602 research outputs found
BBBREM - Monte Carlo simulation of radiative Bhabha scattering in the very forward direction
A fast and simple Monte Carlo program is presented that simulates single
Bremsstrahlung in Bhabha scattering, e+e- --> e+e-gamma, without constraints on
scattering angles. This allows the study of this process at arbitrarily small,
or even vanishing, scattering angles. Experimental cuts can be imposed on an
event-by-event basis, allowing for detailed studies of the process as a
limitation to beam lifetimes, or a luminosity-measuring device, in e+e- storage
rings. As an application, we show that the easy introduction of a cutoff
parameter, corresponding to the characteristic distance between particles in
the bunches, gives a reduced cross section that is in good agreement with
observation.Comment: 14 pages, NIKHEF-H/94-01 and CERN SL/94-03 (OP
Counting tree diagrams: asymptotic results for QCD-like theories
We discuss the enumeration of Feynman diagrams at tree order for processes
with external lines of different types. We show how this can be done by
iterating algebraic Schwinger-Dyson equations. Asymptotic estimates for very
many external lines are derived. Applications include QED, QCD and scalar QED,
and the asymptotic estimates are numerically confronted with the exact results.Comment: 16 page
The Electroweak Standard Model in the Axial Gauge
We derive the Feynman rules of the standard model in the axial gauge. After
this we prove that the fields and do not correspond to
physical particles. As a consequence, these fields cannot appear as incoming or
outgoing lines in Feynman graphs. We then calculate the contribution of these
fields in the case of a particular decay mode of the top quark.Comment: 16 pages, no figures. Added derivation of polarization su
Multiscalar production amplitudes beyond threshold
We present exact tree-order amplitudes for , for final states
containing one or two particles with non-zero three-momentum, for various
interaction potentials. We show that there are potentials leading to tree
amplitudes that satisfy unitarity, not only at threshold but also in the above
kinematical configurations and probably beyond. As a by-product, we also
calculate tree amplitudes at threshold and show that for the unbroken
theory they vanish for , for the Standard Model Higgs they
vanish for and for a model potential, respecting tree-order
unitarity, for even and . Finally, we calculate the imaginary part of
the one-loop amplitude in both symmetric and spontaneously broken
theory.Comment: 11 pages, Latex, 3 figures (available on request
On Measuring the top quark mass using the dilepton decay modes
We demonstrate a new likelihood method for extracting the top quark mass from
events of the type ttbar-->bW(l+nu)bW(l+nu) This method estimates the top quark
mass correctly from an ensemble of dilepton events. The method proposed by
Dalitz and Goldstein [1] is shown to result in a systematic underestimation of
the top quark mass. Effects due to the spin correlations between the top and
anti-top quarks are shown to be unimportant in estimating the mass of the top
quark.Comment: To appear in the proceedings of the 1996 DPF/DPB study on new
directions for High Energy Physics, Snowmass, Colorad
On amplitude zeros at threshold
The occurrence of zeros of 2 to n amplitudes at threshold in scalar theories
is studied. We find a differential equation for the scalar potential, which
incorporates all known cases where the 2 to n amplitudes at threshold vanish
for all sufficiently large , in all space-time dimensions, . This
equation is related to the reflectionless potentials of Quantum Mechanics and
to integrable theories in 1+1 dimensions. As an application, we find that the
sine-Gordon potential and its hyperbolic version, the sinh-Gordon potential,
also have amplitude zeros at threshold, , for and
, independently of the mass and the coupling constant.Comment: 6 pages, Latex, CERN-TH.6762/9
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