Inclusive hadron production in photon-photon collisions at next-to-leading order.

We study inclusive charged-hadron production in collisions of quasireal photons at next-to-leading order (NLO) in the QCD-improved parton model, using fragmentation functions recently extracted from PEP and LEP1 data of e+e− annihilation. We consistently superimpose the direct (DD), single-resolved (DR), and double-resolved (RR) γγ channels. We consider photon spectra generated by electromagnetic bremsstrahlung and/or beamstrahlung off colliding e+ and e beams as well as those which result from backscattering of laser light off such beams. First, we revisit existing single-tag data taken by TASSO at PETRA and by MARK II at PEP (with e+e− energy √ S ≈ 30 GeV) and confront them with our NLO calculations imposing the respective experimental cuts. We also make comparisons with the neutral-kaon to charged-hadron ratio measured by MARK II. Then, we present NLO predictions for LEP2, a next-generation e+e− linear collider (NLC) in the TESLA design with √ S = 500 GeV, and a Compton collider obtained by converting a 500-GeV NLC. We analyze transverse-momentum and rapidity spectra with regard to the scale dependence, the interplay of the DD, DR, and RR components, the sensitivity to the gluon density inside the resolved photon, and the influence of gluon fragmentation. It turns out that the inclusive measurement of small-pT hadrons at a Compton collider would greatly constrain the gluon density of the photon and the gluon fragmentation function. PACS numbers: 13.60.Hb, 13.60.Le, 13.65.+i, 13.87.Fh, 14.40.Aq ∗Supported by Bundesministerium für Forschung und Technologie, Bonn, Germany, under Contract 05 6 HH 93P (5), and by EEC Program Human Capital and Mobility through Network Physics at High Energy Colliders under Contract CHRX–CT93–0357 (DG12 COMA).