C. Roiesnel

Non-perturbative power corrections to ghost and gluon propagators

We study the dominant non-perturbative power corrections to the ghost and gluon propagators in Landau gauge pure Yang-Mills theory using OPE and lattice simulations. The leading order Wilson coefficients are proven to be the same for both propagators. The ratio of the ghost and gluon propagators is thus free from this dominant power correction. Indeed, a purely perturbative fit of this ratio gives smaller value ( 270MeV) of Λ than the one obtained from the propagators separately( 320MeV). This argues in favour of significant non-perturbative ~ 1/q2 power corrections in the ghost and gluon propagators. We check the self-consistency of the method.

On AdS/QCD correspondence and the partonic picture of deep inelastic scattering

Abstract We critically examine the question of scaling of the Deep Inelastic Scattering process in the medium Bjorken x region on a scalar boson in the framework of the AdS/QCD correspondence. To get the right polarization structure of the forward electroproduction amplitude, we show that one needs to add (at least) the scalar to scalar and scalar to vector hadronic amplitudes. This illustrates how the partonic picture may emerge from a simple scenario based on the AdS/QCD correspondence, provided one allows the conformal dimension of the hadronic field to equal 1 and use the concept of “hadron–parton duality”.

On the reduction of hypercubic lattice artifacts

This note presents a comparative study of various options to reduce the errors coming from the discretization of a Quantum Field Theory in a lattice with hypercubic symmetry. We show that it is possible to perform an extrapolation towards the continuum which is able to eliminate systematically the artifacts which break the O(4) symmetry.

Resolution of the η → 3π problem

Abstract It is shown that the discrepancy between experiment and the soft pion current algebra for η → 3 π is mainly due to the square root threshold singularity induced by the strong pion-pion final-state interaction. It is found that Γ ( η → π + π − π 0 ) = 200 eV, in excellent agreement with the experimental result 204 ± 29 eV.

Preliminary Calculation of αS from Green Functions with Dynamical Quarks

We present preliminary results on the computation of the QCD running coupling constant in the scheme and Landau gauge with two flavours of dynamical Wilson quarks. Gluon momenta range up to about 7 GeV (β = 5.6, 5.8 and 6.0) with a constant dynamical-quark mass. This range already allows to exhibit some evidence for a sizable 1/μ2 correction to the asymptotic behaviour, as in the quenched approximation, although a fit without power corrections is still possible with a reasonable χ2. Following the conclusions of our quenched study, we take into account 1/μ2 correction to the asymptotic behaviour. We find = 264(27) MeV × [a−1(5.6,0.1560)/2.19 GeV], which leads to αs(MZ) = 0.113(3)(4). The latter result has to be taken as a preliminary indication rather than a real prediction in view of the systematic errors still to be controlled. Still, being two sigmas below the experimental result makes it very encouraging.

Decay rate of τ into odd number of pions

Abstract Decay rate of τ into 3π and 5π is calculated using current algebra and PCAC. Assuming universality for ordinary leptons and τ, satisfactory agreement with experimental data is obtained with Г tot ( A 1 ) ≈ 300−400 MeV . The branching ratio for π → 5π + ν is found to be 1%.

Optimizing a lattice QCD simulation program

Three computer-time saving techniques for lattice QCD programs are presented. They concern the link updating order, the compacting of the data and the pseudo-heatbath algorithm.

Symmetry-breaking effects in grand unified theories

Abstract We study the threshold effects in a spontaneously broken gauge theory. In particular we present a computation of the symmetry-breaking effects in the SU(5) grand unified theory including those of the breaking of SU(2) × U(1). As an application we give a precise value of the superheavy gauge boson mass M x . We also show that it is possible in SU(5) to define a natural effective weak angle θ w ( μ ) for any scale μ, below as well as above M w , and we give the predicted curve for sin 2 θ w ( μ ).

SU(8) grand unification

Abstract A grand unified gauge theory based on the group SU(8) is presented. The set of rules motivating the choice of SU(8) is given. The particle content of the theory suggests a natural embedding of a horizontal SU(3) symmetry in the SU(8) grand unified symmetry.

Constraints on the IR behaviour of gluon and ghost propagator from Ward-Slavnov-Taylor identities

Abstract.We consider the constraints of the Slavnov-Taylor identity of the IR behaviour of gluon and ghost propagators and their compatibility with solutions of the ghost Dyson-Schwinger equation and with the lattice picture.