The early experiments on photon-photon collisions
The fourth order QED calculation agrees with the experiment and the exact calculation differs from the equivalent photon approximation by less than 30%.
Two methods of seaparating YY events from annihilation events have been experimentally checked: the tagging of the forward e+e and no tagging. Both methods have been successful.
The first indication of hadron production from yy shows that the cross-section for γγ → ππ is not very different from the pointlike cross-section.
From the reaction γγ → nh an upper limit on η′ radiative decay has been derived: γη′→γγ ≤ 16 KeV, 95% C.L.L.
KeywordsStorage Ring Hadron Production Annihilation Event Cerenkov Counter Energy Acceptance
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