New Trends in Hera Physics 2003

PREFACE

CONTENTS

This contribution reviews the main achievements in inclusive measurements made by the H1 and ZEUS collaborations during the first phase of HERA data taking. The QCD analysis of these data by both collaborations are described. The case for a common QCD analysis is briefly discussed, with an emphasis on the possible W mass extraction.

Precise inclusive and dijet measurements are presented that extend the kinematic domain of HERA and cover the transition region from deep inelastic scattering to photoproduction. The rise of the structure function F₂ at low x persists but is shallower at lower Q². Dijet measurements can significantly constrain the parton distribution functions of the virtual photon.

We have calculated the fermionic contributions to the flavour non-singlet structure functions in deep-inelastic scattering (DIS) at third order of massless perturbative QCD. We obtain complete results for the corresponding n_f-parts of the three-loop anomalous dimension and the three-loop coefficient functions for the structure functions F₂ and F_L. Our results agree with all partial and approximate results available in the literature. We discuss their implications for the threshold resummation at the next-to-next-to-leading logarithmic accuracy.

In the first phase of data taking at the HERA ep collider various structure functions were measured. The precision of the measurements allows to make comprehensive analyses in the framework of QCD. Recent results on the gluon distribution function, the strong coupling constant and the longitudinal structure function F_L are reviewed as well as studies of limits of applicability of the perturbative QCD approach.

We outline the impact of the power corrections on the study of deep inelastic scattering and describe the determination of the power corrections from the existing deep inelastic scattering data.

I review some aspects of the measurment of Parton Distribution Functions by QCD global fitting.

An overview is given of selected recent HERMES results obtained from measurements performed during the first running period of HERA. These topics include inclusive g₁(x)-measurements with a NLO QCD analysis, polarized quark distribution extraction, b₁(x)-measurement, double spin asymmetries in vector meson production, ρ⁰-nuclear transparency and finally quark fragmentation in nuclei.

A summary is given of how spin enters in collinearly factorizing processes. Next, theoretical aspects of polarization in processes beyond collinear factorization are discussed in more detail, with special focus on recent developments concerning the color gauge invariant definitions of transverse momentum dependent distribution and fragmentation functions, such as the Sivers and Collins effect functions. This has particular relevance for azimuthal single spin asymmetries, which currently receive much theoretical and experimental attention.

At HERA II, longitudinally polarized electrons and positrons collide with unpolarized protons at centre-of-mass-energies of 318 GeV and at high luminosities. This offers unique opportunities for testing the Standard Model and probing new physics areas. A brief overview is given of the possibilities offered by the use of beam charge and polarization dependent final states at HERA, in particular as regards electroweak structure functions, generalized parton distributions and Λ-baryon production.

Recent measurements of jet and prompt photon production at HERA are reviewed. The results are compared to NLO QCD calculations which describe the data well except in extreme regions of phase space. Studies of the inclusive single jet cross section provide very accurate value of α_s.

The photoproduction of large-p_T charged hadrons and of prompt photons is discussed, for the inclusive case and with an associated jet, using predictions from the NLO partonic Monte Carlo program EPHOX. Comparisons to recent HERA data are also shown.

Recent results from jet production in deep inelastic ep scattering at HERA are reviewed. The values of α_s(M_z) extracted from a QCD analysis of the data are presented.

In the kinematic region where the partons carry a very small fraction of the proton momentum their dynamics is expected to be characterized by initial state cascades which are non-ordered in virtuality of the propagators. In electron-proton collisions the investigation of jet- and single particle production along the direction of the incoming proton provides high sensitivity to such effects. Some recent experimental results from HERA are presented and compared to next-to-leading order calculations and to different QCD models in order to find evidence for the new effects.

The NLO-QCD correction to single hadron production in deep inelastic scattering is calculated. We require the final state meson to carry a non-vanishing transversal momentum, thus being sensitive to perturbative QCD effects. Factorization allows us to convolute the hard scattering process with parton densities and fragmentation functions. The predictions are directly comparable to experimental results at the HERA collider at DESY. The results are sensitive to the gluon density in the proton and allow us to test universality of fragmentation functions.

Production of single hadrons and resonances emerges as a promising field for the study of Quantum Chromodynamics at HERA. Four recent studies are presented here to illustrate the scope and reach of the technique: light mesons and resonances test the universality of hadroproduction, strange particles challenge the hadronisation models or directly probe the quark content of the proton sea while the first observation of K – K resonant states offers a glueball candidate.

The existence of glueballs is predicted in QCD, the lightest one with quantum numbers J^PC = 0⁺⁺, but different calculations do not well agree on its mass in the range below 1800 MeV. Several theoretical schemes have been proposed to cope with the experimental data which often have considerable uncertainties. Further experimental studies of the scalar meson sector are therefore important and we discuss recent proposals to study leading clusters in gluon jets and charmless B-decays to serve this purpose.

New results in open heavy flavour production in ep collisions are presented. Aspects sensitive to the various stages of the production mechanism are compared to the experimental data. Based on the measurements and the comparison with leading-order parton shower Monte Carlo models and next-to-leading order QCD predictions, conclusions related to data alone as well as model initiated issues are drawn.

I review theoretical approaches to open heavy flavour photoproduction and discuss theoretical aspects of a new calculation in a massive theory with subtraction based on the factorization theorem of John Collins with heavy quarks.

This contribution reviews recent experimental results on the production of light and heavy vector mesons from the H1 and ZEUS collaborations during HERA-I data taking. The data are compared in detail with the predictions of theoretical models based on perturbative QCD.

Recent progress is reviewed in our understanding of inclusive diffraction in the deeply inelastic regime at HERA. New precision measurements are now available from both H1 and ZEUS collaborations. A new set of diffractive parton distribution functions, determined from the H1 data, is presented. It can be used to test factorization properties of diffractive scattering in different reactions.

The predictions of diffractive models are compared with H1 measurements of jet production cross sections in diffractive Deep Inelastic Scattering (DIS) and photo-production and with H1 and ZEUS measurements of D*^±(2010) meson production in diffractive DIS.

The Balitsky-Fadin-Kuraev-Lipatov (BFKL) evolution equation is known to be “unstable” with respect to fluctuations in gluon virtuality, transverse momentum and energy requiring to go beyond the leading order BFKL. Still, these instabilities point to fruitful improvements of our deep understanding of QCD. Recent applications to next-leading order and to saturation problems are outlined.

We review some theoretical ideas concerning diffractive processes. We discuss the Regge Ansatz for the pomeron and the two pomeron model. Then we present the results obtained from nonperturbative QCD for high energy scattering. There we can extract from elastic scattering data the parameters describing the QCD vacuum, in particular the string tension.

In this talk some recent developments in the theory of diffractive meson production are reviewed: (1) the issue of end point divergencies and diffractive factorisation; (2) elastic vector meson production in the context of the gluon saturation in the impact parameter plane; (3) diffractive vector meson photoproduction at large momentum transfer; (4) C-even meson photoproduction and the Odderon.

Different formalisms for unintegrated parton densities are discussed, and some results and applications are presented.

We report on the interesting possibility of instanton-driven gluon saturation in lepton-nucleon scattering at small Bjorken-x. Our results crucially involve non-perturbative information from high-quality lattice simulations. The conspicuous, intrinsic instanton size scale 〈ρ〉 ≈ 0.5 fm, as known from the lattice, turns out to determine the saturation scale. A central result is the identification of the “colour glass condensate” with the QCD-sphaleron state.

We review theoretical results on QCD corrections to flavour changing neutral current single top as well as single W boson production at HERA. In both cases the inclusion of QCD corrections has a moderate effect on the size of the cross sections but reduces the QCD scale dependence considerably. This writeup summarizes a talk given at the Ringberg Workshop “New Trends in HERA Physics 2003”.

The H1 and ZEUS Collaborations have almost completed a rich program of searches for physics beyond the Standard Model using the data collected in the HERA I period. The main highlights, such as leptoquarks and signatures with isolated leptons at high energy, are reviewed.

The HERMES experiment¹ has been operational at the HERA accelerator of DESY since 1995.

A sketch is given of the main results obtained with the H1 apparatus at HERA as is a summary of where progress is to be expected as a result of increased HERA luminosity, the measurement of beam charge and polarisation asymmetries, the variation of beam energies, improved systematics and extended kinematic range.

Selected physics topics of the HERA II program are briefly reviewed. An outlook for data taking as of October 2003 and estimated luminosity requirements for some physics topics are given.

A new round of experimentation with the HERA accelerator is discussed.

List of Participants