Open Charm Production at HERA

Measurements of charmed particle cross sections at HERA in the photoproduction and deep inelastic regimes are reviewed. The status of the comparison with perturbative QCD calculations is discussed.Comment: Presented at the Ringberg Workshop on ``New Trends in HERA Physics 2001'' 17-22 June 2001. 12 pages, 11 figure

Production of J/psi Mesons at HERA

Inelastic and diffractive production of J/psi mesons at HERA is reviewed. The data on inelastic photoproduction are described well within errors by the Colour Singlet Model in next-to-leading order. A search for colour octet processes predicted within the NRQCD/factorisation approach is conducted in many regions of phase space. No unambiguous evidence has been found to date. Diffractive elastic production of J/psi mesons has been measured in the limit of photoproduction up to the highest photon proton center of mass energies. The increase of the cross section is described by pQCD models. At larger Q^2, the W dependence is found to be similar to that observed in photoproduction. First analyses of data at high t yield a powerlike dependence on |t|. A LO BFKL calculation gives a good description of the data.Comment: 14 pages, 12 figures, contribution to Ringberg 200

High Q2Q^2 Physics at HERA and Searches for New Particles

Preliminary results from H1 and ZEUS on Deep Inelastic Scattering (DIS) at high momentum transfer squared $Q^2$ are presented. Used are all available $e^+ p$ data accumulated by the H1 and ZEUS experiments between 1994 and 1997, corresponding to integrated luminosities of $37 {pb}^{-1}$ and $47 {pb}^{-1}$, respectively. The anomalies observed at high $Q^2$ in the 1994 to 1996 data still remain, though with less significance. Since this high $Q^2$ domain represents a new frontier in DIS, the same data are used to search for new particles possessing direct couplings to lepton-quark pairs. Assuming that the slight excess of events observed in Neutral Current DIS is due to a statistical fluctuation, preliminary limits on the production of leptoquarks and of squarks in R-parity violating MSSM are presented.Comment: 14 pages, 12 figures, uses iopart style files, contribution to the 3rd UK Phenomenology Workshop on HERA Physics, Durham, Sep 199

Structure functions at large x

Structure function data together with other measurements from fixed-target deep inelastic scattering and hadron-hadron collider experiments which contribute to our knowledge of the parton density functions are reviewed. The inclusive cross-section measurements of neutral and charged current interactions at HERA are presented and their impact on the parton density functions is discussed. Future prospects for an improved knowledge of the parton density functions at large x are briefly mentioned.Comment: Invited talk given at the workshop on ``New Trends in HERA Physics 2001'', Ringberg castle, Germany, June 17-22, 200

A characteristic plot of pomeron-exchanged processes in diffractive DIS

The dependence of the fractal behaviors of the pomeron induced system in deep inelastic lepton-nucleon scattering upon the diffractive kinematic variables is found rather robust and not sensitive to the distinct parameterization of the pomeron flux factor and structure function. A feasible experimental test of the phenomenological pomeron-exchanged model based on the fractal measurement in DESY $ep$ collider HERA is proposed.Comment: 14 pages, 2 Postscript figure

Factorization in hard diffraction

In this talk, I reviewed the role of factorization in diffraction hard scattering.Comment: Talk presented at the Ringberg Workshop on ``New Trends in HERA Physics 2001''. 10 pages, 6 postscript figures. Misprints correcte

Color Glass Condensate and BFKL dynamics in deep inelastic scattering at small x

The proton structure function F_2(x,Q^2) for x < 0.01 and 0.045< Q^2 < 45 GeV^2, measured in the deep inelastic scattering at HERA, can be well described within the framework of the Color Glass Condensate.Comment: 4 pages, 1 figure, incl. IOP style files. Talk given at the 17th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2004), Oakland, CA USA, 11-17 Jan 200

Jets in deep-inelastic scattering at HERA

Jet cross sections in deep-inelastic scattering over a wide region of phase space have been measured at HERA. These cross section measurements provide a thorough test of the implementation of Quantum Chromodynamics in next-to-leading order (NLO) calculations. They also provide the opportunity to test the consistency of the gluon distribution in the proton as extracted from (mainly) inclusive DIS measurements. Comparison of the cross sections with NLO enables accurate extractions of the strong coupling constant, $\alpha_s$, to be made, several of which are reported here.Comment: Talk presented at "New Trends in HERA Physics", Ringberg Workshop, June 2001. 13pages, 12figure

Rapidity gaps at HERA and the Tevatron from soft colour exchanges

Models based on soft colour exchanges to rearrange colour strings in the final state provide a general framework for both diffractive and non-diffractive events in ep and hadron-hadron collisions. We study two such models and find that they can reproduce rapidity gap data from both HERA and the Tevatron. We also discuss the influence of parton cascades and multiple interactions on the results.Comment: 4 pages, 4 EPS figures, presented at UK Phenomenology Workshop on Collider Physics, Durham. Uses iopart.cl