34,584 research outputs found
Critical temperature and Ginzburg region near a quantum critical point in two-dimensional metals
We compute the transition temperature and the Ginzburg temperature
above near a quantum critical point at the boundary of an
ordered phase with a broken discrete symmetry in a two-dimensional metallic
electron system. Our calculation is based on a renormalization group analysis
of the Hertz action with a scalar order parameter. We provide analytic
expressions for and as a function of the non-thermal control
parameter for the quantum phase transition, including logarithmic corrections.
The Ginzburg regime between and occupies a sizable part of
the phase diagram.Comment: 5 pages, 1 figur
Soft-Collinear Messengers: A New Mode in Soft-Collinear Effective Theory
It is argued that soft-collinear effective theory for processes involving
both soft and collinear partons, such as exclusive B-meson decays, should
include a new mode in addition to soft and collinear fields. These
"soft-collinear messengers" can interact with both soft and collinear particles
without taking them far off-shell. They thus can communicate between the soft
and collinear sectors of the theory. The relevance of the new mode is
demonstrated with an explicit example, and the formalism incorporating the
corresponding quark and gluon fields into the effective Lagrangian is
developed.Comment: 22 pages, 5 figures. Extended Section 6, clarifying the relevance of
different types of soft-collinear interaction
Size Matters: Origin of Binomial Scaling in Nuclear Fragmentation Experiments
The relationship between measured transverse energy, total charge recovered
in the detector, and size of the emitting system is investigated. Using only
very simple assumptions, we are able to reproduce the observed binomial
emission probabilities and their dependences on the transverse energy.Comment: 14 pages, including 4 figure
Phase Transitions in a Two-Component Site-Bond Percolation Model
A method to treat a N-component percolation model as effective one component
model is presented by introducing a scaled control variable . In Monte
Carlo simulations on , , and simple cubic
lattices the percolation threshold in terms of is determined for N=2.
Phase transitions are reported in two limits for the bond existence
probabilities and . In the same limits, empirical formulas
for the percolation threshold as function of one
component-concentration, , are proposed. In the limit a new
site percolation threshold, , is reported.Comment: RevTeX, 5 pages, 5 eps-figure
The determinants of direct air fares to Cleveland: how competitive?
Using a model developed to examine the determinants of air fares, the authors discuss the relationship between airline industry competitiveness and fare increases.Airlines ; Competition ; Cleveland (Ohio)
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
Gaining analytic control of parton showers
Parton showers are widely used to generate fully exclusive final states
needed to compare theoretical models to experimental observations. While, in
general, parton showers give a good description of the experimental data, the
precise functional form of the probability distribution underlying the event
generation is generally not known. The reason is that realistic parton showers
are required to conserve four-momentum at each vertex. In this paper we
investigate in detail how four-momentum conservation is enforced in a standard
parton shower and why this destroys the analytic control of the probability
distribution. We show how to modify a parton shower algorithm such that it
conserves four-momentum at each vertex, but for which the full analytic form of
the probability distribution is known. We then comment how this analytic
control can be used to match matrix element calculations with parton showers,
and to estimate effects of power corrections and other uncertainties in parton
showers.Comment: 12 pages, 6 figures, v2: final journal versio
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