• Volume 61, Issue 5

November 2003,   pages  785-1053

• Preface

• Theoretical summary talk of QCD 2002

This is a summary of the talks on QCD, not including QCD at finite temperature or density (which are discussed elsewhere) presented at the QCD 2002 meeting held at IIT, Kanpur. I have attempted to give only an overview of the talks since the details may be found in the individual contributions.

• Partons and jets at the LHC

I review some issues related to short distance QCD and its relation to the experimental program of the large hadron collider (LHC) now under construction in Geneva.

• αs Measurements

The strong coupling constant, αs, has been determined from many pure inclusive and semi-inclusive measurements. All these measurements, measured at different scales, are consistent among each other and the measurements can be combined to give αs(mz)= 0.118±0.003.

• Structure function measurements from HERA

In this paper recent measurements of structure functions from the HERA Collaborations are presented.

• The JLab polarization transfer measurements of proton elastic form factor

The ratio of the electric and magnetic proton form factors,GEpGMp, has been obtained in two Hall A experiments, from measurements of the longitudinal and transverse polarizations of the recoil proton,Pl andPt, in the elastic scattering of polarized electrons,$$\overrightarrow e p \to e\overrightarrow p$$. Together these experiments cover theQ2 range of 0.5 to 5.6 GeV2. A new experiment is currently being prepared, to extend theQ2 range to 9 GeV2 in Hall C.

• Exclusive processes at Jefferson Lab

Mapping the transition from strongly interacting, non-perturbative quantum chromodynamics, where nucleon-meson degrees of freedom are effective to perturbative QCD of quark and gluon degrees of freedom, is one of the most fundamental, challenging tasks in nuclear and particle physics. Exclusive processes such as proton-proton elastic scattering, meson photoproduction, and deuteron photodisintegration have been pursued extensively at many laboratories over the years in the search for such a transition, particularly at Jefferson Lab in recent years, taking the advantage of the high luminosity capability of the CEBAF facility. In this talk, I review recent results from Jefferson Lab on deuteron photodisintegration and photopion production processes and the future 12 GeV program.

• Review of brookhaven nuclear transparency measurements (p,2p) reactions at largeQ2

In this contribution we summarize the results of two experiments to measure the transparency of nuclei in the (p,2p) quasi-elastic scattering process near 90‡ in thepp center-of-mass. The incident momenta went from 6 to 14.4 GeV/c, corresponding to 4.8 &lt;Q2 &lt; 127 (GeV/c)2. First, we describe the measurements with the newer experiment, E850, which has more complete kinematic definition of quasi-elastic events. E850 covers a larger range of incident momenta, and thus provides more information regarding the nature of the unexpected fall in the transparency above 9 GeV/c. Second, we review the techniques used in an earlier experiment, E834, and show that the two experiments are consistent for the carbon data. We use the transparencies measured in the five nuclei from Li to Pb to set limits on the rate of expansion for protons involved in quasi-elastic scattering at large momentum transfer.

• Spin physics at RHIC: Present and future

In 2001–2002 the relativistic heavy-ion collider (RHIC) at the Brookhaven National Laboratory (BNL) was first commissioned for polarized proton collisions. Polarized protons were injected into the RHIC, accelerated to 100 GeV, stored and the two beams were made to collide in four interaction regions. I will review the progress made by the RHIC spin program, followed by the physics goals for the next few years. After that I will present a brief overview of a proposal to build a high intensity polarized electron/positron beam facility at BNL which would enable deep inelastic scattering (DIS) experiments to be pursued at BNL by its collisions with the RHIC hadron beams.

• PHOBOS at RHIC: Some global observations

Particle production in Au+Au collisions has been measured in the PHOBOS experiment at RHIC for a range of collision energies for a large span of pseudorapidities, |η| &lt; 5.4. Three empirical observations have emerged from this data set which require theoretical examination. First, there is clear evidence of limiting fragmentation. Namely, particle production in central Au + Au collisions, when expressed as dN/dη′ ( η′ ≡ – ybeam), becomes energy independent at high energy for a broad region of η′ around η′ = 0. This energy-independent region grows with energy, allowing only a limited region (if any) of longitudinal boost-invariance. Second, there is a striking similarity between particle production in e+eand Au + Au collisions (scaled by the number of participating nucleon pairs). Both the total number of produced particles and the longitudinal distribution of produced particles are approximately the same in e+eand in scaled Au + Au. This observation This presentation is based in large part on the PHOBOS summary talk by M Baker at the16th Int. Conf. on Ultrarelativistic Nucleus- Nucleus Collisions, Quark Matter 2002, Nantes, France was not predicted and has not been explained. Finally, particle production has been found to scale approximately with the number of participating nucleon pairs for (Npart) &gt; 65. This scaling occurs both for the total multiplicity and for highpT particles (3 &lt;pT &lt; 4.5 GeV/c).

• The quark gluon plasma: Lattice computations put to experimental test

I describe how lattice computations are being used to extract experimentally relevant features of the quark gluon plasma. I deal specifically with relaxation times, photon emissivity, strangeness yields, event-by-event fluctuations of conserved quantities and hydrodynamic flow. Finally I give evidence that the plasma is rather liquid-like in some ways.

• Lattice quantum chromodynamics: Some topics

I review some topics in lattice quantum chromodynamics, focusing more on the recent results. These include: (i) the QCD phase diagram in the μ-T plane, (ii) the quark number susceptibilities, and (iii) the screening lengths.

• Connections between quantum chromodynamics and condensed matter physics

Features of QCD can be seen qualitatively in certain condensed matter systems. Recently some of the analyses that originated in condensed matter physics have found applications in QCD. Using examples we discuss some of the connections between the two fields and show how progress can be made by exploiting this connection. Some of the challenges that remain in the two fields are quite similar. We argue that recent algorithmic developments call for optimism in both fields.

• Review of chiral perturbation theory

A review of chiral perturbation theory and recent developments on the comparison of its predictions with experiment is presented. Some interesting topics with scope for further elaboration are touched upon.

• Chiral quark model

In this talk I review studies of hadron properties in bosonized chiral quark models for the quark flavor dynamics. Mesons are constructed from Bethe-Salpeter equations and baryons emerge as chiral solitons. Such models require regularization and I show that the two-fold Pauli-Villars regularization scheme not only fully regularizes the effective action but also leads the scaling laws for structure functions. For the nucleon structure functions the present approach serves to determine the regularization prescription for structure functions whose leading moments are not given by matrix elements of local operators. Some numerical results are presented for the spin structure functions.

• Strong interaction at finite temperature

We review two methods discussed in the literature to determine the effective parameters of strongly interacting particles as they move through a heat bath. The first one is the general method of chiral perturbation theory, which may be readily applied to this problem. The other is the method of thermal QCD sum rules. We show that, when the spectral sides of the sum rules are calculated correctly, they do not lead to any new results, but reproduce those of the vacuum sum rules.

• Anomalies, symmetries and strangeness content of the proton

The matrix elements of the operators of strange quark fieldssГs where Г is 1 orγμγ5 between a proton state is calculated. The sigma term is found to be ≈ 41 MeV and theSU(3) singlet axial matrix element is found to be ≈ 0.22, both in agreement with experiment. The sigma term is found using the trace anomaly, while the determination of the axial vector current matrix element is from QCD sum rules. These correspond to (ie.943-1)≈ 0.12 and for the axial current Δs −0.12, respectively. The role of the anomalies in maintaining flavor symmetry in the presence of substantial differences in quark masses is pointed out. This suggests that there is no need to invoke an intrinsic strange quark component in the proton wave function.

• Some observations on interpolating gauges and non-covariant gauges

We discuss the viability of using interpolating gauges to define the non-covariant gauges starting from the covariant ones. We draw attention to the need for a very careful treatment of boundary condition defining term. We show that the boundary condition needed to maintain gauge-invariance as the interpolating parameter θ varies, depends very sensitively on the parameter variation. We do this with a gauge used by Doust. We also consider the Lagrangian path-integrals in Minkowski space for gauges with a residual gauge-invariance. We point out the necessity of inclusion of an ε-term (even) in theformal treatments, without which one may reach incorrect conclusions. We, further, point out that the ε-termcan contribute to the BRST WT-identities in a non-trivial way (even as ε → 0). We point out that these contributions lead to additional constraints on Green's function that arenot normally taken into account in the BRST formalism that ignores the ε-term, and that they are characteristic of the way the singularities in propagators are handled. We argue that a prescription, in general, will require renormalization; if at all it is to be viable.

• Topological field patterns in the Yang-Mills theory

We present a formalism where the topological configurations of pure Yang-Mills theory are characterised using gauge fields alone. Here, we obtain an expression for the charges of these topologicalSO(3) gauge field configurations in terms of the Abelian vector potentials. In this formalism we analyse the ’t Hooft-Polyakov monopole solution.

• Half-monopoles in the Yang-Mills theory

Using a gauge-invariant characterization of monopoles defined via their centres, we investigate the generic topological field pattern for the three-dimensional Yang-Mills theory. This leads to field patterns with one-half winding number. After presenting the main features through the simpler case of half-vortices, we consider half-monopoles in detail.

• Fermions in light front transverse lattice quantum chromodynamics

We briefly describe motivations for studying transverse lattice QCD. Presence of constraint equation for fermion field on the light front allows different methods to put fermions on a transverse lattice. We summarize our numerical investigation of two approaches using (a) forward and backward derivatives and (b) symmetric derivatives.

• Transversity of quarks in a nucleon

The transversity distribution of quarks in a nucleon is one of the three fundamental distributions, that characterize nucleon's properties in hard scattering processes at leading twist (twist 2). It measures the distribution of quark transverse spin in a nucleon polarized transverse to its (infinite) momentum. It is a chiral-odd twist-two distribution function — gluons do not couple to it. Quarks in a nucleon/hadron are relativistically bound and transversity is a measure of the relativistic nature of bound quarks in a nucleon. In this work, we review some important aspects of this less familiar distribution function which has not been measured experimentally so far.

• The proton electromagnetic form factorF2 and quark orbital angular momentum

We analyse the proton electromagnetic form factor ratioR(Q2) =QF2(Q2)/F1(Q2) as a function of momentum transferQ2 within perturbative QCD. We find that the prediction for (R(Q2) at large momentum transferQ depends on the exclusive quark wave functions, which are unknown. For a wide range of wave functions we find thatQF2F1 ∼ const. at large momentum transfer, which is in agreement with recent JLAB data.

• Nuclear effects in the structure functions

By using a relativistic framework and accurate nuclear spectral function the structure functions F2A andF3A of deep inelastic charged lepton and neutrino scattering are calculated in nuclei and results are presented.

• Nuclear physics at small distances

We report on the study of meson and resonance production in nuclear collisions near the threshold. Because of the large momentum transfer, these reactions occur at length scales less than the size of the hadrons. We explore whether they are best described in terms of the quark-gluon picture or the meson-exchange picture. Comparing our results with the available experimental data we conclude that the spin-averaged cross-sections are best described in meson-exchange picture. The description of the observed nucleon-nucleus and hyperon-nucleus spin-orbit potentials are found to be consistent with the quark-gluon exchange picture.

• Dihyperons in chiral color dielectric model

The mass of the dibaryon having spin, parityJπ = 0+, isospinI = 0 and strangeness—2 is computed using chiral color dielectric model. The bare wave function is constructed as a product of two color-singlet three-quark clusters and then it is properly antisymmetrized by considering appropriate exchange operators for spin, flavor and color. Color magnetic energy due to gluon exchange, meson self energy and energy correction due to center of mass motion are computed. The calculation shows that the mass of the particle is 80 to 160 MeV less than twice λ mass.

• Low energyK+ scattering onN =Z nuclei

The data for the total cross-section ofK+ scattering on various nuclei have been analysed in the Glauber multiple scattering theory. Energy-dependentK+-nucleus optical potential is generated using the forwardK+-nucleon scattering amplitude and the nuclear density distribution. Along with this, the calculated totalK+-nucleus cross-sections using the effectiveK+-nucleon crosssection inside the nucleus are also presented.

• Collective flow in relativistic heavy-ion collisions

A brief introduction is given to the field of collective flow, currently being investigated experimentally at the Relativistic Heavy-Ion Collider, Brookhaven National Laboratory. It is followed by an outline of the work that I have been doing in this field, in collaboration with Nicolas Borghini and Jean-Yves Ollitrault.

• Radial, sideward and elliptic flow at AGS energies

We study the baryon transverse in-plane (sideward) and elliptic flow from SIS to AGS energies for AuAu collisions in a relativistic dynamical simulation model that includes all baryon resonances up to a mass of 2 GeV as well as string degrees of freedom for the higher mass continuum. There are two factors which dominantly determine the baryon flow at these energies: the momentum dependence of the scalar and vector potentials and the resonance-string degrees of freedom. We fix the explicit momentum dependence of the nucleon-meson couplings of NL3(hard) equation of state (EoS) by the nucleon optical potential up to 1 GeV of kinetic energy. We simultaneously reproduce the sideward flow, the elliptic flow and the radial transverse mass distribution of protons data at AGS energies. In order to study the sensitivity of different mean-field EoS, we use NL2(soft) and NL23(medium) along with NL3(hard) momenta-dependent mean-field EoS. We find that to describe data on both sideward and elliptic flow, NL3 model is better at 2 A·GeV, while NL23 model is at 4–8 A·GeV.

• Effect of pre-existing baryon inhomogeneities on the dynamics of quark-hadron transition

Baryon number inhomogeneities may be generated during the epoch when the baryon asymmetry of the universe is produced, e.g. at the electroweak phase transition. These lumps will have a lower temperature than the background. Also the value ofTc will be different in these regions. Since a first-order quark-hadron (Q–H) transition is susceptible to small changes in temperature, we investigate the effect of the presence of such baryonic lumps on the dynamics of the Q–H transition. We find that the phase transition is delayed in these lumps for significant overdensities. Consequently, we argue that baryon concentration in these regions grows by the end of the transition. We mention some models which may give rise to such high baryon overdensities before the Q–H transition.

• Implications of cosmic string-induced density fluctuations at the quark-hadron transition

We show that cosmic strings moving through the plasma at the time of a first-order quark-hadron transition in the early universe generate baryon inhomogeneities, which can survive till the nucleosynthesis epoch. We find out how these inhomogeneities actually affect the calculated values of the light element abundances. Recently a wealth of observational data from various experiments have helped to reduce the uncertainties in the values of these abundances. Using these we show that it is possible to derive constraints in the presence of cosmic strings during the quark-hadron transition.

• Phase space description of the production of quark gluon plasma in heavy-ion collisions

A new source term is proposed to describe the production of quark gloun plasma in heavy-ion collisions.

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