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

Measurement of the reaction gamma* p --> phi p in deep inelastic e+ p scattering at HERA

The production of $\phi$ mesons in the reaction $e^{+}p \rightarrow e^{+} \phi p$ ($\phi \rightarrow K^{+}K^{-}$), for $7 < Q^2 < 25$ GeV$^2$ and for virtual photon-proton centre of mass energies ($W$) in the range 42-134 GeV, has been studied with the ZEUS detector at HERA. When compared to lower energy data at similar $Q^2$, the results show that the \gamma^*p \rightarrow \phid p cross section rises strongly with $W$. This behaviour is similar to that previously found for the $\gamma^*p \rightarrow \rho^0 p$ cross section. This strong dependence cannot be explained by production through soft pomeron exchange. It is, however, consistent with perturbative QCD expectations, where it reflects the rise of the gluon momentum density in the proton at small $x$. The ratio of $\sigma (\phi) / \sigma (\rho^0)$, which has previously been determined by ZEUS to be 0.065 $\pm$ 0.013 (stat.) in photoproduction at a mean $W$ of 70 GeV, is measured to be 0.18 $\pm$ 0.05 (stat.) $\pm$ 0.03 (syst.) at a mean $Q^2$ of 12.3 GeV$^2$ and mean $W$ of $\approx$ 100 GeV and is thus approaching at large $Q^2$ the value of 2/9 predicted from the quark charges of the vector mesons and a flavour independent production mechanism.Comment: 22 pages, including 4 figures. Includes some minor editorial change

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