x / 97 11 02 5 v 1 2 8 N ov 1 99 7 Jet Production at HERA †

Studies on the structure of the photon are presented by means of the extraction of a leading order effective parton distribution in the photon and measurements of inclusive jet differential cross sections in photoproduction. Measurements of the internal structure of jets have been performed and are also presented as a function of the transverse energy and pseudorapidity of the jets. Dijet cross sections have been measured as a function of the dijet mass and centre-of-mass scattering angle. The main source of jets at HERA comes from collisions between protons and quasi-real photons (Q ≈ 0, where Q is the virtuality of the photon) emitted by the positron beam (photoproduction). At lowest order QCD, two hard scattering processes contribute to jet production: the resolved process, in which a parton from the photon interacts with a parton from the proton, producing two jets in the final state; and the direct process, in which the photon interacts pointlike with a parton from the proton, also producing two jets in the final state. The study of high-pT jet photoproduction provides tests of QCD and allows to probe the structure of the photon. In perturbative QCD the cross section for jet production is given by dσ dydxγdxpd cos θ ∝ fγ/e(y) y ∑ ij fi/γ(xγ , Q̂ ) xγ fj/p(xp, Q̂ ) xp |Mij(cos θ )| (1) where fγ/e(y) is the flux of photons from the positron approximated by the Weizsäcker-Williams formula; y = Eγ/Ee is the inelasticity parameter; fi/γ(xγ , Q̂ ) (fj/p(xp, Q̂ )) are the parton densities in the photon (proton) at a scale Q̂; xγ = Ei/Eγ (xp = Ej/Ep) is the fractional momentum of the incoming parton from the photon (proton); and |Mij(cos θ )| are the QCD matrix elements for the parton-parton scattering. Two approaches to the study of the structure of the photon are presented here. One approach is to extract directly from the data an effective parton distribution in the photon. The second approach is to measure jet cross sections that can be calculated theoretically which then provide a testing ground for parametrisations of the photon structure function. The method [1] to extract an effective parton distribution in the photon is based on the use of the leading order (LO) matrix elements for the subprocesses with gluon exchange, which To appear in the Proccedings of the “International Europhysics Conference on High Energy Physics”, Jerusalem, Israel, August 1997. HERA provides collisions between positrons of energy Ee = 27.5 GeV and protons of energy Ep = 820 GeV. The resolved and direct processes are included in fi/γ .