1,035 research outputs found

### 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

### 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

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