Gluon Polarization Research Articles

Polarized and unpolarized intrinsic gluon distributions

Theoretical constraints on both the polarized and unpolarized intrinsic gluon distribution functions are developed. In particular, we study the behavior of the gluon polarization asymmetry in the nucleon in the small- and large- x regions, and relate the intrinsic distribution to the retarded part of the spin-dependent bound-state potential. A simple model for the polarized and unpolarized intrinsic distributions is proposed which incorporates the QCD constraints. The model predicts that the spin carried by intrinsic gluons in the nucleon is approximately 0.5.

On the polarization of gluons in the proton

We analyze the possibility that polarized gluons could make a substantial contribution to the value of ʃ 0 1 dx g 1 p ( x) estimated by the EMC. We show that with a physically motivated kernel for γ ∗g→ q q, where the q are light quarks, a very substantial part of the possible polarized gluon contribution to g 1 p ( x) is in the region x<0.01 not measured by the EMC. This means that the polarized gluon distributions proposed so far are too small to explain the EMC data via their effects on light quarks alone. However, they do generate a significant charm contribution to g 1 p ( x) for x>0.01 which could help explain the data. We present quantitative predictions for this charm cross section asymmetry in future polarized lepton-nucleon scattering experiments.

Probing gluon polarization in hadronic direct photon production.

Inclusive direct photon production at large transverse momentum in proton-proton interactions with a longitudinally polarized beam and target is an incisive probe of the polarized gluon density in a proton. To help motivate future experiments and assist in their design, we provide predictions of the polarization asymmetry for a range of reasonable choices of the polarized gluon density. The cross sections are small at fixed-target energies, but the effort to measure them is necessary if the polarized gluon density $\ensuremath{\Delta}G(x, {Q}^{2})$ is to be determined. In order to obtain our predictions, we parametrize spin-dependent parton densities and compute their evolution with ${Q}^{2}$. Our densities provide good agreement with the spin-dependent structure function ${g}_{1}^{p}(x, {Q}^{2})$ measured in deep-inelastic lepton scattering.

Perturbative and nonperturbative anomalous contributions to the polarization of the proton

Abstract We compute the contribution induced by the chiral anomaly in the total helicity of fermionic constituents of a polarized hadron. We unify previous perturbative and nonperturbative determinations and we show that due to nonperturbative QCD effects the EMC measurements of polarized structure functions may be consistent with a large quark spin component, even in the absence of gluon polarization.

Polarization of gluons

The measurement of the polarization of gluons through gluons jet oblateness is discussed. It is shown that the most suitable process to study the polarization of gluons is p+p̄→(W or Z)+gluon jet+anything.

Monte Carlo simulation of general hard processes with coherent QCD radiation

In this paper we extend our previous work on the simulation of coherent soft-gluon radiation to hard collisions that involve incoming as well as outgoing coloured partons. Existing simulations correctly sum the leading collinear singularities for initial- and final-state radiation, and in some cases the leading infrared contributions from outgoing partons, but not those for incoming (or the interference between incoming and outgoing). Asymptotically, however, the leading infrared and collinear contributions are comparable, the bulk of gluon emission occuring in the soft region. Furthermore, a correct treatment of leading infrared terms is necessary for the inclusive cancellation of singularities in the Sudakov form factor. We show how such a treatment ,ay be formulated in terms of an angular ordering procedure applicable to all hard processes. We then describe a new Monte Carlo program which incorporates this procedure, together with other new features such as azimuthal correlations due to gluon polarization and interference. The program is designed as a general-purpose event generator, simulating hard lepton-lepton, lepton-hadron and hadron-hadron scattering in a single package. Simulation of soft hadronic collisions and underlying events is also included. We present the predictions of the program for a wide variety of processes, and compare them with analytical results and experimental data.

High temperature gluon matter in the background gauge

The polarisation tensor of gluons in high temperature QCD matter is discussed in the back-ground gauge and compared with that in the temporal axialA 0=0 gauge. In both of these the gluon polarisation tensor alone is sufficient to give the asymptotically free sign of the QCD beta function. In the former the beta function and the imaginary part of the polarisation tensor are coupled in such a way that asymptotic freedom implies instability of plasma oscillations. In the latter, due to the non-covariance of the gauge condition, the beta function and the imaginary part are decoupled in such a way that the sign of the imaginary part corresponds to stable plasma oscillations.

Screening and plasmon in QCD on a finite lattice

Finite-size effects on lattice Monte Carlo measurements of the screening mass in high temperature QCD are studied by computing the static limit of the gluon polarization tensor п μν on the lattice. Doing the sum over the discrete frequencies p 0 analytically permits one to write the result in a very simple form as a threefold sum over p i and closely resembling the continuum result. The magnitude of the plasmon term in the pressure of an interacting quark-gluon gas on the lattice is estimated and shown to be small for lattices in current use. The same techniques are applied to the calculation of the energy density and the quark number susceptibility ∂n B (T, μ = 0) ∂ μ for an ideal gas of quarks and gluons on the lattice.

χ c Charmonium — A tool to investigate gluon polarization

Production of particles with hidden charm χ c in polarized parton beams (quarks or gluons) is studied. Parton polarization is shown to cause changes in the angular distributions of the χ c-meson decay products, which allows one to investigate possible gluon polarization in hadrons.

Parton spin asymmetries in polarized hadrons at high energies

The predictions of perturbative QCD for the evolution of quark and gluon distributions for polarized hadrons, in general, and for polarized protons, in particular, are developed. Analytical and numerical studies suggest that quark and gluon polarizations increase with increasing energy at fixed Feynman x. Moreover, the average gluon asymmetry grows as ln Q 2 receiving contributions from both quark and gluon asymmetries at lower energy. This has important implications for the growth of orbital angular momenta of partons and presents a challenge to model building. A new sum rule for a particular moment of the quark and gluon asymmetries is derived. We conclude that more work is needed to determine how useful polarized proton beams might be in the search for and exploration of new physics in lepton-proton, p p and pp colliders.

Quark-gluon transport theory II. Color response and color correlations in a quark-gluon plasma

Starting from a classical kinetic description of the quark-gluon plasma, we derive in the linear response approximation the color response function near thermodynamic equilibrium. From its poles the dispersion relations for the collective color modes (one longitudinal and one transverse) are obtained. The absence of Landau damping in the quark-gluon plasma is shown, and other damping mechanisms are discussed. A common misunderstanding concerning Landau damping in an electron plasma at low momenta is clarified. Finally we find the color correlation function in an equilibrium QCD plasma, using the fluctuation-dissipation theorem. Its relation to the gluon polarization operator in finite temperature QCD is discussed. The coefficient for color transport (color conductivity) is determined within a relaxation time model. Possible applications in the computation of plasma observables are pointed out.

Probing proton spin structure through hadronic reactions

Inclusive and semi-inclusive photon producing polarized proton reactions have been employed to probe the spin structure of the proton. Combinations of cross-sections are suggested which may measure valence quarks polarization and gluon polarization in the proton separately. The general formalism is used to predict numerical results using a model of spin structure based on Altarelli-Parisi equations.

Helicity selection rules in the Born approximation to Compton scattering of massive spin-1 particles on massless fermions

The helicity amplitudes for the reaction $G+q\ensuremath{\rightarrow}\ensuremath{\gamma}+q$, where the gluon $G$ and photon have virtual masses, and where the quarks are massless, are studied. A remarkably simple picture is found to describe the reaction in the Born approximation. Despite the fact that there are three distinct polarization vectors, the transition matrix remains two-dimensional, provided the reaction takes place on a polarized quark. This simplification is achieved by introducing a nonorthogonal basis for the massive gluon and photon states. An interesting aspect is that one particular gluon polarization state cannot produce photons, while a certain massive photon polarization state cannot be produced. Our results represent a generalization to arbitrary-mass spin-1 particles of the usual helicity conservation known to hold for massless vector particles.

Gluon Bremsstrahlung in Charged Lepton-Nucleon Collisions II. Gluon Linear Polarization

The linear polarization of gluons emitted in charged lepton-nucleon scattering, including the effects of weak neutral currents, is calculated. The polarization is given in the target nucleon rest system, as well as in the center of mass system.

QCD at finite temperature

Quantum chromodynamics is studied at finite temperatures and densities using the temperature Green functions method. For the Green functions the renormalized Schwinger-Dyson equations are obtained and their qualitative properties are discussed. The equality of the renormalization constants for the equations obtained at T, μ ≠ 0 with those for quantum field theory is pointed out. General properties of the gluon polarization tensor are investigated at T, μ ≠ 0. The temperature Green functions are calculated within the one-loop approximation using both relativistic and axial gauges. The fulfilment of the Slavnov-Taylor identities is verified. The asymptotic behaviour of the polarization tensor at T, μ ≠ 0 is established and the excitation spectrum of quark-gluon plasma is found. Both Fermi and Bose excitations are considered and the gauge invariance of the spectra is demonstrated. The renormalization group extension of the dispersion laws into the regions of high temperatures and densities is presented. The exact representation of the thermodynamical potential in QCD is found in terms of the temperature Green functions. For the quark-gluon plasma the thermodynamical potential is calculated with the g3-term taken into account. The equation of state of the hot quark-gluon plasma is found and its properties are discussed. The complete evolutional diagram of the hadronic matter is outlined. The phase curve asymptotics, which put bounds on the quark-gluon plasma domain, are found for the two limiting cases (μ = 0, T T0; T = 0, μ μ0). The phase transition of the hot quark-gluon plasma placed in external Abelian field is studied. The instability of such plasma has been found and the program of its stabilization is indicated. The infrared behaviour of the non-Abelian gauge theory is studied for finite temperatures when power divergencies are essential. The propagator of transverse gluons is shown to be singular for momenta |p| ˜ g2T and this cannot be avoided by summing the simplest bubble chains. The infrared asymptotic behaviour of the tree-gluon vertex is found and the results obtained are checked using the Slavnov-Taylor identities. The Green functions asymptotics found indicate either an instability of the quark-gluon plasma in the infrared momentum domain or the inconsistency of the perturbational methods. A non-perturbative approach to the infrared problem in QCD is developed within the axial gauge. The closed equations for the structure functions that determine the gluon polarization tensor are obtained by using the Slavnov-Taylor identities to found approximately the three-gluon vertex. It is shown that the solution of the equations obtained by iterations does not remove the infrared singularity from the temperature Green functions. The nonperturbative solution of such equations is discussed.

The Non-Perturbative Approach to the Infrared Problem in QCD at Finite Temperature

A non-perturbative approach is developed for investigation of the infrared problem in QCD at T ≠ 0 in the ghost-free axial gauge. The problem is solved by using a 3-dimensional theory within the exact Slavnov-Taylor identities and Schwinger-Dyson equations. The system of two non-linear integral equations for the structural functions of the gluon polarization tensor is obtained whose solution determines the infrared behavior of the temperature Green functions in the 4-dimensional QCD. The simplest solution of these equations which is the same as the first term of the perturbation expansion shows the presence of singularities in the gluon propagator at momenta p ∼ g2T, that cannot be eliminated by any choice of the gauge. The infrared instability of QCD at T ≠ 0 caused by these singularities is discussed.

Gluon helicity distributions from hyperon production in proton-proton collisions

We point out a sensitive way of analyzing the gluon polarization inside a polarized proton using hyperon production at largepT inPP collisions, by using initial and final state polarizations.

Polarization asymmetry in dilepton production in perturbative quantum chromodynamics

Quantum-chromodynamics (QCD) evolution equations together with angular momentum sum rules are used to examine the polarization structures of the quark sea and gluons in hadrons. It is shown that it is possible to sustain large polarizations of the flavor-singlet components and they would in turn imply large asymmetries in the dilepton production by polarized beam on polarized target, which can be calculated in perturbative QCD. At high q T for the μ + μ - pair, the main contributing diagram is quark-gluon Compton scattering, and the extent of gluon polarization is expected to be revealed in this kinematic region. At lower q T the quark-antiquark annihilation diagram becomes increasingly important and correspondingly polarized-sea-quark-induced asymmetries are to be found in this region. Further, since the polarization depends on the number of flavors excited, as the available energy in the hadron collision changes, the magnitude of asymmetries should show dramatic variations.

Polarized gluon bremsstrahlung as a test of QCD

Gluon bremsstrahlung emitted in electron-positron annihilation at high energies is found to have a remarkably high degree of linear polarization. It is suggested that measurements of the gluon polarization through angular correlations in the decay of isoscalar vector mesons or possibly through gluon jet oblateness could constitute sensitive test of QCD.