7,273 research outputs found
W=0 Pairing in Carbon Nanotubes away from Half Filling
We use the Hubbard Hamiltonian on the honeycomb lattice to represent the
valence bands of carbon single-wall nanotubes. A detailed symmetry
analysis shows that the model allows W=0 pairs which we define as two-body
singlet eigenstates of with vanishing on-site repulsion. By means of a
non-perturbative canonical transformation we calculate the effective
interaction between the electrons of a W=0 pair added to the interacting ground
state. We show that the dressed W=0 pair is a bound state for resonable
parameter values away from half filling. Exact diagonalization results for the
(1,1) nanotube confirm the expectations. For nanotubes of length ,
the binding energy of the pair depends strongly on the filling and decreases
towards a small but nonzero value as . We observe the existence
of an optimal doping when the number of electrons per C atom is in the range
1.21.3, and the binding energy is of the order of 0.1 1 meV.Comment: 16 pages, 6 figure
Screening Experiments for Simulation: A Review
This article reviews so-called screening in simulation; i.e., it examines the search for the really important factors in experiments with simulation models that have very many factors (or inputs). The article focuses on a most efficient and effec- tive screening method, namely Sequential Bifurcation. It ends with a discussion of possible topics for future research, and forty references for further study.Screening;Metamodel;Response Surface;Design
Certifying and removing disparate impact
What does it mean for an algorithm to be biased? In U.S. law, unintentional
bias is encoded via disparate impact, which occurs when a selection process has
widely different outcomes for different groups, even as it appears to be
neutral. This legal determination hinges on a definition of a protected class
(ethnicity, gender, religious practice) and an explicit description of the
process.
When the process is implemented using computers, determining disparate impact
(and hence bias) is harder. It might not be possible to disclose the process.
In addition, even if the process is open, it might be hard to elucidate in a
legal setting how the algorithm makes its decisions. Instead of requiring
access to the algorithm, we propose making inferences based on the data the
algorithm uses.
We make four contributions to this problem. First, we link the legal notion
of disparate impact to a measure of classification accuracy that while known,
has received relatively little attention. Second, we propose a test for
disparate impact based on analyzing the information leakage of the protected
class from the other data attributes. Third, we describe methods by which data
might be made unbiased. Finally, we present empirical evidence supporting the
effectiveness of our test for disparate impact and our approach for both
masking bias and preserving relevant information in the data. Interestingly,
our approach resembles some actual selection practices that have recently
received legal scrutiny.Comment: Extended version of paper accepted at 2015 ACM SIGKDD Conference on
Knowledge Discovery and Data Minin
Search for technipions in exclusive production of diphotons with large invariant masses at the LHC
We focus on exclusive production of neutral technipion in
collisions at the LHC, i.e. on reaction. The
dependence of the cross section on parameters of recently proposed vector-like
Technicolor model is studied. Characteristic features of the differential
distributions are discussed. For not too large technipion masses the diphoton
decay channel has the dominant branching fraction. This is also the main reason
for an enhanced production of neutral technipions in -fusion
reaction. We discuss potential backgrounds of the QCD and QED origin to the process at large invariant
masses. We conclude that compared to inclusive case the
signal-to-background ratio in the considered exclusive reaction is vary
favorable which thereby could serve as a good probe for Technicolor dynamics
searches at the LHC.Comment: 20 pages, 12 figures, 1 tabl
Simulating Three-Dimensional Hydrodynamics on a Cellular-Automata Machine
We demonstrate how three-dimensional fluid flow simulations can be carried
out on the Cellular Automata Machine 8 (CAM-8), a special-purpose computer for
cellular-automata computations. The principal algorithmic innovation is the use
of a lattice-gas model with a 16-bit collision operator that is specially
adapted to the machine architecture. It is shown how the collision rules can be
optimized to obtain a low viscosity of the fluid. Predictions of the viscosity
based on a Boltzmann approximation agree well with measurements of the
viscosity made on CAM-8. Several test simulations of flows in simple geometries
-- channels, pipes, and a cubic array of spheres -- are carried out.
Measurements of average flux in these geometries compare well with theoretical
predictions.Comment: 19 pages, REVTeX and epsf macros require
The spontaneous breaking Twin Higgs
The Twin Higgs model seeks to address the little hierarchy problem by making
the Higgs a pseudo-Goldstone of a global symmetry that is spontaneously
broken to . Gauge and Yukawa couplings, which explicitly break ,
enjoy a discrete symmetry that accidentally maintains at
the quadratic level and therefore keeps the Higgs light. Contrary to most
beyond the Standard Model theories, the quadratically divergent corrections to
the Higgs mass are cancelled by a mirror sector, which is uncharged under the
Standard Model groups. However, the Twin Higgs with an exact
symmetry leads to equal vevs in the Standard Model and mirror sectors, which is
phenomenologically unviable. An explicit breaking potential must
then be introduced and tuned against the breaking terms to produce a
hierarchy of vevs between the two sectors. This leads to a moderate but
non-negligible tuning. We propose a model to alleviate this tuning, without the
need for an explicit breaking sector. The model consists of two
fundamental Higgses, one whose vacuum preserves and one
whose vacuum breaks it. As the interactions between the two Higgses are turned
on, the breaking is transmitted from the broken to the unbroken
sector and a small hierarchy of vevs is naturally produced. The presence of an
effective tadpole and feedback between the two Higgses lead to a sizable
improvement of the tuning. The resulting Higgs boson is naturally very Standard
Model like.Comment: 17 pages, 6 figures, references update
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