Jet and hadron production in photon - photon collisions ∗

∗Heisenberg-Fellow hadronic fluctuations (“resolved”). Direct and resolved interactions can be separated by measuring the fraction xγ of the photon’s momentum participating in the hard interaction for the two photons. In γγ interactions they are labelled xγ for the two photons. Ideally, the direct γγ events with two bare photons are expected to have xγ = 1 and x − γ = 1, whereas for double-resolved events both values xγ and xγ are expected to be much smaller than one. In photoproduction, the interaction of a bare photon with the proton is labelled ‘direct’ (corresponding to ‘single-resolved’ in γγ) and the interaction of a hadronic photon is called ‘resolved’ (corresponding to ‘double-resolved’ in γγ).