Generalized Gluon Currents and Applications in QCD

We consider the process containing two quark lines and an arbitrary number of gluons in a spinor helicity framework. A current with two off-shell gluons appears in the amplitude. We first study this modified gluon current using recursion relations. The recursion relation for the modified gluon current is solved for the case of like-helicity gluons. We apply the modified gluon current to compute the amplitude for $q \bar q \rightarrow q \bar q gg \cdots g$ in the like-helicity gluon case.Comment: 80 pages, 2 figures (appended in pictex), CLNS 91/112

Small-threshold behaviour of two-loop self-energy diagrams: two-particle thresholds

The behaviour of two-loop two-point diagrams at non-zero thresholds corresponding to two-particle cuts is analyzed. The masses involved in a cut and the external momentum are assumed to be small as compared to some of the other masses of the diagram. By employing general formulae of asymptotic expansions of Feynman diagrams in momenta and masses, we construct an algorithm to derive analytic approximations to the diagrams. In such a way, we calculate several first coefficients of the expansion. Since no conditions on relative values of the small masses and the external momentum are imposed, the threshold irregularities are described analytically. Numerical examples, using diagrams occurring in the Standard Model, illustrate the convergence of the expansion below the first large threshold.Comment: 28 pages (including 23 pages of text in latex and 5 pages with 6 figures in a separate postcript file

Threshold and pseudothreshold values of the sunset diagram

Analytic results for the threshold and pseudothreshold values of the sunset diagram with arbitrary masses are obtained in terms of dilogarithms of ratios of the masses.Comment: 11 pages, plain LaTe

Weyl-van-der-Waerden formalism for helicity amplitudes of massive particles

The Weyl-van-der-Waerden spinor technique for calculating helicity amplitudes of massive and massless particles is presented in a form that is particularly well suited to a direct implementation in computer algebra. Moreover, we explain how to exploit discrete symmetries and how to avoid unphysical poles in amplitudes in practice. The efficiency of the formalism is demonstrated by giving explicit compact results for the helicity amplitudes of the processes gamma gamma -> f fbar, f fbar -> gamma gamma gamma, mu^- mu^+ -> f fbar gamma.Comment: 24 pages, late

WEXTER and ERAFITTER: two programs to fit M_W at LEP2 using the best measurable kinematical variables

In this paper, we present two programs to fit M_W at LEP2 using the best measurable kinematical variables. The theoretical probabilities of observing the final-state kinematical configurations are computed by integrating over the quantities that are not well measured. Therefore, an event-by-event kinematical reconstruction is avoided. M_W is then determined through a maximum likelihood fit.Comment: 19 pages, Latex fil

Two-photon physics with GALUGA 2.0

An extended version of the Monte Carlo program GALUGA is presented for the computation of two-photon production in \e^+\e^- collisions. Functions implemented for the five $\gamma^\star\gamma^\star$ structure functions now include several ans\"atze of the total hadronic cross section based on the BFKL--Pomeron and various Regge-like models. In addition, structure functions for resonance formation are included with full dependence on the two photon virtualities $Q_1^2$ and $Q_2^2$ as given in the constituent-quark model. The six lowest-lying resonances of each of the C-even mesons with $J^{P}= 0^-$, $0^+$, $1^+$, $2^+$ and $2^-$ are provided. The program can also be used to calculate with exact kinematics the effective two-photon luminosity function. Special emphasis is put on a numerically stable evaluation of all variables over the full $Q_i^2$ range while keeping all dependences on the electron mass and $Q_i^2$.Comment: Latex, 32 pages, 7 figures, uses 12pt.sty, epsfig.sty, amsbsy.st