902 research outputs found
Separation of soft and collinear infrared limits of QCD squared matrix elements
We present a simple way of separating the overlap between the soft and
collinear factorization formulae of QCD squared matrix elements. We check its
validity explicitly for single and double unresolved emissions of tree-level
processes. The new method makes possible the definition of helicity-dependent
subtraction terms for regularizing the real contributions in computing
radiative corrections to QCD jet cross sections. This implies application of
Monte Carlo helicity summation in computing higher order corrections.Comment: 14 pages, 1 figure, references adde
Structure of parton showers including quantum interference
It is useful to describe a leading order parton shower as the solution of a
linear equation that specifies how the state of the partons evolves. This
description involves an essential approximation of a strong ordering of
virtualities as the shower progresses from a hard interaction to softer
interactions. If this is to be the only approximation, then the partons should
carry color and spin and quantum interference graphs should be included. We
explain how the evolution equation for this kind of a shower can be formulated.
We discuss briefly our efforts to implement this evolution equation
numerically.Comment: Talk at 2008 Rencontre de Moriond, QCD session. Four page
Effects of subleading color in a parton shower
Parton shower Monte Carlo event generators in which the shower evolves from
hard splittings to soft splittings generally use the leading color (LC)
approximation, which is the leading term in an expansion in powers of
1/N_\Lc^2, where N_\Lc = 3 is the number of colors. In the parton shower
event generator \textsc{Deductor}, we have introduced a more general
approximation, the LC+ approximation, that includes some of the color
suppressed contributions. In this paper, we explore the differences in results
between the LC approximation and the LC+ approximation. Numerical comparisons
suggest that, for simple observables, the LC approximation is quite accurate.
We also find evidence that for gap-between-jets cross sections neither the LC
approximation nor the LC+ approximation is adequate.Comment: 23 pages, 13 figures, published versio
QCD and Monte Carlo event generators
Shower Monte Carlo event generators have played an important role in particle
physics. Modern experiments would hardly be possible without them. In this talk
I discuss how QCD physics is incorporated into the mathematical structure of
these programs and I outline recent developments including matching between
events with different numbers of hard jets and the inclusion of next-to-leading
order effects.Comment: Plenary talk by D. Soper at XIV Workshop on Deep Inelastic Scattering
(DIS2006
Summing threshold logs in a parton shower
When parton distributions are falling steeply as the momentum fractions of
the partons increases, there are effects that occur at each order in
that combine to affect hard scattering cross sections and need to be summed. We
show how to accomplish this in a leading approximation in the context of a
parton shower Monte Carlo event generator.Comment: 83 pages, 8 figure
Jets and threshold summation in Deductor
We explore jet physics in hadron collisions using the parton shower event
generator Deductor. Of particular interest is the one jet inclusive cross
section dsigma/dpT for jets of very high pT. Compared to the Born level, the
cross section decreases substantially because of pT loss from the jet during
showering. We compare to the same effect in Pythia and Dire. The cross section
then increases substantially because of the summation of threshold logarithms
included in Deductor.
We also study the cross section to have a gap with no jets between two hard
jets that are widely separated in rapidity. Here we compare Deductor with
virtuality based ordering with Deductor with kT ordering and we check whether
adding an underlying event and hadronization has a significant effect beyond
that found with just a parton shower.Comment: 47 pages, 11 figures, published versio
Parton distribution functions in the context of parton showers
When the initial state evolution of a parton shower is organized according to
the standard "backward evolution'' prescription, ratios of parton distribution
functions appear in the splitting probabilities. The shower thus organized
evolves from a hard scale to a soft cutoff scale. At the end of the shower, one
expects that only the parton distributions at the soft scale should affect the
results. The other effects of the parton distributions should have cancelled.
This means that the kernels for parton evolution should be related to the
shower splitting functions. If the initial state partons can have non-zero
masses, this requires that the evolution kernels cannot be the usual MSbar
kernels. We work out what the parton evolution kernels should be to match the
shower evolution contained in the parton shower event generator Deductor, in
which the b and c quarks have non-zero masses.Comment: 33 pages, 6 figure
On the transverse momentum in Z-boson production in a virtuality ordered parton shower
Cross sections for physical processes that involve very different momentum
scales in the same process will involve large logarithms of the ratio of the
momentum scales when calculated in perturbation theory. One goal of
calculations using parton showers is to sum these large logarithms. We ask
whether this goal is achieved for the transverse momentum distribution of a
Z-boson produced in hadron-hadron collisions when the shower is organized with
higher virtuality parton splittings coming first, followed successively by
lower virtuality parton splittings. We find that the virtuality ordered shower
works well in reproducing the known QCD result.Comment: 60 pages with three figure
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