Regular Series


Vol. 35 (2004), No. 1, pp. 9 – 510


HBT Interferometry at SPS and Universal Pion Freeze-Out

abstract

Recent pion HBT results from SPS are reviewed and compared to measurements at AGS and RHIC. The evaluation of the energy dependence leads to a universal freeze-out condition for pions. Implications of the HBT results on the dynamical evolution of heavy ion collisions are discussed.


What We Know About HBT at RHIC

abstract

The first measurements of the pion source dimensions for Au–Au collisions at RHIC gave no direct evidence for a large or long-lived source, as previously expected for a deconfinement phase transition. This result has proved to be something of a puzzle when taken in context with indications of jet suppression for central Au–Au collisions. With the goal of illuminating what we do not know, we review aspects of our understanding of the two-particle correlation measurements that have recently yielded to relatively new analysis techniques, in 3-pion correlations, non-identical particle correlations, and self-consistent treatments of the Coulomb interaction.


Predictions from Microscopic Models on Particle Correlations at RHIC

abstract

We review the recent developments on microscopic transport calculations for two-particle correlations at low relative momenta in ultrarelativistic heavy ion collisions at RHIC.


Emission Angle Dependence of HBT Radii: Theoretical Background and Interpretation

abstract

The Wigner function formalism which relates source size parameters to experimental “HBT radii” extracted from two-particle Bose–Einstein correlations is generalized to azimuthally deformed and longitudinally tilted sources. It is explained how this can be used to complement anisotropic flow measurements with relevant space-time information on the source.


Azimuthally-Sensitive Interferometry and the Source Lifetime at RHIC

abstract

Pion interferometry (“HBT”) measurements relative to the reaction plane provide an estimate of the transverse source anisotropy at freeze-out, which probes the system dynamics and evolution duration. Measurements by the STAR Collaboration indicate that the source is extended increasingly out-of-plane with increasing impact parameter, suggesting a short evolution duration, roughly consistent with estimates based on azimuthally-integrated HBT measurements.


Non-Identical Particle Correlations in STAR as a Probe of Emission Asymmetries and Radial Flow

abstract

Non-identical particle correlations offer new methods of probing the dynamics of the heavy-ion collision. In STAR we have performed a correlation analysis of pion–kaon and pion–proton pairs for 130 \(A\)GeV AuAu collisions. The results show that average emission space-time points of these particle species are not the same. These asymmetries are interpreted as a consequence of space-momentum correlations produced by rapid transverse radial expansion of the system. The effects of emission time differences, coming from the decay of resonances, are also investigated with the help of rescattering models, and explain only part of the asymmetry. Therefore our measurements represent an independent confirmation of the existence of transverse radial flow.


Bose–Einstein Correlations in Hadronic \(W\) Decays at LEP

abstract

Both experimentalists and theorists dealing with Bose–Einstein correlations (BEC) in high energy interactions find themselves in an awkward situation. The observation of a very weak BEC signal between two hadronizing sources in the reaction \(e^+e^- \rightarrow W^+W^-\) could well indicate that our understanding of the BEC effect is quite poor. In this paper, an overview of the latest LEP results on BEC between particles from different \(W\)s is given. From the combination of these results, a systematic uncertainty on the measured \(W\) mass in the fully hadronic channel is estimated.


Bose–Einstein Correlations in DIS at HERA

abstract

Bose–Einstein correlations of identical charged pions are studied in deep inelastic \(ep\) scattering events measured with the ZEUS detector at HERA. In one-dimensional analysis the radius of the particle production source and the correlation strength are independent of the exchanged photon virtuality \(Q^2\) in the range between 0.1 and 8000 GeV\(^2\). No significant difference was found between the correlations measured in the target and current region of the Breit frame for \(Q^2\) \(\gt \) 100 GeV\(^2\). The result indicates insensitivity of the radius of the source to the underlying hard process. Two-dimensional analysis performed in the Longitudinally CoMoving System indicates an elongated shape of the source.


Open Questions Related to Bose–Einstein Correlations in \(e^+e^- \to \) Hadrons

abstract

Questions concerning the Bose–Einstein (BEC) and Fermi–Dirac (FDC) correlations of hadrons produced in \(e^+e^-\) collisions are discussed. Among them the emitter dimension \(r\) as a function of \(\sqrt {s_{ee}}\) and the hadron mass, the extension of the BEC by including isospin invariance and the proposed relation between \(r\) and the inter-atomic separation in Bose condensates.


Heavy-Flavor Production Overview

abstract

This talk serves as an introduction to the Heavy-Flavor session of the XXXIII International Symposium on Multiparticle Dynamics. A major focus of this session is on the production of heavy quarks. The talks which follow review the latest results on heavy quark production in strong, electromagnetic, and weak interactions, as well as some of the physics of the heavy quarks themselves. This talk emphasizes what we can learn from the production measurements, both about underlying QCD theory and the partonic nature of the hadrons which we see in the laboratory.


Beauty and Charm Production at Fixed-Target Experiments

abstract

Fixed-target experiments continue to provide insights into the physics of particle production in strong interactions. The experiments are performed with different types of beam particles of varying energies, and many different target materials. Studies of beauty and charm production are of particular interest, since experimental results can be compared to perturbative QCD calculations. It is in this context that recent results from fixed-target experiments on beauty and charm production will be reviewed.


Charmonium Production with Antiproton Gas-Jet Interactions at FNAL

abstract

Precision measurements of charmonium spectroscopy are obtained studying charmonium formed in \(\overline {p}p\) annihilation. Two experiments at FNAL, E760 and E835, adopted this technique. Some of their results are briefly discussed.


Charmonium Production with \(e^+ e^-\)

abstract

We present a new study of double \(c\bar {c}\) production in the \(e^+ e^-\) continuum: many double charmonium final states are observed for the first time; the \(e^+ e^- \to J/\psi c\bar {c}\) cross-section is measured with reduced model dependence.


Heavy Flavour Physics at LEP

abstract

The production of b hadrons in \(e^+e^-\) interactions at the \(Z\) pole allows to perform detailed studies of their properties. Almost a million of \(Z^0\rightarrow {b}\bar {{b}}\) decays have been detected by the four LEP collaborations. In this contribution, some recent results on heavy flavour physics are presented.


Review of Heavy Flavour Production in \(ep\) Collisions

abstract

Measurements of open heavy flavour production in \(ep\) collisions at a center of mass energy of 318 GeV are reviewed in both photoproduction and deep inelastic scattering.


Heavy Flavour Results from the Tevatron

abstract

Given the large number of heavy flavour results from the Tevatron this document can only be a subjective overview. The main selection criterion was to only present results obtained from the analysis of Tevatron Run II data. There is also a preference for cross section measurements, owing to the scope of the conference. The Tevatron and detector upgrades are only discussed briefly. All results are preliminary unless otherwise stated.


Recent Developments in Heavy Quark and Quarkonium Production

abstract

Recent measurements of \(J/\psi \) production in \(e^+e^-\) colliders pose a challenge to the NRQCD factorization theorem for quarkonium production. Discrepancies between leading order calculations of color-octet contributions and the momentum distribution of \(J/\psi \) observed by Belle and BaBar are resolved by resumming large perturbative and nonperturbative corrections that are enhanced near the kinematic endpoint. The large cross sections for \(J/\psi c \bar c \) and double quarkonium production remain poorly understood. Nonperturbative effects in fixed-target hadroproduction of open charm are also discussed. Large asymmetries in the production of charm mesons and baryons probe nonperturbative corrections to the QCD factorization theorem. A power correction called heavy-quark recombination can economically explain these asymmetries with a few universal parameters.


Particle Production and Propagation in Nuclei

abstract

We discuss the effects of gluon radiation by the struck quark and the subsequent absorption of the produced hadron in deep inelastic lepton-nucleus scattering. The theoretical picture is compared with HERMES results on multiplicity ratios.


Attenuation of Hadrons in Nuclei

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. The hadron multiplicity has been measured for helium, neon and krypton relative to that of deuterium. The multiplicity ratio is strongly reduced at high values of the atomic number \(A\). The \(A\)-dependence of HERMES data on krypton and nitrogen is discussed and is compared to two theoretical models. Assuming a simple \(A^\alpha \)-dependence of the nuclear attenuation, the HERMES data are found to be closer to the \(A^{2/3}\)-dependence predicted by some pQCD-based models than to the \(A^{1/3}\)-dependence that follows from models based on nuclear absorption only.


Particle Production in Nuclear Matter: Highlights from \(h+p\) and \(h+A\) Data at SPS Energies

abstract

Various phenomena observed in \(A+A\) collisions can be traced back to more elementary \(h+h\) and \(h+A\) reactions.


Universality of Multiplicity and Transverse Momentum Distributions in the Framework of Percolation of Strings

abstract

In the framework of string percolation, the transverse momentum distributions in heavy ion collisions at all centralities and energies are shown to follow an universal behaviour, the shape of the distributions depending essentially on the transverse density of strings. We find that the relative suppression of intermediate and high \(p_{\rm T}\) production in central nucleus–nucleus collisions has the some origin as the narrowing of multiplicity distributions, the clustering of strings. The clustering of strings also explains naturally the dependence on the centrality of the transverse momentum fluctuations and the strength of the two and three body Bose–Einstein correlations.


Unintegrated Parton Distributions and Particle Production in Hadronic Collisions

abstract

The inclusive distributions of gluons and pions for high-energy \(NN\) collisions are calculated. The results for several unintegrated gluon distributions (UGDs) from the literature are compared. We find huge differences in both rapidity and \(p_{\rm t}\) of gluons and \(\pi \)s in \(NN\) collisions for different models of UGDs. The Karzeev–Levin UGD gives good description of momentum distribution of charged hadrons at midrapidities. We find, however, that the gluonic mechanism discussed does not describe the inclusive spectra of charged particles in the fragmentation region. Some of the missing mechanisms are calculated with the help of unintegrated parton distributions from the solution of the CCFM equation.


On the Status of VHM Physics

abstract

The status of thermodynamical approach in the multiple production processes and especially for very high multiplicity (VHM) domain is discussed. This approach is quite popular in the heavy ion collision physics. It is argued that the “principle of vanishing of correlations” must be used for quantitative estimations of the rate of thermalization. A review of present experimental information is offered.


all authors

Y. Hama, F. Grassi, O. Socolowski Jr., T. Kodama, M. Gaździcki, M.I. Gorenstein

Energy Dependence of the Inverse Slope Parameter in Heavy-Ion Collisions

abstract

We calculate, in a hydrodynamic approach, the \(m_T\) spectra of kaons in central Pb+Pb (Au+Au) collisions, as function of energy. The experimentally observed anomalous behavior of the inverse slope parameter \(T^*\) may be reproduced by a reasonable choice of the critical temperature \(T_{\rm c}\sim 160\) MeV at \(\mu =0\) and an equally reasonable choice of energy-dependent freeze-out temperature \(T_{\rm fo}\), thus supporting the argument that these data are an additional signature of deconfinement transition.


Resonance Production in Au+Au and \(p\)+\(p\) Collisions at \(\sqrt {s_{NN}}= 200\) GeV

abstract

The \(\rho ^0(770)\), \(K^{*0}(892)\), \(f_{0}(980)\), \(\phi (1020)\), and \({\mit \Lambda }(1520)\) production in Au+Au and \(p\)+\(p\) collisions at \(\sqrt {s_{NN}}= 200\) GeV are presented. These resonances are used as a sensitive tool to examine the collision dynamics in the hadronic medium through their decay and regeneration. The modification of resonance mass, width, and shape due to phase space and dynamical effects are also discussed.


Observation of Deconfinement Phase Transition in Nucleus–Nucleus Collisions

abstract

The main results obtained within the energy scan program at the CERN SPS are presented. The anomalies in energy dependence of hadron production indicate that the onset of deconfinement phase transition is located at about 30 \(A\,\)GeV. For the first time we seem to have clear evidence for the existence of a deconfined state of matter in nature.


An Indication for Deconfinement in Au+Au Collisions at RHIC

abstract

We present simultaneous, quality fits to final BRAHMS, PHOBOS, STAR and PHENIX data on particle spectra and two-particle Bose–Einstein or HBT correlations as measured in Au+Au collisions at \(\sqrt {s_{NN}} = 130\) GeV. Using the Buda–Lund hydro model, the best fit is achieved when the most central 1/8th of the particle emitting volume is superheated to \(T \gt T_{\rm c} =170\) MeV. In contrast, we find no indication for such a hot central region in a similar analysis of Pb+Pb data at CERN SPS.


all authors

L. Šándor, F. Antinori, P.A. Bacon, A. Badalà, R. Barbera, A. Belogianni, A. Bhasin, I.J. Bloodworth, M. Bombara, G.E. Bruno, S.A. Bull, R. Caliandro, M. Campbell, W. Carena, N. Carrer, R.F. Clarke, A. Dainese, A.P. de Haas, P. de Rijke, D. Di Bari, S. Di Liberto, R. Divia, D. Elia, D. Evans, K. Fanebust, F. Fayazzadeh, G.A. Feofilov, R.A. Fini, P. Ganoti, B. Ghidini, G. Grella, H. Helstrup, M. Henriquez, A.K. Holme, A. Jacholkowski, G.T. Jones, P. Jovanovic, A. Jusko, R. Kamermans, J.B. Kinson, K. Knudson, A.A. Kolozhvari, V. Kondratiev, I. Králik, A. Kravčáková, P. Kuijer, V. Lenti, R. Lietava, G. Løvhøiden, V. Manzari, G. Martinská, M.A. Mazzoni, F. Meddi, A. Michalon, M. Morando, E. Nappi, F. Navach, P.I. Norman, A. Palmeri, G.S. Pappalardo, B. Pastirčák, J. Pišút, N. Pišútová, F. Posa, E. Quercigh, F. Riggi, D. Röhrich, G. Romano, K. Šafařík, E. Schillings, G. Segato, M. Sené, R. Sené, W. Snoeys, F. Soramel, M. Spyropoulou-Stassinaki, P. Staroba, T.A. Toulina, R. Turrisi, T.S. Tveter, J. Urbán, F.F. Valiev, A. van den Brink, P. van de Ven, P. Vande Vyvre, N. van Eijndhoven, J. van Hunen, A. Vascotto, T. Vik, O. Villalobos, L. Vinogradov, T. Virgili, M.F. Votruba, J. Vrláková, P. Závada

Strange Baryon Production in Pb–Pb Interactions at CERN SPS

abstract

Recent results on strange baryon and antibaryon production in Pb–Pb collisions at 160 GeV per nucleon from the NA57 experiment are reported. Strangeness enhancements and the transverse mass spectra properties are described.


Resonance Production in a Thermal Model

abstract

We discuss the \(\pi ^+ \pi ^-\) invariant-mass correlations and the resonance production in a thermal model with single freeze-out. The predictions are confronted with the recent data from the STAR Collaboration.


Statistical Hadronization and Microcanonical Ensemble

abstract

We present a Monte Carlo calculation of the microcanonical ensemble of the of the ideal hadron-resonance gas including all known states up to a mass of 1.8 GeV, taking into account quantum statistics. The computing method is a development of a previous one based on a Metropolis Monte Carlo algorithm, with a the grand-canonical limit of the multi-species multiplicity distribution as proposal matrix. The microcanonical average multiplicities of the various hadron species are found to converge to the canonical ones for moderately low values of the total energy. This algorithm opens the way for event generators based for the statistical hadronization model.


Introduction to Low \(x\) Physics and Saturation

abstract

The idea of saturation of parton densities in small \(x\) physics is briefly introduced. Some aspects of saturation are described, mainly focusing on the status of our knowledge on the non-linear equations describing the high parton density regime. Implications of saturation ideas on the description of nuclear collisions at the Relativistic Heavy Ion Collider are discussed.


Recent Results on Proton Structure Functions Measurements at Low \(Q^2\) at HERA

abstract

Recent results on \(F_2\) and \(F_{\rm L}\) proton structure functions measurements in \(e^+p\) deep inelastic scattering (DIS) at low \(Q^2\) at HERA are presented.


Forward Jets and Particles at HERA

abstract

It is possible to study the low \(x\) behavior of the proton structure at HERA for \(x\) values as low as \(\sim 10^{-5}\). One of the most interesting aspects of low \(x\) proton structure is the study of QCD dynamics — the evolution of partons between kinematic regimes. In \(ep\) DIS, this can be studied inclusively by measuring the proton structure functions — \(F_2\), \(F_{\rm L}\), etc., or exclusively by studying processes in the target region of the proton — forward going jets and particles. In this paper, various measurements made at HERA by the H1 and ZEUS experiments are presented and compared to MC models and fixed-order QCD calculations. While DGLAP-based evolution is able to describe the inclusive, i.e. \(F_2\), data over the whole HERA kinematic region, a consistent picture of deviations from DGLAP models and fixed-order calculations in exclusive measurements is emerging. MC models and calculations based on BFKL or BFKL-like \(x\)-dependent parton evolution are able to describe most of the exclusive results.


Experimental Status of Parton Saturation at RHIC

abstract

We review the basic phenomenological predictions of parton saturation models and evaluate their relevance in light of the data from Au + Au collisions at RHIC.


Measurements of the Photon and Electron Structure Functions at LEP

abstract

The progress in the measurements of the photon and electron structure functions at LEP is discussed. The short introduction to the kinematics and theoretical framework of the structure functions measurements is given first. Then follow presentations of the recent measurements of the hadronic structure function of the photon, and in particular the charm structure function, which have been updated by using the full LEP2 statistics. The first measurement of the electron structure function is also presented.


Recent Results from CCFM Evolution

abstract

Recent developments of the small \(x\) CCFM evolution are described, including improvements of the splitting function. The resulting unintegrated gluon densities are used for predictions of hadronic final state measurements like jet production at HERA and heavy quark production at the Tevatron.


Multiple Interactions in Low \(x\) DIS and High Energy \(pp\) Collisions

abstract

The LDC formalism for DIS can describe HERA structure functions, and also agrees well with CTEQ and MRST gluon distributions. It is also suitable for hadronic collisions, and provides a strong connection between \(ep\) and \(pp\) reactions. Some preliminary results are presented.


Nonlinear Evolution and Saturation


Geometrical Scaling Due to Critical Behavior Near the Light Cone

abstract

At low \(x\) a transition from a dilute parton gas to a dense parton liquid takes place. We derive geometrical scaling for the structure function in deep inelastic scattering at low \(x\) from a diverging correlation length \(\xi (x)\) of Wilson lines near the light cone. QCD (SU(3)) in \(2+1\) space–time dimensions near the light cone becomes a critical theory in the limit of \(x \rightarrow 0\) with a diverging correlation length \(\xi (x) \propto x^{-{1/2 \lambda _2}}\) where the exponent \(\lambda _2=2.52\) is obtained from the center group Z(3) of SU(3).


Event-by-Event Fluctuations

abstract

An overview of the physics of event by event fluctuations in heavy ion collisions is provided. Several observables are discussed.


Analysis of Transverse Momentum Distributions Observed at RHIC by a Stochastic Model in Hyperbolic Space

abstract

To describe large momentum distributions of charged hadrons observed at RHIC, a diffusion equation in the three dimensional hyperbolic space is introduced.


Charge Fluctuations in \(\pi ^{+}p\) and \(K^{+}p\) Collisions

abstract

It reports on charge fluctuations in \(\pi ^{+}p\) and \(K^{+}p\) collisions at 250 GeV/\(c\). The dependence of these fluctuations on the size of rapidity window and on the multiplicity are presented systematically. The results are compared with those obtained from current theoretical expectations for a hadronic gas, a QGP phase, a stochastic model and a PYTHIA simulation.


The Description of \(pp\)-Interactions with Very High Multiplicity at 70 GeV/\(c\)

abstract

Collective behavior of secondary particles in \(pp\)-interactions at 70 GeV/\(c\) is studied. A Two Stage Gluon Model is offered to describe processes with very high multiplicity. An active role of gluons is shown in multiparticle dynamics. The analysis of multiplicity distributions has revealed a possibility of a thermodynamic interpretation of these interactions. A mechanism of the soft photon production as a signature of the quark–gluon system is proposed.


Learning from RHIC Data with DPMJET-III

abstract

The dual parton model event generator Dpmjet-III is compared to some selected data from the RHIC collider. The aim is to find out, which features of the two-component dual parton model have to be modified in such collisions.


New Physics Signals in Global Event Properties in \(pp\) Collisions at the LHC

abstract

The properties of a new possible class of events, characterised by large hadronic densities and very few clans, in proton-proton collisions at LHC energy are explored within the mechanism of the weighted superposition of different substructures. The most surprising feature is the appearance of an “elbow structure” in the high multiplicity tail, leading to very large expected multiplicity fluctuations.


Anomalous Phenomena in Terms of the Quantum Statistics

abstract

We argue that data coming from the Japanese–American cooperative emulsion experiment (JACEE) on the Si + AgBr event at 4 TeV/nucleon reveal a remarkable regularity (without adjusting any free parameter) which is in accord with the quantum statistical expectation.


Theory of Hadronic Interactions and Its Application to Modeling of Cosmic Ray Hadronic Showers

abstract

Cosmic ray showers offer a mean to study flux, particle type and arrival direction of primary cosmic rays at high energy. Such showers are formed by successive and competing particle interaction and decay processes. Detailed understanding of hadronic and electromagnetic particle production processes is needed to link the observed shower characteristics to properties of the primary particle. In this contribution we discuss some aspects of hadronic interaction models used in extensive air shower simulation and the importance and complementary nature of accelerator and cosmic ray measurements for improving these models.


all authors

A. Haungs, T. Antoni, W.D. Apel, F. Badea, K. Bekk, A. Bercuci, H. Blümer, H. Bozdog, I.M. Brancus, C. Büttner, A. Chilingarian, K. Daumiller, P. Doll, R. Engel, J. Engler, F. Feßler, H.J. Gils, R. Glasstetter, D. Heck, J.R. Hörandel, K.-H. Kampert, H.O. Klages, G. Maier, H.J. Mathes, H.J. Mayer, J. Milke, M. Müller, R. Obenland, J. Oehlschläger, S. Ostapchenko, M. Petcu, S. Plewnia, H. Rebel, A. Risse, M. Risse, M. Roth, G. Schatz, H. Schieler, J. Scholz, T. Thouw, H. Ulrich, J. van Buren, A. Vardanyan, A. Weindl, J. Wochele, J. Zabierowski, S. Zagromski

Astrophysics of the Knee in the Cosmic Ray Energy Spectrum

abstract

A brief review is given on the astrophysics of cosmic rays in the PeV primary energy range, i.e.  the region of the knee.


all authors

J. Milke, T. Antoni, W.D. Apel, A.F. Badea, K. Bekk, A. Bercuci, H. Blümer, H. Bozdog, I.M. Brancus, C. Büttner, A. Chilingarian, K. Daumiller, P. Doll, R. Engel, J. Engler, F. Feßler, H.J. Gils, R. Glasstetter, A. Haungs, D. Heck, J.R. Hörandel, K.-H. Kampert, H.O. Klages, G. Maier, H.J. Mathes, H.J. Mayer, M. Müller, R. Obenland, J. Oehlschläger, S. Ostapchenko, M. Petcu, S. Plewnia, H. Rebel, A. Risse, M. Risse, M. Roth, G. Schatz, H. Schieler, J. Scholz, T. Thouw, H. Ulrich, J. van Buren, A. Vardanyan, A. Weindl, J. Wochele, J. Zabierowski, S. Zagromski

Test of Hadronic Interaction Models with KASCADE

abstract

The interpretation of extensive air shower (EAS) measurements depends on the comparison with EAS simulations. These calculations rely on hadronic interaction models which have to extrapolate into kinematical and energy regions not covered by present-day collider experiments. Therefore, it is necessary to check the reliability of the interaction models used. For the EAS simulations the program CORSIKA is used with several hadronic event generators embedded. Different observables, measured with the KASCADE experiment simultaneously for each air shower event, are investigated as well as their correlations. The consistency of the models applied is checked by comparison of experimental and simulated results.


New Physics from Ultrahigh Energy Cosmic Rays

abstract

Observations of cosmic rays with energies above \(\sim 4\times 10^{10}\) GeV have inspired several speculative suggestions concerning their origin. The crucial question is whether or not the spectrum exhibits the expected ‘GZK cutoff’ at this energy — concerning which there are presently contradictory results. If there is indeed a cutoff, then the sources are cosmologically distant and rather exotic in nature. If there is no cutoff then new physics is required.


Recent Results in Jet Physics

abstract

I present some recent experimental results in jet physics. The general themes are color reconnection, rapidity gaps, particle multiplicities in jets, and differences between gluon and quark jets. Data from \(e^+e^-\), \( p\overline { p}\) and \(ep\) collisions are presented.


Review of Jet Results from HERA

abstract

Some of the most recent results on jet production at HERA are reviewed. The measurements are confronted with predictions from next-to-leading order Quantum Chromodynamics. In regions where uncertainties are small, the strong coupling, \(\alpha _{\rm s}\), has been extracted. Other topics of current interest are also highlighted.


QCD at LEP

abstract

Several preliminary QCD results from \(e^+e^-\) interactions at LEP are reported. These include studies of event shape variables, which are used to determine \(\alpha _{\mathrm s}\) and for studies of the validity of power corrections. Further, a study of color reconnection effects in 3-jet \(Z\) decays is reported.


Recent QCD Results from the CDF and DØ Experiments at the Tevatron

abstract

Recent QCD result obtained from the Tevatron Run II data are reported. The CDF and DØ experiments at the Tevatron has used \(p\bar {p}\) collisions at \(\sqrt {s} = 1.96 \textrm { TeV}\) to measure the inclusive jet and dijet mass spectra.


Soft QCD Phenomena in Events with High-\(E_{\rm T}\) Jets at Tevatron

abstract

Presented are results from the recent jet fragmentation and underlying event structure studies carried out by the CDF Collaboration. Multiplicities of charged particles in quark and gluon jets are measured and their ratio is compared to pQCD predictions. The results are compared to the earlier measurements obtained at \(e^+ e^-\) colliders. Average charged particle multiplicities and energy flow in the underlying event are compared to PYTHIA Monte Carlo event generator; the necessary tuning of PYTHIA parameters needed to make it match the data is discussed.


Unbiased Gluon Jets from \(e^+e^-\) Annihilations Using the Boost Algorithm

abstract

We present the first experimental results based on the jet boost algorithm, a technique to define unbiased gluon jet samples in \(e^+e^-\) annihilations, i.e. events free of any bias from event selection or jet finding criteria. The boost algorithm has been tested using the Herwig Monte Carlo event generator and then applied to hadronic \(Z^0\) decays observed with the OPAL detector at the LEP collider. The gluon jet charged particle multiplicity distribution has been measured in seven jet energy bins from 5 to 20 GeV. From these distributions, the mean value and the first two non trivial normalized factorial moments have been extracted. The gluon jet fragmentation function has been measured in two jet energy bins between 14 and 20 GeV. The results have been compared with existing QCD predictions. In general, good agreement has been found between theory data.


Automated Resummation of Jet Observables in QCD

abstract

We build a computer code that fully automates the resummation of jet-observable distributions at next-to-leading logarithmic accuracy. As an application we present results for a jet shape in hadronic dijet production.


Jet Calculus Problems of the Perturbative Quantum Chromodynamics

abstract

The perturbative quantum chromodynamics (pQCD) has been extremely successful in the prediction and description of main properties of quark and gluon jets. There are, however, some problems of the jet calculus with the higher order corrections of the modified perturbative expansion which should be resolved to get more precise statements. Some of them are discussed here.


Oscillating Hadron and Jet Multiplicity Moments

abstract

Recently, the moments of multiplicity distributions in \(e^+e^-\) annihilation and the ratios \(H_q\) (cumulant over factorial moments \(K_q~/~F_q\)) have been determined both for the hadronic final state and for jets at variable resolution. These ratios show an oscillatory behaviour as function of \(q\) with strong dependence of the amplitude and length of oscillation on the jet resolution parameter \(y_{\rm cut}\). The recent explanation of this phenomenon based on perturbative QCD calculations is discussed.


Simulations of Minimum Bias Events and the Underlying Event, MC Tuning and Predictions for the LHC

abstract

Minimum bias and underlying event data from the \(Sp\bar {{p}}S\) and the Tevatron have been compared to PYTHIA and PHOJET simulations. The data have been used to tune the PYTHIA multiparton model and good agreement is found. PHOJET also gives good agreement. Predictions are made for the multiplicities in minimum bias and the underlying events at the LHC. PYTHIA predicts increases by factors of order two to three from Tevatron energies, while PHOJET predicts more modest increases by a factor of 1.5. The effect of different levels of underlying event activity on the central jet veto used in Higgs searches in the vector boson fusion channel is investigated.


Two-Mode Squeezed and Entangled Gluon States

abstract

We study non-perturbative evolution of the gluon states during small time. Fluctuations of the gluons are less than those for coherent states that is indication of the gluon squeezed states. We show that the two-mode squeezed and entangled states of the gluon fields can appear as a result of the non-perturbative gluon selfinteraction.


Diffraction at HERA

abstract

Studies of diffractive events performed by the H1 and ZEUS Collaborations at the HERA \(ep\) collider are presented. The data of vector meson and photon production, inclusive deep inelastic scattering and hadronic final states with open charm and jets are confronted with the predictions of the resolved Pomeron and colour dipole models.


Diffractive Physics Results at CDF

abstract

Forward detectors are described together with the first physics results from Run II. Using new data and dedicated diffractive triggers, a measurement of single diffractive dijet production rate, with particular focus on the diffractive structure function of the antiproton, is discussed. Upper limits on the exclusive dijet and \(\chi ^0_c\) production cross sections are also presented.


ZEUS Results on Inclusive Diffraction

abstract

Dissociation of virtual photons, \({\gamma }^* p \rightarrow Xp\), has been studied in \(ep\) interactions with the ZEUS detector at HERA. The diffractive cross section, \(d\sigma _{\gamma ^*p}~/~dM_X\), is presented as a function of the photon virtuality, \(Q^2\). The \(W\) dependence of the diffractive and the total cross section is compared. The dependence of the diffractive structure function, \(F_2^{D(3)}\), on \(\beta \), the Pomeron momentum fraction probed by photon, is also studied. The data are compared to predictions based on dipole models.


Proton-Dissociative Vector Meson Production at HERA

abstract

The double diffractive photoproduction of \(J/\psi \) mesons, \(\gamma p \rightarrow J/\psi \) \(Y\), has been studied in \(ep\) collisions with the ZEUS and H1 detectors at HERA. The differential cross section \(d\sigma /dt\), where \(t\) is the four-momentum transfer squared at the proton vertex, has been measured for \(|t|\lt 30\) \({\rm GeV^2}\). The total cross section has been presented as a function of the photon-proton centre-of-mass energy, \(W_{\gamma p}\), in the kinematic range \(50\lt W_{\gamma p}\lt 245\) \({\rm GeV}\). The results are compared with perturbative QCD models based on BFKL and DGLAP evolution.


Diffractive Charmonium Production in Collisions of 920 GeV Protons with Nuclei

abstract

We present preliminary results on the diffractive charmonium production at the HERA-B experiment. An excess of the events with a large rapidity gap between \(J/\psi \) and the outgoing proton system is observed, while a much smaller excess of that between \(J/\psi \) and the outgoing nucleus system is found. The \(A\)-dependence for the events with the large rapidity gap is measured. The search for the Double Pomeron Exchange charmonium production results in the upper limits on the proton–carbon cross-section times the radiative branching fraction of \(\sigma _{\chi _{c0}}^{\rm DPE} \times {\rm BR\,} \lt 0.5\)nb and \(\sigma _{\chi _{c1,2}}^{\rm DPE} \times {\rm BR\,} \lt 1.1\)nb at 90% C.L. The upper limits cover part of the region of the theoretical expectations.


Saturation and Diffractive DIS

abstract

We review QCD based descriptions of diffractive deep inelastic scattering emphasizing the role of models with parton saturation. These models provide natural explanation of such experimentally observed facts as the constant ratio of \(\sigma ^{\rm diff}/\sigma ^{\rm tot}\) as a function of the Bjorken variable \(x\) and Regge factorization of diffractive parton distributions.


Conference Summary

abstract

In this summary I briefly discuss some of the many interesting talks presented at this symposium.


top

ver. 2024.03.17 • we use cookies and MathJax