Dijet cross-sections in photoproduction at HERA

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 xg, 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 xg $\ge 0.75$ and xg $< 0.75$. The former is sensitive to the gluon momentum density in the proton. The latter is sensitive to the gluon in the photon. The cross sections are corrected for detector acceptance and compared to leading order QCD calculations.Comment: 21 pages, latex, 4 figures appended as uuencoded fil

Study of D*(2010)+- Production in ep Collisions at HERA

We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel D*(+) --> (D-0 --> K(-)pi(+)) pi(+) (+ c.c.). Clear signals in the mass difference Delta M=M(D*)-M(D-0) as well as in the M(K pi) distribution at the D-0 mass are found. The ep cross section for inclusive D*(+/-) production with Q(2) c $($) over bar cX)=(0.45+/-0.11(-0.22)(+0.37)) mu b at root s=296 GeV and [W]=198 GeV. The average gamma p charm cross section sigma(gamma p --> c $($) over bar cX) is found to be (6.3+/-2.2(-3.0)(+6.3)) mu b at [W]=163 GeV and (16.9+/-5.2(-8.5)(+13.9)) mu b 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

Comparison of energy flows in deep inelastic scattering events with and without a large rapidity gap

Energy flows in deep inelastic electron-proton scattering are investigated at a center-of-mass energy of 296 GeV for the range Q2 greater-than-or-equal-to 10 GeV2 using the ZEUS detector. A comparison is made between events with and without a large rapidity gap between the hadronic system and the proton direction. The energy flows, corrected for detector acceptance and resolution, are shown for these two classes of events in both the HERA laboratory frame and the Breit frame. From the differences in the shapes of these energy flows we conclude that QCD radiation is suppressed in the large-rapidity-gap events compared to the events without a large rapidity gap

Observation of jet production in deep inelastic scattering with a large rapidity gap at HERA

Events with a large rapidity gap in deep inelastic scattering with Q2 ≥ 10 GeV2 have been studied in the ZEUS detector. The properties of these events with W &gt; 140 GeV are consistent with a leading twist diffractive production mechanism. In the laboratory frame, with ETjet ≥ 4 GeV, 15% of the events are of the 1-jet type with negligible 2-jet production. The single jet is back-to-back in azimuth with the scattered electron. No energy flow is observed between the jet and the proton direction. With a lower jet transverse energy cut 2-jet production is observed both in the laboratory and the γ∗ p centre-of-mass systems, demonstrating the presence of hard scattering in the virtual photon proton interactions that give rise to large rapidity gap events