Prompt Photons and Particle Momentum Distributions At

Recent results, obtained by the H1 and ZEUS collaborations, are presented on differential cross sections , for inclusive prompt photon production in DIS and for photoproduction of prompt photons accompanied by a hadronic jet. Also presented are cross sections of normalised scaled momentum distribution of charged final state hadrons, measured by H1 in DIS ep collisions at high Q 2 in the Breit frame of reference. Isolated high transverse energy photons in the final state are a powerful tool for detailed studies of the Quantum Chromody-namics (QCD) in hard interaction processes and of the hadronic structure of the incoming particles. The photons are called " prompt " if they are directly coupled to the interacting quarks, instead of being produced as hadronic decay products. In contrast to jet measurements, where the partonic structure is obscured by the non-perturbative hadronisation process, prompt photons at large transverse energy E γ T can be directly related to the partonic event structure. Furthermore, the experimental uncertainties connected with the energy determination of an electromagnetic shower initiated by a photon are smaller compared to the measurement of a hadron jet. However, the cross section for prompt pho-ton production is small and the identification of photons in the detector is not trivial. Preliminary results for two analyses are presented here: An H1 study of inclusive prompt photons in deep inelastic scattering (DIS) 1 , and a ZEUS study of prompt photons with an accompanying jet in photoproduction 2. The main experimental difficulty is the separation of the prompt photons from hadronic background, in particular from signals due to π 0 mesons. In the H1 analysis photons are identified in the liquid argon calorimeter by a compact electromagnetic cluster with no track pointing to it. The photon transverse energy E γ T and pseudorapidity η γ are restricted to 3 GeV < E γ T < 10 GeV and −1.2 < η γ < 1.8. In order to compare with perturbative QCD (pQCD) calculations , the photon isolation requirement is defined in an infrared-safe way, using: z = E γ /E photonjet > 0.9, i.e. z is the ratio of the photon energy to the energy of the jet, which contains the photon. The photon signal is extracted by a shower shape analysis which uses six discriminating shower shape functions in a likelihood method. In the ZEUS analysis photons are identified using the barrel preshower detector (BPRE). …