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G. I. Poulis

Publications and source records attributed to G. I. Poulis.

12 recordsLinked to original sources
arXiv1999

Chiral symmetry and the Delta-Nucleon transition form factors

The three complex form factors entering the $Δ\to Nγ^\ast$ vertex are calculated to ${\cal O}(ε^3)$ in the framework of a chiral effective theory with explicit Δ(1232) degrees of freedom included. It is shown that the low q^2 behavior of the form factors is governed by $πN$, $πΔ$ loop effects. Predictions are given for the q^2-dependence of the three transition-multipoles $M1(q^2), E2(q^2), C2(q^2)$. Furthermore, the role of presently unknown low energy constants that affect the values of the multipole ratios EMR(q^2) and CMR(q^2) is elucidated.

hep-ph↗
arXiv1998

N Delta - Transition Form Factors at Low Momentum Transfer

The three complex form factors entering the $Δ\to Nγ^\ast$ vertex are calculated to ${\cal O}(ε^3)$ in the framework of a chiral effective theory with explicit $Δ$(1232) degrees of freedom. Furthermore, the role of presently unknown low energy constants that affect the values of EMR and CMR is elucidated.

hep-ph↗
arXiv1997

Deconfinement at finite chemical potential

In a confining, renormalisable, Dyson-Schwinger equation model of two-flavour QCD we explore the chemical-potential dependence of the dressed-quark propagator, which provides a means of determining the behaviour of the chiral and deconfinement order parameters, and low-energy pion observables. We find coincident, first order deconfinement and chiral symmetry restoration transitions at μ_c = 375 MeV. f_πis insensitive to μuntil μ\approx μ_0 = 0.7 mu_c when it begins to increase rapidly. m_πis weakly dependent on μ, decreasing slowly with μand reaching a minimum 6% less than its μ=0 value at μ=μ_0. In a two-flavour free-quark gas at μ=μ_c the baryon number density would be approximately 3 ρ_0, where ρ_0=0.16 fm^{-3}; while in such a gas at μ_0 the density is ρ_0.

nucl-th↗
arXiv1997

Scaling and confinement aspects of tadpole improved SU(2) lattice gauge theory and its abelian projection

Using a tadpole improved SU(2) gluodynamics action, the nonabelian potential and the abelian potential after the abelian projection are computed. Rotational invariance is found restored at coarse lattices both in the nonabelian theory and in the effective abelian theory resulting from maximal abelian projection. Asymptotic scaling is tested for the SU(2) string tension. Deviation of the order of $6%$ is found, for lattice spacings between 0.27 and 0.06 fm. Evidence for asymptotic scaling and scaling of the monopole density in maximal abelian projection is also seen, but not at coarse lattices. The scaling behavior is compared with analyses of Wilson action results, using bare and renormalized coupling schemes. Using extended monopoles, evidence is found that the gauge dependence of the abelian projection reflects short distance fluctuations, and may thus disappear at large scales.

hep-lat↗
arXiv1997

Parity-violating quasifree electron-deuteron scattering in a covariant approach

The role of relativistic corrections associated with lower Dirac components of the deuteron wavefunction, is examined for parity-violating (PV) electron scattering. The relation between these corrections and negative energy components of the struck nucleon in relativistic PWIA is elucidated. The model dependence induced by describing such effects, using different deuteron vertex functions and prescriptions for the half-off-shell nucleon vertices, is compared against the precision required for studies of nucleon strangeness in quasielastic PV $e-d$ scattering.

nucl-th↗
arXiv1996

Sidewise dispersion relations and the structure of the nucleon vertex

We revisit sidewise dispersion relations as a method to relate the nucleon off-shell form factor to observable quantities, namely the meson-nucleon scattering phase shifts. It is shown how for meson-nucleon scattering a redefinition of the intermediate fields leaves the scattering amplitude invariant, but changes the behavior of the off-shell form factor as expressed through dispersion relations, thus showing representation dependence. We also employ a coupled-channel unitary model to test the validity of approximations concerning the influence of inelastic channels in the sidewise dispersion relation method.

nucl-th↗
arXiv1994

Gauge Fixing and Extended Abelian Monopoles in SU(2) Gauge Theory in 2+1 Dimensions

Extended Abelian monopoles are investigated in SU(2) lattice gauge theory in three dimensions. Monopoles are computed by Abelian projection in several gauges, including the maximal Abelian gauge. The number $N_m$ of extended monopoles in a cube of size $m^3$ (in lattice units) is defined as the number of elementary ($1^3$) monopoles minus antimonopoles in the cube ($m=1,2,\ldots$). The distribution of $1^3$ monopoles in the nonlocal maximal Abelian gauge is shown to be essentially random, while nonscaling of the density of $1^3$ monopoles in some local gauges, which has been previously observed, is shown to be mainly due to strong short-distance correlations. The density of extended monopoles in local gauges is studied as a function of $β$ for monopoles of fixed physical ``size'' ($m / β= {\rm fixed}$); the degree of scale violation is found to decrease substantially as the monopole size is increased. The possibility therefore remains that long distance properties of monopoles in local gauges may be relevant to continuum physics, such as confinement.

hep-lat↗
arXiv1993

Interference Effects in Relativistic Deuteron Electrodisintegration

We extend the relativistic plane--wave impulse approximation formalism to incorporate a specific class of relativistic interference effects for use in describing inclusive electrodisintegration of $^2$H. The role of these ``exchange'' terms for the various response functions accessible in parity--conserving and --violating inclusive processes is investigated and shown, especially for the latter, to have important consequences for experiment. An extension to a simple quasi--deuteron model is also considered.

nucl-th↗
arXiv1993

Polarized Deformed Nuclei Studied via Coincidence Polarized Electron Scattering: The case of 21 Ne

Coincidence reactions of the type $\svec{A}(\svec{e},e'N)B$ involving the scattering of polarized electrons from deformed polarized targets are discussed within the context of the plane--wave impulse approximation. A general expression for the polarized spectral function for transitions leaving the residual nucleus in discrete states is presented. General properties and angular symmetries exhibited by the polarization observables are discussed in detail. Results for unpolarized cross sections as well as for polarization ratios (asymmetries) are obtained for typical quasi--free kinematics. The dependences of the polarization observables on the bound neutron momentum, target polarization orientation, nuclear deformation and value of the momentum transfer $q$ are discussed in detail for various different kinematical situations.

hep-ph↗
arXiv1993

Effective Hamiltonians with Relativistic Corrections I: The Foldy--Wouthuysen transformation versus the direct Pauli reduction

Two different methods of obtaining ``effective $2\times 2$ Hamiltonians'' which include relativistic corrections to nonrelativistic calculations are discussed, the standard Foldy--Wouthuysen transformation and what we call the ``direct Pauli reduction''. We wish to investigate under which circumstances the two approaches yield the same result. Using a generic interaction with harmonic time dependence we show that differences in the corresponding effective S--matrices do arise beyond first--order perturbation theory. We attribute them to the fact that the use of the direct reduction effective Hamiltonian involves the additional approximation of neglecting contributions from the negative--energy intermediate states, an approximation which is unnecessary in the Foldy--Wouthuysen case as there the $4\times 4$ Hamiltonian does not connect positive-- and negative--energy states. We conclude that at least in the cases where the relativistic Hamiltonian is known, using the direct Pauli reduction effective Hamiltonian introduces spurious relativistic effects and therefore the Foldy--Wouthuysen reduction should be preferred.

nucl-th↗
arXiv1993

Effective Hamiltonians with Relativistic Corrections II: Application to Compton Scattering by a Proton

We discuss two different methods of obtaining ``effective $2 \times 2$ Hamiltonians'' of the electromagnetic interaction which include relativistic corrections. One is the standard Foldy--Wouthuysen transformation which we compare with the Hamiltonian obtained from a direct reduction of the matrix element of the interaction Hamiltonian between positive--energy solutions of the free Dirac equation. The two approaches are applied to Compton scattering by a proton for which a low--energy theorem exists. It is found that the Foldy--Wouthuysen Hamiltonian yields the same result as a covariant calculation. This is not true for the direct reduction method which will in general lead to incorrect results even after restoring the gauge invariance property of the Hamiltonian. Furthermore, it is shown that an identification of the Z--diagrams of the usual Dirac representation with the contact graphs of the Foldy--Wouthuysen representation is incorrect beyond the order of the low--energy theorem.

nucl-th↗