Regular Series


Vol. 33 (2002), No. 10, pp. 2727 – 3347


QCD and Proton and Photon Structure Since DIS2001

abstract

Results on proton and photon structure that have become available since the DIS2001 meeting are summarized.


High Energy QCD at \(e^+e^-,\, pp\) and \(ep \) Colliders

abstract

We review recent high energy QCD data and phenomenological developments on small-\(x\) and BFKL like effects. Data of \(ep\), \(e^+e^-\) and \(pp\) colliders are discussed, and prospects for future colliders are given.


Saturation Effects in DIS at Low \(x\)

abstract

We review basic ideas related to saturation of parton densities at small value of the Bjorken variable \(x\). A special emphasis is put on interpreting the results from the \(ep\) deep inelastic scattering experiments at HERA.


QCD Tests Through Hadronic Final-State Measurements

abstract

Modern-day ‘testing’ of (perturbative) QCD is as much about pushing the boundaries of its applicability as about the verification that QCD is the correct theory of hadronic physics. This talk gives a brief discussion of a small selection of topics: factorisation and jets in diffraction, power corrections and event shapes, the apparent excess of \(b\)-production in a variety of experiments, and the matching of event generators and NLO calculations.


Status and Perspectives of Spin Physics

abstract

The status of our understanding the spin–flavor structure of the nucleon is discussed based on most recent data from inelastic scattering experiments at SLAC, JLAB and DESY. Recent insights from current measurements and promising directions for future measurements are highlighted.


Measurement of the Inclusive Deep Inelastic Scattering Cross Section at \(Q^2 \sim 1\ {\rm GeV}^2\) with the H1 Experiment

abstract

A preliminary measurement is presented of the inclusive \(ep\) scattering cross section in the kinematic region of very low Bjorken \(x\) and four-momentum transfer squared \(Q^2 \sim 1\ {\rm GeV}^2\), in the transition region from the non-perturbative to the deep-inelastic domain. The cross-section measurement is used to obtain new data on the proton structure function \(F_2(x,Q^2)\).


The Rise of the Proton Structure Function \(F_2\) Towards Low \(x\)

abstract

Results on the derivative of \(\log (F_2)\) with respect to \(\log (x)\) at fixed \(Q^2\) are presented. The measured derivatives are within errors independent of \(x\) for \(Q^2 \geqq 0.85\) GeV\(^2\) and increase linearly with \(\log (Q^2)\) for \(10^{-4} \leqq x \leqq 0.01\) and \(Q^2 \gtrapprox 3\) GeV\(^2\). The results are based on preliminary and published H1 data which at \(Q^2\) below 2 GeV\(^2\) are combined with NMC and ZEUS data.


\(F_2^{ep}\) in the Deep Sea and DGLAP

abstract

The consistency of the DGLAP equations is tested in the deep sea region using the HERA \(F_2\) data.


DGLAP Evolution in the Saturation Model

abstract

A modification of the saturation model of deep inelastic scattering at small \(x\) which includes the Altarelli-Parisi (DGLAP) evolution is presented Significant improvement of the description of the structure function \(F_2\) at large \(Q^2\) is achieved and a good description of diffractive data is preserved.


The Color Glass Condensate RHIC and HERA

abstract

In this talk, I discuss a universal form of matter, the Color Glass Condensate. It is this matter which composes the low \(x\) part of all hadronic wavefunctions. The experimental programs at RHIC and HERA, and future programs at LHC and eRHIC may allow us to probe and study the properties of this matter.


Fractal Structure of the Proton at Low Bjorken \(x\)

abstract

A description of the structure function \( F_{2}(x,Q^2)\,\) at low Bjorken \(x\) reflecting a fractal self-similar proton structure is proposed. The specific parameterisation obtained provides an excellent description of all low \(x\) data extending in four momentum transfer squared from the non-perturbative region, \(Q^2 \gtrsim 0.05\) GeV\(^2\), into the deep inelastic domain of \(Q^2 \lesssim 100\) GeV\(^2\).


Status of the NLO Corrections to the Photon Impact Factor

abstract

We present the status of the programme of calculating the next-to-leading order corrections to the virtual photon impact factor. In particular, we discuss new results for the transversely polarized photon. We briefly outline the definition of infrared finite terms and the subtraction of the leading logarithmic parts.


H1 High \(Q^2\) Results in Neutral and Charged Current \(e^+p\) and \(e^-p\) Scattering

abstract

Inclusive neutral and charged current cross sections in deep inelastic electron proton scattering at high four momentum transfer \(Q^2\) have been measured by the H1 experiment at HERA. The structure functions \(F_2\), \(xF_3\) and \(F_L\) are extracted from neutral current cross sections. Charged current cross sections are used to determine the \(W\) propagator mass. The valence quark distributions are derived using exclusively H1 data. All results, including those at highest \(Q^2\), are found to be in good agreement with the Standard Model.


High \(Q^2\) Neutral Current Scattering at ZEUS

abstract

The cross sections for neutral current deep inelastic scattering have been measured in \(e^-p\) and \(e^+p\) collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 16 pb\(^{-1}\) and 63 pb\(^{-1}\), respectively. Single and double differential cross sections at high Q\(^2\) are presented and compared to the predictions of the Standard Model. The structure function \(xF_3\) has been extracted by combining the \(e^-p\) data with the recent ZEUS measurement of \(e^+p\) neutral current Deep Inelastic Scattering (DIS) at a centre-of-mass energy of 300 GeV. The measured \(xF_3\) is well described by the Standard Model using recent proton parton density functions.


Measurements of the Charged Current Cross Sections with the ZEUS Detector

abstract

A new measurement of the charged current cross section in \(e^{-}p\) scattering is presented in the range of \(Q^2 \gt 200\) GeV\(^2\), using the 1998 and 1999 data with an integrated luminosity of 16.4 pb\(^{-1}\). This cross section is compared to the preliminary charged current cross section in \(e^{+}p\) scattering using the 1999 and 2000 data with an integrated luminosity of 61.0 pb\(^{-1}\), and to predictions of the Standard Model using PDFs extracted from fits to NC data. Finally, the mass of the W boson determined from a fit to \(d\sigma \)/\(dQ^{2}\) of the \(e^{-}p\) scattering data is presented.


Measurability of the Proton Structure Functions at Very High \(x\) at HERA II

abstract

In this paper we address a question of measuring a proton Structure Functions (SF) at Bjorken \(x\sim 1\). A new method which allows for a measurement of SF at high \(x\) to be done with large data samples is proposed. Expected measurements after HERA luminosity upgrade are estimated using a Monte Carlo simulation. This shows that \(\sim 30\%\) precision can be achieved for the highest \(x\)-bins with \(1\,{\rm fb}^{-1}\) of data in so far experimentally unconstrained region. Impact of this measurement on constraining the fundamental physics is discussed. A measurement with proton beam energy lowered to half (460 GeV) is discussed as well.


High-\(P_{\rm T}\) Multi-Electron Production at HERA

abstract

Multi-electron production has been measured at high transverse momentum in 115 pb\(^{-1}\) of positron- and electron–proton collisions collected by the H1 experiment at HERA. A good overall agreement is found with the Standard Model predictions, dominated by photon–photon interactions. Six events are observed with a di-electron mass above 100 GeV, a domain where the Standard Model prediction is low.


QCD Results from the Tevatron

abstract

We present results from QCD studies at the Tevatron from Run 1 data, including jet and direct photon production, and a measurement of the strong coupling constant.


\(B\)-Physics at Tevatron

abstract

During the run period (1992–1995) CDF experiment has collected \(110~{\rm pb}^{-1}\) of \(p\overline {p}\) collisions at 1.8 TeV center of mass energy. The highlights of the CDF \(B\) physics results are described, among them the most important are the measurement of \(B\) hadron masses, lifetimes and mixing as well as the observation of the \(B_c\) meson and the measurement of the CP violation parameter \(\sin 2\beta \). Run IIa will give a unique opportunity of extended \(B\) physics studies due to the high luminosity of 2 fb\(^{-1}\) of \(p\overline {p}\) collisions and the major upgrades of both Tevatron experiments, CDF and D0. The perspectives for the \(B\) measurements, specially the studies of heavier \(B\) hadrons are described, together with the Run I highlights.


Two-Loop Amplitudes for \( e^+ e^- \to q {\bar q} g\): the \(n_f\)-Contribution

abstract

We discuss the calculation of the \(n_f\)-contributions to the two-loop amplitude for \( e^+ e^- \rightarrow q g \bar {q}\). The calculation uses an efficient method based on nested sums. The result is presented in terms of multiple polylogarithms with simple arguments, which allow for analytic continuation in a straightforward manner.


Update of MRST Parton Distributions

abstract

We discuss the latest update of the MRST parton distributions in response to the most recent data. We discuss the areas where there are hints of difficulties in the global fit, and compare to some other updated sets of parton distributions, particularly CTEQ6. We briefly discuss the issue of uncertainties associated with partons.


New Generation of Parton Distributions with Uncertainties from Global QCD Analysis

abstract

A new generation of parton distribution functions with increased precision and quantitative estimates of uncertainties is presented. This work includes a full treatment of available experimental correlated systematic errors for both new and old data sets and a systematic and pragmatic treatment of uncertainties of the parton distributions and their physical predictions. The new gluon distribution is considerably harder than that of previous standard fits. Extensive results on the uncertainties of parton distributions at various scales, and on parton luminosity functions at the Tevatron RunII and the LHC, are obtained. The latter provide the means to quickly estimate the uncertainties of a wide range of physical processes at these high-energy hadron colliders, such as the production cross sections of the \(W,Z\) at the Tevatron and the LHC, and that of a light Higgs.


Strong Coupling Constant from the Photon Structure Function

abstract

We extract the value of the strong coupling constant \(\alpha _{\rm s}\) from a single-parameter pointlike fit to the photon structure function \({F_2^\gamma }\) at large \(x\) and \(Q^2\) and from a first five-parameter full (pointlike and hadronic) fit to the complete \({F_2^\gamma }\) data set taken at PETRA, TRISTAN, and LEP. In next-to-leading order and the \({\overline {\rm MS}} \) renormalization and factorization schemes, we obtain \(\alpha _{\rm s}(m_Z)=0.1183\pm 0.0050\)(exp.)\(^{+0.0029}_{-0.0028}\)(theor.) [pointlike] and \(\alpha _{\rm s}(m_Z)=0.1198\pm 0.0028\)(exp.)\(^{+0.0034}_{-0.0046}\)(theor.) [pointlike and hadronic]. We demonstrate that the data taken at LEP have reduced the experimental error by about a factor of two, so that a competitive determination of \(\alpha _{\rm s}\) from \({F_2^\gamma }\) is now possible.


A Study of QCD Coupling Constant and Power Corrections in the Fixed Target Deep Inelastic Measurements

abstract

We reanalyze deep inelastic scattering data of BCDMS Collaboration by including proper cuts of ranges with large systematic errors. We perform also the fits of high statistic deep inelastic scattering data of BCDMS, SLAC, NM and BFP Collaborations taking the data separately and in combined way and find good agreement between these analyses. We extract the values of both the QCD coupling constant \(\alpha _{\rm s}(M^2_Z)\) up to NLO level and of the power corrections to the structure function \(F_2\).


Mass Corrections and Neutrino DIS

abstract

Including the effects of the \({\cal {O}}(\gtrsim 1\ {\rm GeV})\) masses of the charm quark, \(\tau \) lepton and target nucleon in DIS phenomenology is discussed with applications to CC neutrino DIS: Neutrino data for \(F_2\) are revisited within the global analysis framework. A fully differential calculation refines the CC charm production process as a gate to extract \(\{ s(x), {\bar s} (x) \}\). New results are presented for a “heavy quark” version of the CTEQ6 set of PDFs and for (\(\nu _{\mu } \rightarrow \nu _{\tau }\) oscillation-signal) \(\tau \) neutrino cross sections.


Summary of the Photon Structure Functions — Measurements at LEP

abstract

The present status of the photon structure functions measurements at LEP is discussed. The short introduction to the kinematics and theoretical framework of the structure functions measurements at LEP is given first. Then follow presentations of the most important measurements, ranging from the QED photon structure function, through the hadronic structure functions of real and virtual photons, and at the end the first measurement of the electron structure function is shown.


Jets and Prompt Photon Production in Two-Photon Collisions at LEP

abstract

Di-jet and prompt photon production is studied in collisions of quasi-real photons radiated by the LEP beams at \(e^+ e^-\) centre-of-mass energies \(\sqrt {s_{ee}}\) from 183 to 209 GeV. The data have been taken with the OPAL and DELPHI detectors.


Photon Structure from HERA

abstract

Some recent measurements of real and virtual photon structure at HERA are reviewed. The cross section for dijet events in photoproduction is compared to NLO QCD calculations. In low \(Q^2\) DIS, the production of dijets is shown to be sensitive to the structure of longitudinally polarised photons and the \(Q^2\) evolution modified in charm-enriched events.


New Radiatively Generated LO Quark (\(u,d,s,c,b\)) and Gluon Densities in Real Photon

abstract

New, radiatively generated, LO quark (\(u,d,s,c,b\)) and gluon densities in real photon are presented. We [1] perform a global fit, based on LO DGLAP evolution equations, to all available data for the structure function \(F_2^{\gamma }(x,Q^2)\). We adopt a new theoretical approach called ACOT(\(\chi \)) to deal with heavy quark thresholds (CJKL model). For comparison we perform a standard fit using the Fixed Flavour Number Scheme (FFNS\(_{\rm cjkl}\) model), updated with respect to previous fits of this type. We show the superiority of the CJKL model over the FFNS\(_{\rm cjkl}\) one and other LO parton parametrizations for \(F_2^{\gamma }(x,Q^2)\). Both our models describe equally well the gluon density extracted from HERA data.


Charm Production and \(F_2^c\) at HERA

abstract

H1 and ZEUS results on open charm production in deep inelastic \(ep\) scattering at HERA are reviewed. The corresponding results for the proton structure function \(F_2^c (x,Q^2)\) are discussed.


Gluon Distribution Functions in the \(k_{\perp }\)-Factorization Approach

abstract

At small \(x\), the effects of finite transverse momenta of partons inside a hadron become increasingly important, especially in analyses of jets and heavy-quark production. These effects can be systematically accounted for in a formalism based on \(k_{\perp }\)-factorization and unintegrated distribution functions. We present results for the integrated and unintegrated distribution functions obtained within the framework of the Linked Dipole Chain model. Comparisons are made to results obtained within other approaches.


Investigation of Next-to-Leading Effects in CCFM

abstract

The effect of formally next-to-leading contributions to the CCFM evolution equation are discussed.


Forward Jets and BFKL at Hadron Colliders

abstract

We present results on dijet and \(W\)+dijet production at hadron colliders obtained by supplementing the leading log BFKL resummation with energy and momentum conservation. For pure dijet production, the inclusion of the BFKL radiation in the energy conservation leads to a decrease in the parton flux sufficient to counter-act the expected exponential increase in the cross section obtained for the partonic cross section. Other BFKL signatures such as the dijet azimuthal angle decorrelation do still survive.


Jets at High Energies, Factorization and Jet Vertex in NL \(\ln (s)\)

abstract

The next-to-leading corrections to the jet vertex which is relevant for the Mueller–Navelet jets production in hadronic collisions and for the forward jet cross section in lepton–hadron collisions are presented in the context of a \(k_{\rm t}\) factorizazion formula which resums the leading and next-to-leading logarithms of the energy. Both the quark- and gluon-initiated contribution are now computed. This completes the framework for a full phenomenological analysis of Mueller–Navelet jets in NL \(\log (s)\) approximation. Forward jets phenomenology still requires the NL photon impact factor.


Hadron Formation in Nuclei in DIS

abstract

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive DIS has been studied by the HERMES experiment at DESY with a 27.5 GeV positron beam. A large reduction of the differential multiplicity of charged hadrons from krypton relative to that from deuterium is observed. The reduction is larger than that seen in previously published HERMES data on nitrogen. The data are compared to two theoretical models. Both well describe the reduction of the multiplicity ratio at low values of the virtual photon energy \(\nu \) and at high values of the fractional energy transfer \(z\) to the hadron. In order to obtain a better understanding of the hadronization process and possibly be able to distinguish between the two theoretical models the multiplicity ratio on the krypton data has been evaluated separately for pions, kaons and protons.


Towards Deep-Inelastic Structure Functions at Three Loops

abstract

We report on the status of the calculation of deep-inelastic structure functions at three loops in perturbative QCD. The method employed allows to calculate the Mellin moments of structure functions analytically as a general function of \(N\). As an illustration, we present the leading fermionic contributions to the non-singlet anomalous dimension of \(F_2\) at three loops and, as a new result, to the non-singlet coefficient function of \(F_2\) at three loops.


Status of Lattice Structure Function Calculations

abstract

Lattice QCD allows computations of moments of structure functions from first principles. An overview of the present status of the calculations is given. Recent results and future perspectives are discussed.


Analytic Description of Proton Structure Functions in the Whole Kinematic Region

abstract

A new approach based on a search of analytic solutions for the main limiting cases and of linking functions describing intermediate region is developed. In frame of this work, analytical formulas for proton form factors in the limiting cases (photoproduction, quasielastic scattering and low-\(x\) limit) have been obtained. Connections and transitions between these limiting cases have been studied and linking functions have been found. The obtained dependencies of proton structure functions for the whole kinematic region of variables \(Q^2\) and \(x\) contain practically no free parameters but they are in a good agreement with experimental data.


Running Coupling and BFKL Pomeron

abstract

We show that in the case of the BFKL pomeron with running coupling the diffusion pattern is strongly modified and is characterised by the sudden tunneling transition to the non-perturbative regime. We suggest that by using the \(b\)-expansion method one can suppress the non-perturbative Pomeron and isolate purely perturbative part of the gluon Green’s function.


Saturation Model for 2\(\gamma \) Physics

abstract

We introduce a saturation model for photon-photon interactions, based on a QCD dipole picture of high energy scattering. The two-dipole cross-section is assumed to satisfy the saturation property. This pomeron-like contribution is supplemented with QPM and non-pomeron reggeon contributions. The model gives a very good description of the data on the \(\gamma \gamma \) total cross-section, on the photon structure function \(F_2^{\gamma }(x,Q^2)\) at low \(x\) and on the \(\gamma ^* \gamma ^*\) cross-section.


Measurement of \(R = \sigma _{\rm L} / \sigma _{\rm T}\) in Deep-Inelastic Scattering on Nuclei

abstract

Cross section ratios for deep-inelastic scattering from \(^3\)He, \(^{14}\)N and \(^{84}\)Kr with respect to \(^2\)H have been measured by the HERMES Collaboration at DESY using a 27.5 GeV positron beam. The data cover a range in the Bjorken scaling variable \(x\) from 0.010 to 0.65 while \(Q^2\) varies from 0.5 to 15 GeV\(^2\). From the dependence of the data on the virtual photon polarisation parameter \(\varepsilon \), values for \(R_A\)/\(R_{\rm D}\) have been derived, where \(R\) is the ratio \(\sigma _{\rm L}\)/\(\sigma _{\rm T}\) of longitudinal to transverse DIS cross sections. The helium-3 and nitrogen data were published in 1999. Recently, those data were found to be subject to an \(A\)-dependent tracking inefficiency of the HERMES spectrometer, which was not recognised in the previous analysis. The resulting correction of the cross section ratios is significant at low values of \(x\) and \(Q^2\) and substantially changes the interpretation of those data.


Saturation from Nonlinear pQCD at Small \(x\) in \(ep\) and \(eA\) Processes

abstract

In this talk we discuss inclusive \(ep,eA\) scattering in the framework of nonlinear, small \(x\), pQCD, in particular the natural emergence of nuclear shadowing within this framework through simple rescaling of the natural scaling variable \(\tau \), in this approach, by \(A^{\delta }\). We then compare this approach to other popular approaches to nuclear shadowing like the eikonal approximation or leading twist calculations.


Generalized Parton Distributions at Next-to-Leading Order

abstract

This talk discusses Generalized Parton Distributions (GPDs), which encode various types of non-perturbative information relevant to the QCD description of exclusive processes. Results on their Next-to-Leading Order (NLO) QCD evolution are presented. We find that models for the input GPDs based on double distributions require some modification in order to reproduce the available data on deeply virtual Compton scattering.


Polarization as a Tool at HERA

abstract

The HERA ring has been upgraded to fivefold its luminosity and will provide longitudinal lepton beam polarization to the HERA experiments. The accurate polarization measurement and high luminosity opens a completely new field for HERA physics.


The Structure Function Working Group Summary

abstract

A summary of the experimental and theoretical presentations in the Structure Function Working Group on the proton and photon unpolarized structure functions is given.


Measurement of Single Inclusive High \(e_{\rm T}\) Jet Cross-Sections in Photoproduction at HERA

abstract

Inclusive jet cross-sections for the reaction \(e^+ p \rightarrow {\rm jet} + X\) for \(Q^2 \lt 1\) GeV\(^2\) and photon-proton energies in the interval \(95 \leq W \leq 285\) GeV have been measured. Jets with transverse energies \(E_{\rm T}^{\rm jet} \gt 21\) GeV in the pseudorapidity range \(-1 \leq {\eta }^{\rm jet} \leq 2.5\) are searched for using the inclusive \(k_{\rm T}\) cluster algorithm in the laboratory frame. Cross-sections as a function of \(\eta ^{\rm jet}\) and \(E_{\rm T}^{\rm jet}\) are compared with NLO QCD calculations using different sets of photon parton density functions as input. The cross-sections are in good agreement with NLO QCD up to the highest measured values of \(E_{\rm T}^{\rm jet} = 75\) GeV.


Real Photon Structure via Dijet Photoproduction

abstract

The dijet cross section in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb\(^{-1}\). The events were required to have a virtuality of the incoming photon, Q\(^2\), of less than 1 GeV\(^2\) and a photon–proton centre-of-mass energy in the range 134 \(\lt W_{\gamma p} \lt \) 277 GeV. Each event contains at least two jets satisfying transverse-energy requirements of \(E_{{\rm T}}^{{\rm jet1}}\gt \) 14 GeV and \(E_{\rm T}^{\rm jet2} \gt \) 11 GeV and pseudorapidity requirements of \(-1\lt \eta ^{\rm jet1,2}\lt \)2.4. The measurements are compared to next-to-leading-order QCD predictions. The data show particular sensitivity to the density of partons in the photon, allowing the validity of the current parameterisations to be tested.


Study of Virtual Photon Structure in Charm Production at HERA

abstract

The dependence of the virtual-photon structure on its virtuality, \(Q^2\), has been studied in events containing dijets and a charm quark with the ZEUS detector at HERA using an integrated luminosity of 103.7 pb\(^{-1}\). The measurement was performed in the kinematic range \(Q^2 \lt {\rm 5\times 10^3 \ GeV^2}\) using events containing a \(D^{*\pm } (2010)\) meson. Events having two jets with large transverse energies were selected using the \(k_{\rm T}\) clustering algorithm. The ratio of dijet cross sections at low \(x_{\gamma }^{\rm obs}\) (“resolved photon”) relative to high \(x_{\gamma }^{\rm obs}\) (“direct photon”) was measured as a function of \(Q^2\), where \(x_{\gamma }^{\rm obs}\) is the fractional momentum of the photon participating in the dijet production, and is compared to the predictions of leading order pQCD. The ratio does not change significantly with \(Q^2\), in marked contrast to previous dijet measurements which did not require the presence of charm.


Dijets in Photoproduction

abstract

Dijet cross sections as functions of several jet observables are measured in photoproduction using the H1 detector at HERA. The data sample comprises \(ep\) data with an integrated luminosity of 34.9 \(\textrm {pb}^{-1}\). Jets are selected using the inclusive \(k_{\rm t}\) algorithm with a minimum transverse energy of 25 GeV for the leading jet. The phase space covers longitudinal proton momentum fraction \(x_p\) and photon longitudinal momentum fraction \(x_\gamma \) in the ranges \(0.05\lt x_p\lt 0.6\) and \(0.1\lt x_\gamma \lt 1\). The predictions of next-to-leading order perturbative QCD, including recent photon and proton parton densities, are found to be compatible with the data in a wide kinematical range.


Multijets in Photoproduction at HERA

abstract

Cross-sections for the photoproduction of events containing four high-\(E_{\rm T}\) jets have been measured with the ZEUS detector using an integrated luminosity of 38.7 pb\(^{-1}\). At low invariant mass of the four-jet system, \(m_{\rm 4J}\), the observed event distributions are sensitive to the underlying event model. The inclusion of multi-parton interactions in Monte Carlo simulations lead to a significantly better description of the data. For \(m_{\rm 4J} \gt 50 \) GeV the data is adequately described by models with no simulation of an underlying event, suggesting that this is a kinematic region suitable for comparison with perturbative QCD calculations.


Dijet Production at Low \(Q^2\)

abstract

Triple differential dijet cross-sections in \(e^\pm p\) interactions measured with the H1 detector at HERA are presented. The data are compared to Monte Carlo simulations based on the DGLAP and CCFM parton evolution schemes. Effects of longitudinally polarized virtual photons are investigated.


Random Walk Through Recent CDF QCD Results

abstract

We present recent results on jet fragmentation, jet evolution in jet and minimum bias events, and underlying event studies.


Jet Results at the DØ Experiment

abstract

In this paper I will present a selection of recent results concentrating on the measurement of jets as measured by the DO experiment at the Fermilab Tevatron \(\bar {p}p\) collider. The results presented here are a comparison of the inclusive jet cross-section as measured using the cone and \(k_{\rm T}\) jet algorithms, the measurement of sub-jet multiplicity of quark and gluon jets, a measurement of thrust, and a measurement of low \(p_{\rm T}\) jets.


Electroproduction of Dijets at Small Jet Separation

abstract

Dijet production in deep-inelastic scattering (DIS) in the range \(150 \lt Q^{2} \lt 35000\) GeV\(^{2}\) has been measured by the H1 collaboration using the Durham jet algorithm in the laboratory frame. QCD calculations in next-to-leading order (NLO) are found to give a good description of the data when requiring a small minimum jet separation, which selects a dijet sample containing 1/3 of DIS events in contrast to approximately 1/10 with more typical jet analyses.


Three-Jet Production in Deep-Inelastic Scattering at HERA

abstract

Three-jet production has been studied in deep-inelastic positron–proton scattering. The measurement carried out with the H1 detector at HERA covers a large range of four-momentum transfer squared \(5 \lt Q^2 \lt 5\,000\,{\rm GeV}^2\) and invariant three-jet masses \(25 \lt M_{\rm 3jet} \lesssim 140\,{\rm GeV}\). Jets are defined by the inclusive \(k_{\perp }\) algorithm in the Breit frame. The size of the three-jet cross section and the ratio of the three-jet to the dijet cross section \(R_{3/2}\) are described over the whole phase space by the predictions of perturbative QCD in next-to-leading order. The shapes of angular jet distributions deviate significantly from a uniform population of the available phase space but are well described by the QCD calculation.


The Internal Structure of Jets at HERA

abstract

Measurements of the internal structure of jets in deep-inelastic scattering in terms of the jet shape and the mean subjet multiplicity are used to determine \(\alpha _{\rm s}\). The data were taken with the ZEUS detector at the HERA collider. From photoproduction dijet events and a sample of charm–quark jets the jet shapes of quark and gluon jets have been statistically extracted to study differences between these types of jets.


Combining Matrix Elements and Parton Showers

abstract

A method is suggested to combine tree level QCD matrix for the production of multi jet final states and the parton shower in hadronic interactions.


Combining Matrix Elements and the Dipole Cascade Model

abstract

An algorithm is presented in which the Colour–Dipole Cascade Model as implemented in the Ariadne program is corrected to match the fixed order tree-level matrix elements for \(e^+e^- \rightarrow n\) jets. For the results presented here, matrix elements were used up to second order in \(\alpha _{\rm s}\), but the scheme is applicable also for higher orders.


\(D^{*}\mu \) Correlations and \(D\) Meson Production in \(ep\) Scattering at HERA

abstract

\(D^{*}\mu \) coincidences are studied using data taken with the H1 detector at HERA during the years 1997 to 2000, corresponding to an integrated luminosity of about \(90\;\textrm {pb}^{-1}\). With the aid of charge and angle correlations between the \(D^{*}\) and the muon, a separation of charm and beauty production contributions is possible. Total charm and beauty production cross sections are shown as well as differential cross sections of several \(D^{*}\mu \) quantities and compared with the prediction of Leading Order (LO) Monte Carlo calculations. In a separate analysis inclusive charm production is studied using \(e^{+}p\) data taken with the H1 detector during the years 1999 to 2000, corresponding to an integrated luminosity of about \(48\;\textrm {pb}^{-1}\). For the charmed mesons \(D^{*}\), \(D^{0}\), \(D_{\rm s}\) and \(D^{+}\) total production cross sections are determined and differential distributions are measured for the \(D^{+}\) and \(D^{0}\) mesons, whereby lifetime information is used to improve the signal qualities. The results are compared with predictions, based on LO Monte Carlo simulations. The measured production cross sections are used to determine fragmentation ratios, which are then compared with values measured e.g. at \(e^+ e^-\) colliders.


\(D^{*\pm }\) Production in \(e^-p\) and \(e^+p\) Deep Inelastic Scattering at HERA

abstract

Inclusive production of \(D^{*\pm } (2010)\) mesons in deep inelastic scattering has been measured using \(e^+p\) and \(e^-p\) data obtained with the ZEUS detector at HERA using integrated luminosities of \(16.7\) and \(65.2 \rm pb^{-1}\), respectively. The decay channel \(D^{*+} \rightarrow D^{0} \pi ^{+}\) with \(D^{0} \rightarrow K^{-} \pi ^{+}\) and corresponding antiparticle decays were used to identify \(D^{*\pm }\) mesons. The \(D^{*\pm }\) cross sections in \(e^-p\) and \(e^+p\) interactions agree with NLO QCD predictions, although the \(D^{*\pm }\) cross section in \(e^-p\) is slightly higher than that in \(e^+p\).


Charm Fragmentation and Dijet Angular Distributions

abstract

Charm fragmentation and dijet angular distributions have been measured in \(D^*\) photoproduction at HERA. Charm fragmentation and its property of universality is evaluated in terms of measurement of \(P_v\), the ratio of vector/(vector + pseudoscalar) mesons. Angular distributions of dijets, with at least one of the jets associated with a \(D^{*\pm }\) meson, have been measured for samples enriched in direct or resolved photon events. The differential cross section shows a steep rise for resolved events in the photon direction, providing strong evidence that the bulk of the resolved photon cross section is due to the charm content of the photon. The shallower rise for direct events as well as for resolved photon events in the proton direction are consistent with the quark exchange diagrams.


Beauty Photoproduction

abstract

Beauty photoproduction in \(ep\) collisions has been studied with the ZEUS detector at HERA, at a center-of-mass energy of \( 300~\textrm {GeV}\). Beauty quarks are tagged via their semi-leptonic decay into a muon or an electron. Events are selected by requiring the presence of two high-transverse energy jets and one high-\(p_{\rm T}\) lepton in the final state. The discrimination between beauty and background events is done by a fit to the \(p_{\rm T}^{\rm {rel}}\) variable, the transverse momentum of the lepton with respect to the jet axis. Total and differential cross sections are presented and compared to leading order Monte Carlo simulations and next-to-leading order QCD predictions.


Inelastic \(J/\psi \) Photo- and Electroproduction

abstract

Recent results from the H1 experiment on inelastic \(J\)/\(\psi \) photo- and electroproduction in \(ep\) collisions are presented. The complete HERA I data, corresponding to an integrated luminosity of \(\sim 80\,{\rm pb}^{-1}\), are compared to Colour Singlet Model (CSM) predictions and calculations based on non-relativistic QCD. First measurements of the polarisation of the \(J\)/\(\psi \) meson are shown.


Charm in Diffractive Deep Inelastic Scattering

abstract

Diffractive production of \(D^{*\pm }(2010)\) mesons in deep inelastic scattering has been studied with the ZEUS detector at HERA using an integrated luminosity of 82 pb\(^{-1}\). Diffractive events were identified by the presence of a large rapidity gap in the final state. The \(D^{*\pm }\) mesons were reconstructed in the decay channel \(D^{*+} \rightarrow D^0 \pi _s^+\) with \(D^0 \rightarrow K^- \pi ^+ (+{\rm c.c.})\). Differential cross sections were measured in the kinematic region \(Q^2 \gt 1.5\,\)GeV\(^2\), \(0.02 \lt y \lt 0.7\), \(x_{\Bbb {P}} \lt 0.035\), \(\beta \lt 0.8\), \(P_{\rm T}^{D^*} \gt 1.5\,\)GeV and \(|\eta ^{D^*}| \lt 1.5\). The measured integrated and differential cross sections are compared with theoretical predictions.


Jets and \(E_t\) Flow in Diffraction at HERA

abstract

Recent measurements of diffractive multi-jet production and transverse energy flow in Deep-Inelastic Scattering (DIS) at HERA are presented. The data are used to investigate the factorisation properties of diffractive DIS and to examine its quantum chromodynamic (QCD) structure.


Leading Neutron Production and \(F_2^{\pi }\) at HERA

abstract

New results on leading neutron production at HERA are presented for cross sections in photoproduction, in deep inelastic scattering and in an intermediate \(Q^2\) range. The data with medium to high photon virtuality are presented in terms of structure functions. Vertex factorization is tested for the semi-inclusive leading neutron data, as well as for events with a dijet system in the final state.


Inclusive Diffractive Dissociation in Photoproduction at HERA

abstract

A new measurement of the differential cross section \(M_X^2 \; d \sigma \)/\(d M_X^2\) for the process \(\gamma p \rightarrow X Y\), with a large rapidity gap between the systems \(X\) and \(Y\) and with \(Y\) a proton or a low mass proton excitation, is presented at three centre-of-mass energies \(\left \lt W \right \gt = 91\), 187 and 231 GeV. The slope parameter \(\alpha _{\rm eff}(0)\) obtained from a single regge type parameterization with a single effective trajectory is presented as a function of \(M_X^2\). A combined triple regge fit is performed over these data together with leading proton data from the same experiment and lower energy fixed target data. The pomeron intercept \(\alpha _{\Bbb {P}}(0)\) is extracted from this fit and was found to be \(\alpha _{\Bbb {P} }(0)\)= 1.127\(\pm \)0.004(stat)\(\pm \)0.025(syst)\(\pm \)0.046(mod).


Modeling Diffractive Final States

abstract

I will discuss some aspects of modeling diffractive final states with event generators. In the first part of the talk I will discuss multiple scatterings in hard diffraction in hadron collisions and its effects on gap survival and hadronization corrections. In the second part I will argue that the study of the ’pomeron remnant’ in hard diffraction at HERA may be the best way of distinguishing resolved pomeron models from dipole models and soft colour interaction models.


Polarization of Prompt \(J/\psi \) and \({\mit \Upsilon }(nS)\)

abstract

We review predictions, based on the nonrelativistic QCD factorization framework, for the polarizations of prompt \(J\)/\(\psi \)’s and \({\Upsilon }(nS)\)’s produced at the Fermilab Tevatron. We also discuss the effect of relativistic corrections on the theoretical prediction for the polarization of prompt \(J\)/\(\psi \)’s at the Tevatron.


Heavy Flavour Production in Two-Photon Collisions at LEP

abstract

New results from the experiments ALEPH, DELPHI, L3, and OPAL on heavy quark production in \(\gamma \gamma \) collisions are presented. Inclusive charm and bottom production are investigated at LEP 2 energies. The total and differential cross sections for charm quarks are now measured by all four LEP collaborations, the total bottom by two. Charmonia are studied inclusively via the muonic decay of the \(J\)/\(\psi \) and separated for the resolved and diffractive processes. New results are available for exclusive production of the \(\eta _{ c}\) meson. First searches for exclusive \(\eta _{ b}\) production are presented.


Heavy-Quark Production in Photon–Nucleon and Photon–Photon Collisions

abstract

I discuss mechanisms of heavy quark production in (real) photon–nucleon and (real) photon–(real) photon collisions. In particular, I focus on application of the Saturation Model. In addition to the main dipole–nucleon or dipole–dipole contribution included in recent analyses, I propose how to calculate within the same formalism the hadronic single-resolved contribution to heavy quark production. At high photon–photon energies this yields a sizeable correction of about 30–40 % for inclusive charm production and 15–20 % for bottom production.


Associated \(D^*\) and Dijet Production at HERA as a Test of the \(k_{\rm T}\)-Factorization Approach

abstract

In the framework of the semi-hard (\(k_{\rm T}\)-factorization) approach, we analyze the various charm production processes in the kinematic region covered by the HERA experiments.


Search for QCD Instanton Induced Processes at HERA

abstract

A search is presented for QCD instanton-induced events in Deep-Inelastic Scattering (DIS) at the electron-proton collider HERA in a kinematic region defined by the Bjorken-scaling variables \(x \gt 10^{-3}\), \(0.1 \lt y \lt 0.6\) and photon virtualities \(10 \lesssim Q^2 \lt 100\) GeV. Several observables characterising hadronic final state properties of QCD instanton-induced events are exploited to identify a potentially instanton-enriched domain. While an excess of events with instanton-like topology over the expectation of the standard DIS background is observed it cannot be claimed to be significant given the uncertainty in the predictions for the background. Upper limits on the cross-section for instanton-induced processes are set and compared to non-perturbative lattice simulations of the QCD vacuum.


Contributions Due to the Longitudinal Virtual Photon in the Semi-Inclusive \(ep\) Collision at HERA

abstract

The importance of contributions due to the longitudinally polarised virtual photon, \(d \sigma _{\rm L}\) and the interference term \(d \tau _{\rm LT}\), in the unpolarised \(e p\) collisions is discussed. The numerical calculations for the Compton process \(e p \rightarrow e \gamma X\) at the HERA collider were performed in the Born approximation. The various distributions in the CM\(_{ep}\) and Breit frames are presented. These cross sections are dominated by the transversely polarised intermediate photon, even for large \(Q^2\).


Muon Pair Production

abstract

Events containing pairs of isolated muons at high invariant masses have been detected at HERA with the H1 detector in a data sample corresponding to an integrated luminosity of 70.9 pb\(^{-1}\) of \(e^{\pm }p\) scattering at \(\sqrt {s} = 318\) GeV. The results are well described by the Standard Model prediction which is dominated by photon–photon collisions.


Bose–Einstein Correlations in DIS

abstract

Measurements of Bose–Einstein correlations (BEC) are presented for deep inelastic \(ep\) scattering using the ZEUS detector at HERA using an integrated luminosity of 83 pb\(^{-1}\). The two-particle correlation functions were studied as a function of the virtuality of the exchanged photon, \(Q^2\), assuming a Gaussian shape of the BEC source. The results do not show a dependence of the BEC effect in the range from \(25\) to \(5000\) GeV\(^2\).


Forward \(\pi ^0\) Production in DIS at HERA

abstract

The dynamics of QCD evolution at low values of Bjorken-\(x\) are studied via the measurement of hard \(\pi ^0\) probes in deeply inelastic positron proton scattering with the H1 experiment. The \(\pi ^0\) mesons are produced close to the fragmented proton direction which is called forward region. Measurements of the forward \(\pi ^0\) cross sections and the accompanying transverse energy flow around the \(\pi ^0\) are presented and used to discriminate between different schemes for QCD evolution.


Production of Protons in Photoproduction at HERA

abstract

A measurement of the inclusive cross-section for the photoproduction of protons in the central fragmentation region at HERA is presented. The measured cross-section is compared with the prediction of the PYTHIA MC model. The obtained value of the diquark suppression factor in the Lund string model is different from that obtained in \(e^+e^-\) data. The dependence of the cross-section as a function of the transverse mass \(M_{\rm T}\) is shown to be similar for different hadrons produced in photoproduction at HERA.


Nonpartonic Effects in Pion Electroproduction in the HERMES Kinematical Region

abstract

The presentation is concerned with higher twist corrections of nonpartonic origin to semi-inclusive observables in the kinematical region relevant for the HERMES experiment. We demonstrate a strong impact of the VDM-like interaction and the exclusive production of \(\rho ^0\) meson on the extraction of the \(\bar d\)–\(\bar u\) asymmetry from charged pion DIS multiplicities. We also show that it is the exclusive-\(\rho ^0\) channel which accounts for the experimentally observed effect of the excess of charged over neutral pions produced at large \(z\).


Multiplicities and Correlations at LEP

abstract

A brief review on recent charge multiplicity and correlation measurements at LEP is given. The measurements of unbiased gluon jet multiplicity are discussed. Recent results on charged particle Bose–Einstein and Fermi–Dirac correlations at LEP1 are reported. New results on two-particle correlations of neutral pions are given. Correlations of more than two particles (high-order correlations) obtained using different methods are performed. Recent Bose–Einstein correlation measurements at LEP2 are discussed.


DIS Event-Shape Resummations and Spin-Offs

abstract

We present results from a recently completed project to calculate next-to-leading logarithmic resummed distributions for a variety of event shapes in the \(1+1\)-jet limit of DIS. This allows fits for the strong coupling and for non-perturbative effects using the large amount of data on these observables from HERA. Spin-offs include the discovery of a new class of logs for certain final state observables (non-global observables); a program that allows a speed-up by an order of magnitude of certain fixed-order calculations in DIS with DISENT or DISASTER++; and the development of state-of-the-art PDF evolution code.


Out-of-Plane QCD Radiation in DIS Events with High \(P_{\rm T}\) Jets

abstract

We present the QCD analysis of the out-of-event-plane momentum distribution in DIS events with high \(p_t\) jets. The achieved accuracy allows the measurement of the running coupling and the study of non-perturbative effects, in particular the test of universality of power corrections in a new experimental regime.


Azimuthal Correlation in DIS

abstract

We introduce a new angular correlation in DIS process and study its differential distribution in the region in which the observable is small. We perform a perturbative resummation at single logarithmic accuracy and estimate leading non-perturbative power corrections.


Multi-Hadron Final States

abstract

This summary aims to highlight major results and insights gained from recent studies of hadronic final states in \(ep\), \(p\overline {p}\), \(e^+e^-\), as well as relevant theoretical developments, presented in the Multi-Hadron Final States parallel sessions of the DIS2002 workshop held in Kraków, Poland in May 2002.


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