1,143 research outputs found
Final-state QED Multipole Radiation in Antenna Parton Showers
We present a formalism for a fully coherent QED parton shower. The complete
multipole structure of photonic radiation is incorporated in a single branching
kernel. The regular on-shell 2 to 3 kinematic picture is kept intact by
dividing the radiative phase space into sectors, allowing for a definition of
the ordering variable that is similar to QCD antenna showers. A modified
version of the Sudakov veto algorithm is discussed that increases performance
at the cost of the introduction of weighted events. Due to the absence of a
soft singularity, the formalism for photon splitting is very similar to the QCD
analogon of gluon splitting. However, since no color structure is available to
guide the selection of a spectator, a weighted selection procedure from all
available spectators is introduced.Comment: 33 pages, 12 figures. Added subsection 4.3 and some comments and
references per reviewer request. Version accepted by JHE
Competing Sudakov Veto Algorithms
We present a way to analyze the distribution produced by a Monte Carlo
algorithm. We perform these analyses on several versions of the Sudakov veto
algorithm, adding a cutoff, a second variable and competition between emission
channels. The analysis allows us to prove that multiple, seemingly different
competition algorithms, including those that are currently implemented in most
parton showers, lead to the same result. Finally, we test their performance and
show that there are significantly faster alternatives to the commonly used
algorithms.Comment: 16 pages, 1 figur
Discrepancy-based error estimates for Quasi-Monte Carlo. III: Error distributions and central limits
In Quasi-Monte Carlo integration, the integration error is believed to be
generally smaller than in classical Monte Carlo with the same number of
integration points. Using an appropriate definition of an ensemble of
quasi-randompoint sets, we derive various results on the probability
distribution of the integration error, which can be compared to the standard
Central Limit theorem for normal stochastic sampling. In many cases, a Gaussian
error distribution is obtained.Comment: 15 page
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
Singular Cross Sections in Muon Colliders
We address the problem that the cross section for the collisions of unstable
particles diverges, if calculated by standard methods. This problem is
considered for beams much smaller than the decay length of the unstable
particle, much larger than the decay length and finally also for pancake-
shaped beams. We find that in all cases this problem can be solved by taking
into account the production/propagation of the unstable particle and/or the
width of the incoming wave packets in momentum space.Comment: 12 pages, 3 figures. References corrected. Removed one sentence about
a fact that was known. Added explaination why one of our graphs is different
as compared to one of the references. Clearified explaination in sec. 3.
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
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