ATLAS

Abstract:

Events with a final-state proton carrying a large fraction of the proton beam momentum, x L >0.6, and the square of the transverse momentum p T ² <0.5 GeV ² , have been studied in e ⁺ p collisions with the ZEUS detector at HERA. Data with different photon virtualities were used: Q ² <0.02 GeV ² , 0.12 <0.74 GeV ² and 32 <254 GeV ² , corresponding to integrated luminosities of 0.9, 1.85 and 3.38 pb ^-1 . The cross sections are given as a function of x L , p T ² , Q ² and the Bjorken scaling variable, x. The ratio of the cross section for leading proton production to the inclusive e ⁺ p cross section shows only a mild dependence on Q ² and on x. In the region 0.6L <0.97, the relative yield of protons is only a weak function of x L . © 2003 Elsevier Science B.V. All rights reserved.

Abstract:

Differential cross sections for jet photoproduction in the reaction ep → e jet X have been measured with the ZEUS detector at HERA using 82.2 pb^-1 of integrated luminosity. Inclusive jet cross sections are presented as a function of the jet transverse energy, ET^jet, for jets with ET^jet > 17 GeV and pseudorapidity -1 <η^jet <2.5, in the γp centre-of-mass-energy range 142 < Wγp < 293 GeV. Scaled jet invariant cross sections are presented as a function of the dimensionless variable xT ≡ 2ET^jet/Wγp for (Wγp) = 180 and 255 GeV. Next-to-leading-order QCD calculations give a good description of the measured differential cross sections in both magnitude and shape. The ratio of scaled jet invariant cross sections at the two (Wγp) values shows clear non-scaling behaviour. A value for the strong coupling constant of αs(MZ) = 0.1224 ± 0.0001 (stat.)-0.0019^+0.0022(exp.)-0.0042 ^+0.0054(th.) has been extracted from a QCD analysis of the measured dσ/dET^jet. The variation of αs with ET^jet is in good agreement with the running of αs as predicted by QCD. © 2003 Published by Elsevier Science B.V.