QCD calculations in the light-cone gauge

The non-singlet quark structure function is calculated in the leading logarithm approximation in an axial gauge with n² = 0, the light-cone gauge. The choice n² = 0 leads to a simple identity for loop integrals involving the extra n · k denominators. We compare the results graph by graph with both Feynman gauge QCD and a scalar gluon theory. The leading diagrams are the same “rainbow” diagrams as for the case of the scalar theory.The techniques are also applied to quark-quark scattering at large transverse momentum. The leading diagrams have the same dressed ladder-form factor structure.     

Precise hadron mass calculations from lattice QCD

By using scalar as opposed to spin-$\text{1}\text{2}$ quarks and treating spin effects perturbatively, the masses of the lowest lying 0^- and 1^- mesons above 1 GeV (the J/Ψ, η_c, D, D¹, F, F¹, and φ) are calculated to 1%. The masses of the K¹, ?, and K are respectively obtained to 3%, 8% and 30%. Certain (spin-averaged) linear combinations of baryon masses are also computed. The nucleon-delta result differs from experiment by 8%. For heavier baryons the error is smaller. Scalar lattice QCD seems to be a promising approach to the strong interactions.     

Automating dipole subtraction for QCD NLO calculations

In this publication the construction of an automatic algorithm to subtract infrared divergences in real QCD corrections through the Catani–Seymour dipole subtraction method [1,2] is reported. The resulting computer code has been implemented in the matrix element generator AMEGIC++ [3]. This will allow for the automatic generation of dipole subtraction terms and their integrals over the one-parton emission phase space for any given process. If the virtual matrix element is provided as well, this then directly leads to an NLO QCD parton level event generator.     

Loop calculations in confined QCD.I

We present a method for calculating one loop corrections in QCD confined to a spherical cavity with bag boundary conditions. We use euclidean Greens functions separated into free space and boundary parts and use oldfashioned perturbation theory. As a first application we calculate that part of the quark self energy that gives rese to the confining mirror charge potential in the nonrelativistic approximation. We obtain a weaker potential for relativistic massive quarks. Some consequences are discussed.     

Determination of freeze-out conditions from lattice QCD calculations

Freeze-out conditions in Heavy Ion Collisions are generally determined by comparing experimental results for ratios of particle yields with theoretical predictions based on applications of the Hadron Resonance Gas model. We discuss here how this model dependent determination of freeze-out parameters may eventually be replaced by theoretical predictions based on equilibrium QCD thermodynamics.     

On fixing the scale in QCD from lattice calculations

Recent results of lattice Monte-Carlo calculations in QCD are used to estimate the value of . Special attention is paid to the role played by the light quarks in the construction of the continuum limit in QCD. The resulting value of turns out to be strongly dependent onn _f , the number of light quarks taken into account.Dedicated to the 30th anniversary of the Joint Institute for Nuclear Research.On leave of absence from theInstitute of Physics, Czechosl. Accd. Sci., Na Slovance 2, 180 40 Praha 8, Czechoslovakia.     

Lattice QCD calculations for the physical equation of state

In this report we consider the numerical simulations at finite temperature using lattice QCD data for the computation of the thermodynamical quantities including the pressure, energy density and the entropy density. These physical quantities can be related to the equation of state for quarks and gluons. We shall apply the lattice data to the evaluation of the specific structure of the gluon and quark condensates at finite temperature in relation to the deconfinement and chiral phase transitions. Finally we mention the quantum nature of the phases at lower temperatures.     

Variational calculations in the gluon sector of the lattice QCD

The partition function ofd dimensional systems is estimated with the help of a variational Ansatz in terms of quasi-(d-1)-dimensional systems. The internal energy and the string tension derived in this framework agree reasonably with the Monte Carlo data.     

Reweighting in nonequilibrium simulations

A simple reweighting scheme is proposed for Monte Carlo simulations of interacting particle systems, permitting one to study various parameter values in a single study, and improving efficiency by an order of magnitude. Unlike earlier reweighting schemes, the present approach does not require knowledge of the stationary probability distribution, and so is applicable out of equilibrium. The method is applied to the contact process in two and three dimensions, yielding the critical parameter and spreading exponents to unprecedented precision.     

Multiplicity fluctuations in one- and two-dimensional angular intervals compared with analytic QCD calculations

Multiplicity fluctuations in rings around the jet axis and in off-axis cones have been measured by the DELPHI collaboration in e⁺e⁻ annihilations into hadrons at LEP energies. The measurements are compared with analytical perturbative QCD calculations for the corresponding multiparton system, using the concept of Local Parton Hadron Duality. Some qualitative features are confirmed by the data but substantial quantitative deviations are observed.     

Nonperturbative calculations in the framework of variational perturbation theory in QCD

Physics of Atomic Nuclei - We discuss applications of the method based on the variational perturbation theory to perform calculations down to the lowest energy scale. The variational series is...     

Effective Hamiltonains and degenerate perturbation theory calculations of the mass spectrum in lattice QCD

Hamiltonian quantum chromodynamics has been formulated on a discrete lattice, in an attempt to calculate the hadronspectrum using strong coupling expansions. Previous attempts have involved including an irrelevant four-fermion interaction term in the Hamiltonian. As well as making the quark content of the zeroth order states the same as that of the simple additive quark model, this term splits the infinite zeroth order degeneracies of the original Hamiltonian. In this paper we calculate the masses of low lying states in the absence of such a term, using an effective Hamiltonian method to solve the degeneracy problem. We discuss the problems of continuing our series to the continuum limit in the presence of level repulsion.     

Testing the symmetric Cabibbo-Kobayashi-Maskawa (CKM) matrix with lattice QCD calculations

We examine quantitatively the expectation of a symmetric quark mixing using recent lattice QCD calculations on the various hadronic matrix elements. We present a general parametrization of the symmetric CKM matrix which has directly measurable parameters. The direct measurements of the CKM matrix elements, unitarity and CP violation are used to constrain the parameters. The symmetric CKM matrix is also confronted with ε in the kaon system,x _d of \(B_d^0 - \bar B_d^0 \) mixing, and ε′/ε of ΔS=1 CP-violation in the kaon system. Our analyses reveal that the symmetric CKM matrix is consistent with the present experiments, providedm _t>130 GeV. The factorf _Bd ² B _Bd plays an important role in reaching the conclusions.     

Hadron scattering in two-dimensional QCD: (I). Formalism and leading order calculations

Two-dimensional quantum chromodynamics in the 1/N_c expansion is explored as a model for hadronic interactions. In meson-meson scattering, order 1/N_c terms exhibit embryonic “Regge” behaviour, with the crossed channel factorization, signature and exchange degeneracy properties found in dual Regge models. “Regge” intercepts are additive in the quark quantum numbers exchanged, and have a lower limit for large quark masses. Residues also factorize in the direct channel. An analogue for fixed angle scattering is presented which has deviations from dimensional counting rules related to form factors and Regge intercepts.