Soft gluons and factorization

We clarify the role of soft gluons in hard inclusive hadron-hadron cross sections, emphasizing the mechanisms by which they decouple from the incoming hadrons and cancel.     

Soft gluons in semi-inclusive deep inelastic experiments

The longitudinal and transverse properties of the QCD contribution to semi-inclusive deep inelastic scattering is worked out within the soft gluon resummation procedure. The Λ parameter needed to reproduce the data is in agreement with the value extracted from scaling violations. The intrinsic transverse momentum squared of a quark inside a proton is around 0.45 GeV². Predictions for the transverse correlations $\text{〈}\text{p}{}_{\mathrm{1\perp }}\text{·}\text{p}{}_{\mathrm{2\perp }}\text{〉}$ of two outgoing pions are guven.     

Soft gluons and the normalization of the Drell-Yan cross section

We analyze the sensitivity of the inclusive Drell-Yan cross section to soft gluons, using a previously developed summation procedure which includes nonleading logaritmic effects. By varying an infrared cutoff in gluon energies, we study the importance of soft, but still perturbative, gluons to the normalization of the cross section or, equivalently, the “K-factor.” The results are strongly dependent on the kinematic range being considered, as well as on the parton distributions of the incoming hadrons.     

Lekner summations and Ewald summations for quasi-two-dimensional systems

The purpose of this work is to carry out a comparison between the Ewald quasi-2D and Lekner summation methods. These methods were derived to treat the long-range electrostatic interactions in systems periodic in two directions, but bound in the third. The comparison is performed by Monte Carlo simulations on a very simple system, a bilayer of point ions; samplings of the phase space, average energies and structure functions are compared. When correctly implemented, the Lekner summation method is found to be in close agreement with the Ewald quasi-2D method; otherwise, a very complicated bias may plague computations.     

Collective excitations of gluons

Three gauge invariant antisymmetric tensor fields are introduced in the nonabelian gauge theories. They are certain non-linear combinations of the conjugate field tensor and they obey O(3) algebra. An effective chiral lagrangian for these fields is derived. It describes 3 vector and 3 axial mesons with vacuum quantum numbers. The masses are generated by spontaneous restoration of Lorentz invariance.     

Confinement models for gluons

Confinement model for gluons using a ‘colour super current’ is formulated. An attempt has been made to derive a suitable dielectric function corresponding to the current confinement. A simple inhomogeneous dielectric confinement model for gluons is studied for comparison. The model Hamiltonians are second quantized and the glueball states are constructed. The spurious motion of the centre of confinement is accounted for. The results of the current confinement scheme is found in good agreement with experimental candidates.     

Dynamics of confined gluons

Propagation of gluons in the confining vacuum is studied in the framework of background perturbation theory, where nonperturbative background contains confining correlators. Two settings of the problem are considered. In the first, the confined gluon evolves in time together with the static quark and antiquark forming the one-gluon static hybrid. The hybrid spectrum is calculated in terms of string tension and is in agreement with earlier analytic and lattice calculations. In the second setting, the confined gluon is exchanged between quarks and the gluon Green’s function is calculated, giving rise to the Coulomb potential modified at large distances. The resulting screening radius of 0.5 fm presents a problem when confronted with lattice and experimental data. A possible solution of this discrepancy is discussed.     

Spin-dependence and coherent summations in two-nucleon transfer reactions

The differential cross sections of the ¹⁹F(p, ³He)¹⁷O and ¹⁹F(p, t)¹⁷F reactions leading to the ground and first three excited states have been measured at an incident energy of 42.4 MeV. The polarization analysing powers of two of the (p, ³He) reactions have been measured at a higher energy of 49.5 MeV with a polarized incident beam. The experimental results have been analysed in terms of conventional DWBA theory, and the importance of interference terms arising from the proper coherent summations contained in the definition of the differential cross section and analysing power, have been investigated. It is found that the interference term arising from the coherent summation over the transferred spin S is significant and should not normally be neglected.     

Integrating out gluons in flow equations

We present an exact nonperturbative flow equation for the average action for quarks which incorporates the effects of gluon fluctuations. With suitable truncations this allows one to compute effective multi-quark interactions in dependence on an infrared scalek. Our method amounts to integrating out the gluons with momenta larger thank.     

Quarks and gluons in color fields

Production and collective motion ofq \(\bar q\) and gluon pairs in strong color fields is studied using boost-invariant version of collisionless Boltzmann-Vlasov transport equations. It is shown that, for realistic fields, the effective formation time for pair production according to Schwinger mechanism is a small fraction of 1 fermi. Collective oscillations of quarkgluon gas, although damped by pair production and expansion of the system, remain an important feature of the process. The considered mechanism produces quark and gluons in a state which is rather far from equilibrium and thus strong collision effects (collision time ?1 fm) are needed to achieve equilibrated distributions at formation time. Finally, it is observed that, if such equilibrated distribution is produced at later times, the expansion of the system modifies dramatically its properties, as is explicitly shown for small oscillations of quark-gluon plasma in a weak external field.     

Topological “vector gluons”

It is shown that the axion can be made invisible in the family-unifyingSU(9) model and that all the light fermions become massive due to the radiative correction.     

Properties of gluons in color superconductors

I give an introduction to color superconductivity in cold, dense quark matter. I focus in particular on how the solution to the gap equation is qualitatively different in ordinary BCS theory with local four-fermion interactions, in theories with non-local, but screened boson exchange, and in QCD, where magnetic gluon exchange is not screened at zero temperature. I argue that a reliable computation of the gap parameter requires knowledge of the gluon self-energy in a color superconductor. As a first step to determine the gluon self-energy, I report calculations of the Debye and Meissner masses in two- and three-flavor color superconductors.     

Elastic diffraction and non-perturbative gluons

Zeitschrift für Physik C Particles and Fields - We consider a simple model for the propagator of a non-perturbative gluon and derive the rules determining its coupling to perturbative gluons...     

Quarks and gluons in dense two-colour QCD

We compute quark and gluon propagators in 2-colour QCD at large baryon chemical potential μ. The gluon propagator is found to be antiscreened at intermediate μ and screened at large μ. The quark propagator is drastically modified in the superfluid region as a result of the formation of a superfluid gap.     

Medium-induced emissions of hard gluons

We present a derivation of the medium-induced gluon radiation spectrum beyond the current limitation of soft gluon emission. Making use of the path integral approach to describe the propagation of high-energy particles inside a medium, we study the limiting case of a hard gluon emission. Analytical and numerical results are presented and discussed within the multiple soft scattering approximation. An ansatz interpolating between soft and hard gluon emissions is provided. The Landau–Pomeranchuk–Migdal effect is observed in the expected kinematic region.