ATLAS

Abstract:

We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel D^*+→ (D⁰→ K^-π⁺)π⁺(+c.c.). Clear signals in the mass difference ΔM=M(D^*)-M(D⁰) as well as in the M(Kπ) distribution at the D⁰mass are found. The ep cross section for inclusive D^*±production with Q²< 4 GeV²in the γp centre-of-mass energy range 115 < W < 275 GeV has been determined to be (32±7-7 ⁺⁴) nb in the kinematic regionpt(D^*) ≥ 1.7 GeV, |η(D^*)| < 1.5. Extrapolating outside this region, assuming a mass of the charm quark of 1.5 GeV, we estimate the ep charm cross section to be σ(ep → ccX) = (0.45 ± 0.11-0.22 ^+0.37) μb at s=296 GeV and 〈W〉 = 198 GeV. The average γp charm cross section σ(γp → ccX) is found to be (6.3 ± 2.2-3.0 ^+6.3) μb at 〈W〉 = 163 GeV and (16.9 ± 5.2-8.5 ^+13.9) μb at 〈W〉 = 243 GeV. The increase of the total charm photoproduction cross section by one order of magnitude with respect to low energy data experiments is well described by QCD NLO calculations using singular gluon distributions in the proton. © 1995.

Abstract:

Dijet production by almost real photons has been studied at HERA with the ZEUS detector. Jets have been identified using the cone algorithm. A cut on xγ ^OBS, the fraction of the photon energy participating in the production of the two jets of highest transverse energy, is used to define cross sections sensitive to the parton distributions in the proton and in the photon. The dependence of the dijet cross sections on pseudorapidity has been measured for xγ ^OBS ≥ 0.75 and xγ ^OBS < 0.75. The former is sensitive to the gluon momentum density in the proton. The latter is sensitive to the ginon in the photon. The cross sections are corrected for detector acceptance and compared to leading order QCD calculations. © 1995.