2,022 research outputs found
One-dimensional classical adjoint SU(2) Coulomb Gas
The equation of state of a one-dimensional classical nonrelativistic Coulomb
gas of particles in the adjoint representation of SU(2) is given. The problem
is solved both with and without sources in the fundamental representation at
either end of the system. The gas exhibits confining properties at low
densities and temperatures and deconfinement in the limit of high densities and
temperatures. However, there is no phase transition to a regime where the
string tension vanishes identically; true deconfinement only happens for
infinite densities and temperatures. In the low density, low temperature limit,
a new type of collective behavior is observed.Comment: 6 pages, 1 postscript figur
On the Spectrum of QCD(1+1) with SU(N_c) Currents
Extending previous work, we calculate in this note the fermionic spectrum of
two-dimensional QCD (QCD_2) in the formulation with SU(N_c) currents. Together
with the results in the bosonic sector this allows to address the as yet
unresolved task of finding the single-particle states of this theory as a
function of the ratio of the numbers of flavors and colors, \lambda=N_f/N_c,
anew. We construct the Hamiltonian matrix in DLCQ formulation as an algebraic
function of the harmonic resolution K and the continuous parameter \lambda.
Amongst the more surprising findings in the fermionic sector chiefly considered
here is that the fermion momentum is a function of \lambda. This dependence is
necessary in order to reproduce the well-known 't Hooft and large N_f spectra.
Remarkably, those spectra have the same single-particle content as the ones in
the bosonic sectors. The twist here is the dramatically different sizes of the
Fock bases in the two sectors, which makes it possible to interpret in
principle all states of the discrete approach. The hope is that some of this
insight carries over into the continuum. We also present some new findings
concerning the single-particle spectrum of the adjoint theory.Comment: 21 pp., 13 figures, version published in PR
The local structure of topological charge fluctuations in QCD
We introduce the Dirac eigenmode filtering of topological charge density
associated with Ginsparg-Wilson fermions as a tool to investigate the local
structure of topological charge fluctuations in QCD. The resulting framework is
used to demonstrate that the bulk of topological charge in QCD does not appear
in the form of unit quantized lumps. This means that the mixing of "would-be"
zeromodes associated with such lumps is probably not the prevalent microscopic
mechanism for spontaneous chiral symmetry breaking in QCD. To characterize the
coherent local behavior in topological charge density at low energy, we compute
the charges contained in maximal coherent spheres enclosing non-overlapping
peaks. We find a continuous distribution essentially ending at ~0.5. Finally,
we study, for the first time, the overlap-operator topological-charge-density
correlators and find consistency with non-positivity at nonzero physical
distance. This represents a non-trivial check on the locality (in gauge paths)
of the overlap Dirac operator for realistic gauge backgrounds.Comment: 3 pages, 4 figures, talk, Lattice2002(topology
ProbCD: enrichment analysis accounting for categorization uncertainty
As in many other areas of science, systems biology makes extensive use of statistical association and significance estimates in contingency tables, a type of categorical data analysis known in this field as enrichment (also over-representation or enhancement) analysis. In spite of efforts to create probabilistic annotations, especially in the Gene Ontology context, or to deal with uncertainty in high throughput-based datasets, current enrichment methods largely ignore this probabilistic information since they are mainly based on variants of the Fisher Exact Test. We developed an open-source R package to deal with probabilistic categorical data analysis, ProbCD, that does not require a static contingency table. The contingency table for
the enrichment problem is built using the expectation of a Bernoulli Scheme stochastic process given the categorization probabilities. An on-line interface was created to allow usage by non-programmers and is available at: http://xerad.systemsbiology.net/ProbCD/. We present an analysis framework and software tools to address the issue of uncertainty in categorical data analysis. In particular, concerning the enrichment analysis, ProbCD can accommodate: (i) the stochastic nature of the high-throughput experimental techniques and (ii) probabilistic gene annotation
`Oumuamua as a messenger from the Local Association
7 pages, one table, two figures, accepted for publication by ApJL. © 2018. The American Astronomical Society. All rights reserved.With a hyperbolic trajectory around the Sun, 'Oumuamua is the first confirmed interstellar object. However, its origin is poorly known. By simulating the orbits of 0.23 million local stars, we find 109 encounters with periastron less than 5 pc. 'Oumuamua's low peculiar velocity is suggestive of its origin from a young stellar association with similar velocity. In particular, we find that 'Oumuamua would have had slow encounters with at least five young stars belonging to the Local Association, thus suggesting these as plausible sites for formation and ejection. In addition to an extremely elongated shape, the available observational data for 'Oumuamua indicates a red color, suggestive of a potentially organic-rich and activity-free surface. These characteristics seem consistent with formation through energetic collisions between planets and debris objects in the middle part of a young stellar system. We estimate an abundance of at least 6.0 × 10 -3 au -3 for such interstellar objects with mean diameter larger than 100 m and find that it is likely that most of them will be ejected into the Galactic halo. Our Bayesian analysis of the available light curves indicates a rotation period of 6.96 +1.45 -0.39, which is consistent with the estimation by Meech et al. and shorter than those in other literature. The codes and results are available on GitHub (https://github.com/phillippro/Oumuamua).Peer reviewe
Diagonal deformations of thin center vortices and their stability in Yang-Mills theories
The importance of center vortices for the understanding of the confining
properties of SU(N) Yang-Mills theories is well established in the lattice.
However, in the continuum, there is a problem concerning the relevance of
center vortex backgrounds. They display the so called Savvidy-Nielsen-Olesen
instability, associated with a gyromagnetic ratio for the
off-diagonal gluons.
In this work, we initially consider the usual definition of a {\it thin}
center vortex and rewrite it in terms of a local color frame in SU(N)
Yang-Mills theories. Then, we define a thick center vortex as a diagonal
deformation of the thin object. Besides the usual thick background profile,
this deformation also contains a frame defect coupled with gyromagnetic ratio
, originated from the charged sector. As a consequence, the
analysis of stability is modified. In particular, we point out that the defect
should stabilize a vortex configuration formed by a pair of straight components
separated by an appropriate finite distance.Comment: 20 pages, LaTe
Probing crunching AdS cosmologies
Holographic gravity duals of deformations of CFTs formulated on de Sitter spacetime contain FRW geometries behind a horizon, with cosmological big crunch singularities. Using a specific analytically tractable solution within a particular single scalar truncation of N=8 supergravity on AdS_4, we first probe such crunching cosmologies with spacelike radial geodesics that compute spatially antipodal correlators of large dimension boundary operators. At late times, the geodesics lie on the FRW slice of maximal expansion behind the horizon. The late time two-point functions factorise, and when transformed to the Einstein static universe, they exhibit a temporal non-analyticity determined by the maximal value of the scale factor a_max. Radial geodesics connecting antipodal points necessarily have de Sitter energy E < a_max, while geodesics with E > a_max terminate at the crunch, the two categories of geodesics being separated by the maximal expansion slice.The spacelike crunch singularity is curved ``outward'' in the Penrose diagram for the deformed AdS backgrounds, and thus geodesic limits of the antipodal correlators do not directly probe the crunch. Beyond the geodesic limit, we point out that the scalar wave equation, analytically continued into the FRW patch, has a potential which is singular at the crunch along with complex WKB turning points in the vicinity of the FRW crunch. We then argue that the frequency space Green's function has a branch point determined by a_max which corresponds to the lowest quasinormal frequency
Polyhedral units and network connectivity in calcium aluminosilicate glasses from high-energy x-ray diffraction
Structure factors for Cax/2AlxSi1-xO2 glasses (x=0,0.25,0.5,0.67) extended to
a wave vector of magnitude Q= 40 1/A have been obtained by high-energy x-ray
diffraction. For the first time, it is possible to resolve the contributions of
Si-O, Al-O and Ca-O coordination polyhedra to the experimental atomic pair
distribution functions (PDF). It has been found that both Si and Al are
four-fold coordinated and so participate in a continuous tetrahedral network at
low values of x. The number of network breaking defects in the form of
non-bridging oxygens (NBO's) increases slowly with x until x=0.5 (NBO's ~ 10%
at x=0.5). By x=0.67 the network breaking defects become significant as
evidenced by the significant drop in the average coordination number of Si. By
contrast, Al-O tetrahedra remain free of NBO's and fully integrated in the
Al/Si-O network for all values of x. Calcium maintains a rather uniform
coordination sphere of approximately 5 oxygen atoms for all values of x. The
results suggest that not only Si/Al-O tetrahedra but Ca-O polyhedra, too, play
a role in determining the glassy structure
- …