New Trends in HERA Physics 2005

Preface.

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Contents.

Proton structure measurements at high Q² and x, performed by the H1 and ZEUS Collaborations at the HERA accelerator, are reviewed. Neutral and charge current deep inelastic scattering cross sections and structure function measurements are presented. A new method employed to measure the NC cross section up to Bjorken-x values equal to one is described. The review also discusses the ZEUS-JETS PDF fits which include jet data giving direct access to the gluon densities. An estimate on the impact of HERA II data on the PDFs and their influence on the LHC measurements are mentioned.

Results obtained at HERA concerning electroweak physics in deep inelastic electron-proton scattering are reviewed. At HERA I electroweak unification has been demonstrated. In a global fit to the measured proton structure functions QCD and electroweak parameters are determined simultaneously. The availability of polarised electrons at HERA II allows to determine the chirality properties of NC and CC interactions at high spacelike momentum transfers.

Inclusive ep scattering measurements at low virtualities of the exchanged boson, Q², allow precision tests of perturbative QCD at high gluon densities, as well as studies of the transition from the perturbative to non-perturbative QCD domain. Measurements in the transition region require special experimental approaches due to the limited detector acceptance. The current status and results of low Q² measurements at HERA are summarised.

We present a small x resummation for the GLAP anomalous dimension and its corresponding dual BFKL kernel, which includes all the available perturbative information and nonperturbative constraints.

Recent, precise HERA data on diffractive photoprocesses are analysed in the context of colour dipole models. The data on the deep inelastic structure function F₂ is used to determine the dipole cross section, and shown to prefer parameterisations which include saturation effects. The resulting dipole cross sections are then used to predict cross sections for DVCS and DDIS, which are in good agreement with experiment.

Hard exclusive reactions are the tool to learn about generalized parton distributions, which provide a more complete parameterization of the nucleon than the ordinary parton distribution functions. Measurements performed by the HERMES collaboration of the exclusive production of photons, i.e., Deeply-Virtual Compton Scattering, and of mesons are briefly reviewed, focusing on new results which may allow the determination of the total angular momentum of quarks in the nucleon.

In the first phase of the HERA II running period the HERMES collaboration is taking data with a transversely polarized Hydrogen target. It was installed during the HERA shutdown in the fall of 2000. The transverse target extends the spin physics program of HERMES to the topic of transversity. It allows to investigate the distribution of transversely polarized quarks in a transversely polarized nucleon. In the data evidence is found not only for a non-zero signal on transversity but also on the Sivers mechanism that can be related to non-zero orbital angular momenta of the quarks in a transversely polarized nucleon.

The present understanding of the spin structure of the nucleon in view of recent experiments is briefly reviewed. The main focus is on parton helicity distributions, orbital angular momentum of partons as defined through generalized parton distributions, as well as single spin asymmetries and time-reversal odd correlation functions.

Use of HERA jet production cross-sections can extend our knowledge of the gluon parton distribution function and yield accurate measurements of α_S(M_Z) in addition to illustrating the running of α_S with scale.

In this paper we outline a new parton shower algorithm based on the Catani-Seymour dipole factorization. Our motivation is to have an algorithm which can naturally cooperate with the NLO calculations.

No abstract received.

I summarize the theoretical and experimental status of multijet production in DIS. I present the state-of-the-art theoretical predictions and compare those to the corresponding experimental results obtained by analysing the data collected by the H1 and ZEUS collaborations at HERA. I also show new predictions for three-jet event-shape distributions at the NLO accuracy.

We discuss the physics underlying an all-order resummation of logarithmic enhanced contributions to dijet cross sections, and present preliminary results for the distribution in the dijet transverse energy difference in DIS.

Recent measurements of deep inelastic events containing jets and π° mesons, produced close to the proton remnant in the laboratory frame, are discussed. The data are used to discriminate between QCD models with different parton evolution approximations.

We review recent results on the inclusive electroproduction of light hadrons at next-to-leading order in the parton model of quantum chromodynamics implemented with fragmentation functions and present updated predictions for HERA experiments based on the new AKK set. We also discuss phenomenological implications of these results.

Recent results from HERA are presented on a range of topics: charged multiplicities, production of non-strange mesons and strange particles, charm fragmentation, baryons decaying to strange particles, antideuteron production, Bose-Einstein correlations, and new interpretations of results on prompt photon production in DIS.

We define a general scheme for the evolution of fragmentation functions which resums soft gluon logarithms in a manner consistent with fixed order evolution. We present an explicit example of our approach in which double logarithms are resummed using the Double Logarithmic Approximation. We show that this scheme reproduces the Modified Leading Logarithm Approximation in certain limits, and find that after using it to fit quark and gluon fragmentation functions to experimental data, a good description of the data from the largest x_p values to the peak region in ξ = In(1/x_p) is obtained. In addition, we develop a treatment of hadron mass effects which gives additional improvements at large ξ.

We review one-particle inclusive production of heavy-flavored hadrons in a framework which resums the large collinear logarithms through the evolution of the FFs and PDFs and, at the same time, retains the full dependence on the heavy-quark mass without additional theoretical assumptions. The main focus is on the production of D* mesons in deep inelastic electron–proton scattering at HERA. We show results, neglecting for the time being the heavy-quark mass terms, for deep inelastic D* meson production at finite transverse momenta. Work to implement this process in the above mentioned massive QCD framework is in progress.

Measurements of charm production in deep inelastic scattering (DIS) and photo-production have been carried out by the ZEUS and H1 collaborations at HERA. Protons at 920 GeV are collided with positrons of 27.5 GeV. Results using integrated luminosities up to 120 pb⁻¹ for HERA I and 35 pb⁻¹ for HERA II are presented. Single and double differential cross sections are compared to perturbative QCD (pQCD) predictions. Charm DIS and photoproduction cross sections are in reasonable agreement with pQCD calculations. Charm and jet angular correlations show regions of phase space where NLO is not sufficient to describe these data sets, indicating the need for improved calculations such as MC@NLO or NNLO predictions. Charm fragmentation ratios and fractions generally support the hypothesis that fragmentation proceeds independently from the hard sub-process in e⁺e⁻ and ep collisions. This test of charm universality is confirmed by the measured D meson ground states at HERA and LEP. Stringent tests of QCD have been performed; theoretical errors dominate or are equivalent to experimental errors in the comparisons.

A review is given on beauty production at HERA. Many new results have recently become available based on various tagging techniques and covering altogether a wide kinematic phase space in photon virtualities and b quark transverse momenta. Differential beauty production cross sections are presented and compared to perturbative QCD predictions. The first measurements of the structure function are shown.

In this paper, we report on the calculation of the cross section of J/Ψ inclusive production in direct γγ collisions at next-to-leading order within the factorization formalism of nonrelativistic quantum chromodynamics (NRQCD). Theoretical predictions for the future e⁺e⁻ international linear collider ILC are also presented.

One of the outstanding problems in J/Ψ physics is a systematic understanding of the differential photo-production cross section dσ/dz(γ + p → J/Ψ + X), where z = E_Ψ/E_γ in the proton rest frame. The theoretical prediction based on the non-relativistic QCD (NRQCD) factorization formalism has a color-octet contribution which grows rapidly in the endpoint region, z → 1, spoiling perturbation theory. In addition there are subleading operators which are enhanced by powers of 1/(1 − z) and they must be resummed to all orders. Here an update of a systematic analysis is presented. The approach used to organize the endpoint behavior of the photo-production cross section is based on a combination of NRQCD and soft collinear effective theory. While a final result is not yet available, an intermediate result indicates that better agreement between theory and data will be achieved in this framework.

This talk describes the measurement of F₂ and inclusive and exclusive diffractive cross sections in the low-x region by HERA experiments. The abundance of diffractive reactions observed at HERA indicates the presence of perturbative multi-ladder exchanges. The exclusive diffractive vector-meson and diffractive dijet production are discussed in terms of dipole models which connect the measurement of F₂ with diffractive processes and in which multiple exchanges and saturation processes are natural. The diffractive dijets are also discussed within the diffractive parton density approach. Good description of diffractive dijets in the dipole picture and in the diffractive parton density approach indicates that these two seemingly different views on diffraction are not really distinct.

Perturbative QCD calculations of diffractive production of vector mesons are discussed. The MRT approach which uses k_T-factorization and Parton Hadron Duality is explained in some detail, and the uncertainties of the predictions are scrutinized. Skewing corrections turn out to be important. With recent data from HERA the strong sensitivity on the input gluon distribution at small x and small to intermediate scales will allow to better constrain the gluon.

Results are given on the measurements of the hard diffractive interactions at HERA ep collider. The structure of the diffractive exchange in terms of partons and the factorisation properties are discussed, in particular by comparing the QCD predictions for dijets and D* with measurements both in the photo and electroproduction regimes.

When comparing recent experimental data from the H1 and ZEUS Collaborations at HERA for diffractive dijet production in deep-inelastic scattering (DIS) and photoproduction with next-to-leading order (NLO) QCD predictions using diffractive parton densities, good agreement is found for DIS. However, the dijet photoproduction data are overestimated by the NLO theory, showing that factorization breaking occurs at this order. While this is expected theoretically for resolved photoproduction, the fact that the data are better described by a global suppression of direct and resolved contribution by about a factor of two comes as a surprise. We therefore discuss in some detail the factorization scheme and scale dependence between direct and resolved contributions and propose a new factorization scheme for diffractive dijet photoproduction.

We discuss the perturbative QCD description of diffractive deep-inelastic scattering and extract diffractive parton distributions from recent HERA data. The asymptotic collinear factorisation theorem has important modifications in the sub-asymptotic HERA regime. In addition to the usual resolved Pomeron contribution, the direct interaction of the Pomeron must also be accounted for. The diffractive parton distributions are shown to satisfy an inhomogeneous evolution equation, analogous to the parton distributions of the photon.

An overview of experimental results on searches for new phenomena at HERA is presented. The complementarity with searches performed at other experiments is discussed and the prospects for a discovery, using the full HERA data to be delivered until mid-2007, are presented.

HERA-I studies on new resonance searches in the hadronic final state in ep collisions are reviewed. The searches were performed both in meson and baryon resonances. Pentaquark candidates were observed in system in ZEUS and in D*p system in H1. The two experiments are compared and the compatibility is discussed.

We briefly review a variety of theoretical and phenomenological indications for the probable importance of powerful diquark correlations in hadronic physics. We demonstrate that the bulk of light hadron spectroscopy can be organized using three simple hypotheses: Regge-Chew-Frautschi mass formulae, feebleness of spin-orbit forces, and energetic distinctions among a few different diquark configurations. Those hypotheses can be implemented in a semi-classical model of color flux tubes, extrapolated down from large orbital angular momentum L. We discuss refinements of the model to include the effects of tunneling, mass loading, and internal excitations. We also discern effects of diquark correlations in observed patterns of baryon decays. Many predictions and suggestions for further work appear.

I investigate what a direct measurement of the longitudinal structure function F_L (x, Q²) could teach us about the structure of the proton and the best way in which to use perturbative QCD for structure functions. I assume HERA running at a lowered beam energy for approximately 4-5 months and examine how well the measurement could distinguish between different theoretical approaches. I conclude that such a measurement would provide useful information on how to calculate structure functions and parton distributions at small x.

The theoretical interest in the longitudinal inclusive F_L (x, Q²) and diffractive Structure Functions are briefly mentioned. A simulation based on running HERA for three months with a reduced proton beam energy shows that measurements are possible with five and three sigma significance for F_L and , respectively.

This report summarizes some of the main results of the one year long workshop on HERA and the LHC.

List of Participants.