Order Perturbative Corrections Research Articles

Heavy quark correlations ine + e ? annihilations

In heavy quark jets the quark mass acts as a regulator of collinear singularities, making the quark momentum an infra-red safe variable in perturbative QCD. This allows a direct comparison of measured heavy hadron momentum spectra with perturbative calculations. We exploit the factorisation of heavy quark fragmentation to derive QCD predictions for momentum correlations between heavy hadrons produced ine+e− annihilations. We study the practical feasibility and model sensitivity of our approach using Monte Carlo simulations. Higher order perturbative corrections and contributions from non-perturbative effects are found to be at the level of 10%.

New jet cluster algorithms: next-to-leading order QCD and hadronization corrections

New algorithms have been proposed as alternatives to the standard JADE-type algorithms for quantifying jet production in e+e− annihilation. We study in detail the “Durham” algorithm, based on clustering according to relative transverse momentum, and introduce a new “Geneva” variation which is expected to exhibit similar theoretical properties of exponentiation at small jet resolution. In particular, we calculate the next-to-leading order perturbative corrections to the three-jet rate for the new algorithms, and we study non-perturbative corrections using the JETSET parton-shower Monte Carlo. In each case we compare to the results obtained with the usual JADE-type algorithms. We conclude that the new algorithms exhibit somewhat smaller perturbative corrections compared to the standard algorithms, and that the non-perturbative corrections are of comparable size. Thus the new algorithms appear to be at least as well suited to precision measurements of the strong coupling.

Benchmark cross sections forp $$\bar p$$ collisions at 1.8 TeV

High-energyp\(\bar p\) collisions provide many quantitative tests of the standard model. Of particular interest are “hard scattering” processes, which test not only standard model matrix elements and higher order perturbative corrections, but also the distributions of quarks and gluons in the colliding hadrons. We present detailed comparisons of data from the CERNSp\(\bar p\)S collider with theory, incorporating up-to-date parton distributions derived from recent deep inelastic scattering data. Encouraged by the excellent agreement between data and theory at\(\sqrt s \)=630GeV, we present a complete set of “benchmark” predictions for the FNALp\(\bar p\) collider at\(\sqrt s \)=1.8TeV.

A new technique for the evaluation of Feynman diagrams in the high energy, large momentum transfer limit

The problem of finding scattering amplitudes in the high energy, large momentum transfer limit is reconsidered. We propose a novel technique of evaluating Feynman diagrams in this regime with higher order perturbative corrections in mind. The relation to other methods is discussed and the ladder diagrams in φ3 theory, recalculated up to sixth order, serve as examples.

Non-renormalization theorems for non-perturbative effects in susy gauge theories

Supersymmetric gauge theories with Higgs mechanism are considered. After all heavy fields are integrated out we are left with the instanton-induced effective action for light fields. It is demonstrated that the one-loop instanton result is not modified by higher order perturbative corrections. The peculiarity of the case considered is that the background scalar fields do not possess definite chirality, and the bosonic and fermionic modes are not degenerate for this reason.

NEXT-TO-LEADING LOGARITHM CORRECTIONS IN CERTAIN SUPERSYMMETRIC PROCESSES

Higher order perturbative corrections in certain processes of Supersymmetric QCD are determined. The structure and pattern of the correction terms are studied and compared to those of similar processes in conventional QCD.

Fayet-Iliopoulos terms in string theory

For vacua of string theory which leave unbroken N = 1 supersymmetry in the leading approximation, non-renormalization theorems guarantee that higher order (string or sigma model) perturbative corrections do not modify the space-time superpotential. If the low energy gauge group contains U(1) factors, this is not enough to ensure that the low energy physics is perturbatively stable. It is still possible to generate a Fayet-Iliopoulos D-term. We show that under certain conditions such a term is generated, and destabilizes the vacuum. This is a counterexample to various claims about the universal absence of quantum tadpoles when expanding around a classical solution with unbroken supersymmetry. Whether, in a given model, the generation of a D-term will destabilize the vacuum (or merely change the pattern of symmetry breaking) can be determined from properties of the low energy effective action.

Solution of the coupled-channel schrödinger equation using constant, linear and quadratic reference potentials

Systems of coupled second order differential equations arise in many problems in molecular quantum physics. In this paper we investigate approximate potential methods that solve these differential equations. A series method for the evaluation of the propagators is developed for the multichannel Schrodinger equation where the reference potential is expressed as a polynomial. The case of a linear approximate potential is treated in detail and the results are compared with the Magnus and Bessel methods which approximate the potential as a constant in each interval. The importance of fitting the potentials at the zeroes of a Legendre polynomial is shown. Optimum fitting of the potential results in second and fourth order global convergence respectively for constant and linear fits. Richardson extrapolation is used to increase the accuracy of the low order methods. Explicit formulae are presented for the first order perturbative corrections to the Magnus propagators. The importance of adding the leading terms ...

What lattice calculations can tell us about the gluon gas

Higher order perturbative and nonperturbative corrections to the grand potential of hot QCD are considered qualitatively Comparing with lattice results, it is argued that the nonperturbative parts are small but that the O( g 4) term in Ω is large and positive.

Perturbative QCD corrections in ep scattering

The lowest order perturbative QCD corrections in deep-inelastic ep scattering are calculated. We discuss how cancellation of the infrared divergences occurs. The remaining logarithmic mass singularity is absorbed in the input parton distributions. In a phenomonological analysis the gluon-corrected structure functions are compared with the AF-corrected ones. The pattern of scaling violation is interpreted by the Feynman diagrams.