the detection efficiency and calculated cross section to deduce the following meson partial widths: Γπdegree → γγ = 7.7 ± 0.5 ± 0.5 eV; Γη → γγ = 0.514 ± 0.017 ± 0.035 keV; Γη' → γγ = 5.6 ± 0.06 ± 0.6 keV. The π degree result is in agreement with the direct measurement of the π degree lifetime. This is the first case in which a width measured using two-photon collisions can be checked against a precise direct measurement. The η and η' results are in agreement with other photon-photon collision experiments. The η partial width is the most precise obtained to date