On the Field Renormalization Constant for Unstable Particles

A recently proposed normalization condition for the imaginary part of the self-energy of an unstable particle is shown to lead to a closed expression for the field renormalization constant Z. In turn, the exact expression for Z is necessary, in some important cases, in order to avoid power-like infrared divergences in high orders of perturbation theory. In the same examples, the width plays the r\^ole of an infrared cutoff and, consequently, Z is not an analytic function of the coupling constant.Comment: 7 pages (Latex

Considerations Concerning the QCD Corrections to Δρ\Delta\rho

Using recent results of Avdeev et al. and an expansion for \mu_t/\mms ($M_t$ is the pole mass and $\mu_t\equiv \hat{m_t}(\mu_t)$ ), it is shown that when deltarho is expressed in terms of $\hat{m_t}^2(M_t)$, the QCD correction is only $(2-3)\times 10^{-3}$ in the NLO approximation. As a consequence, in terms of $M_t^2$ the correction to \dr is almost entirely contained in \mmss/M_t^2, a pure QCD effect. The latter is studied using various optimization procedures, and the results compared with the expansion proposed by Avdeev et al.. Implications for \ew physics are discussed. Threshold effects are analyzed on the basis of a simple sum rule.Comment: 11 pages plain LaTeX including 2 table

Dependence of Electroweak Parameters on the Definition of the Top-Quark Mass

The QCD corrections to electroweak parameters depend on the renormalization scheme and scales used to define the top-quark mass. We analyze these dependences for the W-boson mass predicted via Delta r to O(alpha alpha_s) and O(alpha alpha_s^2) in the on-shell and MS-bar schemes. These variations provide us with a hint on the magnitude of the unknown higher-order QCD effects and contribute to the theoretical error of the prediction.Comment: 27 pages (Latex), 6 figures (uuencoded compressed tarred Postscript). Extended and updated version, which incorporates the recently calculated next-to-leading-order QCD corrections to Delta rho and Delta

Inclusive electroproduction of light hadrons with large p_T at next-to-leading order

We review recent results on the inclusive electroproduction of light hadrons at next-to-leading order in the parton model of quantum chromodynamics implemented with fragmentation functions and present updated predictions for HERA experiments based on the new AKK set.Comment: 4 pages, 5 figures, to appear in the proceedings of the XIII International Workshop on Deep Inelastic Scattering (DIS 2005

D-meson production in the GM-VFN scheme

We study the inclusive hadrodroduction of D^0, D^+, D^{*+}, and D_s^+ mesons at next-to-leading order in the parton model of quantum chromodynamics endowed with universal non-perturbative fragmentation functions (FFs) fitted to e^+e^- annihilation data from CERN LEP1. Working in the general-mass variable-flavor-number scheme, we resum the large logarithms through the evolution of the FFs and, at the same time, retain the full dependence on the charm-quark mass without additional theoretical assumptions. In this way, the cross section distributions in transverse momentum recently measured by the CDF Collaboration in run II at the Fermilab Tevatron are described within errors.Comment: 4 pages, 4 figures, to appear in the Proceedings of the XIV International Workshop on Deep Inelastic Scattering (DIS 2006), 20-24 April 2006, Tsukuba City, Japa