17,142 research outputs found
GenEvA (I): A new framework for event generation
We show how many contemporary issues in event generation can be recast in
terms of partonic calculations with a matching scale. This framework is called
GenEvA, and a key ingredient is a new notion of phase space which avoids the
problem of phase space double-counting by construction and includes a built-in
definition of a matching scale. This matching scale can be used to smoothly
merge any partonic calculation with a parton shower. The best partonic
calculation for a given region of phase space can be determined through physics
considerations alone, independent of the algorithmic details of the merging. As
an explicit example, we construct a positive-weight partonic calculation for
e+e- -> n jets at next-to-leading order (NLO) with leading-logarithmic (LL)
resummation. We improve on the NLO/LL result by adding additional
higher-multiplicity tree-level (LO) calculations to obtain a merged NLO/LO/LL
result. These results are implemented using a new phase space generator
introduced in a companion paper [arXiv:0801.4028].Comment: 60 pages, 22 figures, v2: corrected typos, added reference
Mixtures of Shifted Asymmetric Laplace Distributions
A mixture of shifted asymmetric Laplace distributions is introduced and used
for clustering and classification. A variant of the EM algorithm is developed
for parameter estimation by exploiting the relationship with the general
inverse Gaussian distribution. This approach is mathematically elegant and
relatively computationally straightforward. Our novel mixture modelling
approach is demonstrated on both simulated and real data to illustrate
clustering and classification applications. In these analyses, our mixture of
shifted asymmetric Laplace distributions performs favourably when compared to
the popular Gaussian approach. This work, which marks an important step in the
non-Gaussian model-based clustering and classification direction, concludes
with discussion as well as suggestions for future work
Mixtures of Skew-t Factor Analyzers
In this paper, we introduce a mixture of skew-t factor analyzers as well as a
family of mixture models based thereon. The mixture of skew-t distributions
model that we use arises as a limiting case of the mixture of generalized
hyperbolic distributions. Like their Gaussian and t-distribution analogues, our
mixture of skew-t factor analyzers are very well-suited to the model-based
clustering of high-dimensional data. Imposing constraints on components of the
decomposed covariance parameter results in the development of eight flexible
models. The alternating expectation-conditional maximization algorithm is used
for model parameter estimation and the Bayesian information criterion is used
for model selection. The models are applied to both real and simulated data,
giving superior clustering results compared to a well-established family of
Gaussian mixture models
A Fixed-Point Algorithm for Closed Queueing Networks
In this paper we propose a new efficient iterative scheme for solving closed queueing networks with phase-type service time distributions. The method is especially efficient and accurate in case of large numbers of nodes and large customer populations. We present the method, put it in perspective, and validate it through a large number of test scenarios. In most cases, the method provides accuracies within 5% relative error (in comparison to discrete-event simulation)
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