118692296
Applying Optimized Perturbation Theory to QCD at Low Energies
We discuss the use of the optimization procedure based on the Principle of Minimal Sensitivity to the third-order calculation of {\mbox{${R_{e^+e^-}}$}}. The effective coupling constant remains finite allowing us to apply the Poggio-Quinn-Weinberg smearing method down to energies below 1 GeV, where we find good agreement between theory and experiment. The couplant freezes to a value of $\alpha_s/\pi = 0.26$ at zero energy which is in remarkable concordance with values obtained phenomenologically.
120633210
Comments on the Sterman-Weinberg jet formula
Abstract A simple derivation of the Sterman-Weinberg jet formula (the QCD perturbation theory formula for the 2-jet cross section) is described. Conditions on the jet parameters ϵ and δ for this formula to be applicable are discussed and the corrections to the formula up to O(ϵ 2 ), O(δ 2 ) are presented.
117188910
The renormalization‐scheme‐dependence problem and its solution
I explain, in ‘‘uncompromising’’ terms, the solution to the renormalization‐scheme‐dependence problem.
121312073
Gluon enhancements in charmed-meson decays
We address tests of nonspectator diagrams in charmed-meson decays. In particular, we explore the consequences of a model in which such diagrams are enhanced by the emission of a single gluon. For a lifetime ratio tau(D/sup +/)/tau(D/sup 0/)=5, we predict (i) tau(F/sup +/)/tau(D/sup +/) approx. = 0.6, (ii) an enhanced ..delta..S=0 rate for D/sup +/, (iii) a substantial multipion branching fraction for the F/sup +/, (iv) an F/sup +/ ..-->.. tau/sup +/..nu.. branching fraction of order 10 --20%, and (v) restrictive bounds on the D/sup 0/ ..-->.. K-bar/sup 0/..pi../sup 0/ mode.
119880793
Optimized perturbation theory in the Gross-Neveu model
17579087
The Banks-Zaks Expansion and "Freezing" in Perturbative QCD
The recent calculation of the four-loop beta function in QCD provides further evidence that the Banks-Zaks expansion in 16half-n_f is sufficiently well behaved to be useful even for n_f=2 light flavours. This expansion inherently predicts "freezing" of the QCD couplant at low energies as a perturbative effect. We consider the e+e- and Bjorken-sum-rule cases as examples.
123614631
Pushing nuclei to the extreme
The ancient Greek philosopher Epicurus reportedly said that "atoms come together in different order and position, like the letters which, though they are few, produce innumerable words". Much the same could be said of the way that neutrons and protons form atomic nuclei. The nucleus is at the heart of every atom and makes up essentially all of its mass. Yet there is still a surprising amount we do not know about the behaviour of nuclei.
17333676
Isoscalar and Isovector Giant Monopole Resonances from a Continuum Hartree-Fock Method
We motivate and summarise some recent results in the application of formally exact boundary conditions in nuclear time-dependent Hartree-Fock calculations, mak- ing use of Laplace transformations to calculate the values of the wave functions at the boundaries. We have realised the method in the case of giant monopole resonances of spherically-symmetric nuclei, and present strength functions of 16 O and 40 Ca using a sim- plified version of the Skyrme force, showing that no artefacts from discretisation occur as contaminatnts
17501117
Cause of the charge radius isotope shift at the $N$=126 shell gap
We discuss the mechanism causing the ‘kink’ in the charge radius isotope shift at the N = 126 shell closure. The occupation of the 1 i 11/2 neutron orbital is the decisive factor for reproducing the experimentally observed kink. We investigate whether this orbital is occupied or not by different Skyrme effective interactions as neutrons are added above the shell closure. Our results demonstrate that several factors can cause an appreciable occupation of the 1 i 11/2 neutron orbital, including the magnitude of the spin-orbit field, and the isoscalar effective mass of the Skyrme interaction. The symmetry energy of the effective interaction has little influence upon its ability to reproduce the kink.
120382191
Densities of superheavy nuclei at closed shells
We employed the Skynne-Hartree-Fock + BCS model to investigate the density distributions of even-even superheavy nuclei around the predicted rnagic numbers of Z = 114, 120, 126 and N = 172, 184. The central depressions in densities are shown in the heaviest nuclei with Z <= 120 and N <= 178, which is due to high-j orbits occupied. However, densities become flatter when deformations occur. The superheavy nuclei with N = 180-188 and Z = 126 have no central depressions, due to that low-j orbits become occupied. Shell closures in the superheavy mass region are isospin dependent.
16386160
Extension of the continuum time-dependent Hartree-Fock method to proton states.
This paper deals with the solution of the spherically symmetric time-dependent Hartree-Fock approximation applied to nuclear giant monopole resonances in the small amplitude regime. The problem is spatially unbounded as the resonance state is in the continuum. The practical requirement to perform the calculation in a finite-sized spatial region yields an artificial boundary, which is not present physically. The question of how to ensure the boundary does not interfere with the internal solution, while keeping the overall calculation time low is studied. Here we propose an absorbing boundary condition scheme to handle the conflict. The derivation, via a Laplace transform method, and implementation is described. An inverse Laplace transform required by the absorbing boundaries is calculated using a method of nonlinear least squares. The accuracy and efficiency of the scheme is tested and results presented to support the case that they are an effective way of handling the artificial boundary.