3,204 research outputs found
Color Superconductivity at Large N: A New Hope
At zero density, the `t Hooft large N_c limit often provides some very useful
qualitative insights into the non-perturbative physics of QCD. However, it is
known that at high densities the `t Hooft large N_c world looks very different
from the N_c=3 world, which is believed to be in a color superconducting phase
at high densities. At large N_c, on the other hand, the DGR instability causes
a chiral-density wave phase to dominate over the color superconducting phase.
There is an alternative large N_c limit, with the quarks transforming in the
two-index antisymmetric representation of the gauge group, which at N_c=3
reduces to QCD but looks quite different at large N_c. We show that in this
alternative large N_c limit, the DGR instability does not occur, so that it may
be plausible that the ground state of high-density quark matter is a color
superconductor even when N_c is large. This revives the hope that a large N_c
approximation might be useful for getting some insights into the high-density
phenomenology of QCD.Comment: 8 pages, 1 figur
Gyromagnetic Ratios of Bound Particles
A new approach to calculation of the binding corrections to the magnetic
moments of the constituents in a loosely bound system, based on the
Bargmann-Michel-Telegdi equation, is suggested. Binding corrections are
calculated in this framework, and the results confirm earlier calculations
performed by other methods. Our method clearly demonstrates independence of the
binding corrections on the magnitude of the spin of the constituents.Comment: 10 pages, RevTe
On Toroidal Horizons in Binary Black Hole Inspirals
We examine the structure of the event horizon for numerical simulations of
two black holes that begin in a quasicircular orbit, inspiral, and finally
merge. We find that the spatial cross section of the merged event horizon has
spherical topology (to the limit of our resolution), despite the expectation
that generic binary black hole mergers in the absence of symmetries should
result in an event horizon that briefly has a toroidal cross section. Using
insight gained from our numerical simulations, we investigate how the choice of
time slicing affects both the spatial cross section of the event horizon and
the locus of points at which generators of the event horizon cross. To ensure
the robustness of our conclusions, our results are checked at multiple
numerical resolutions. 3D visualization data for these resolutions are
available for public access online. We find that the structure of the horizon
generators in our simulations is consistent with expectations, and the lack of
toroidal horizons in our simulations is due to our choice of time slicing.Comment: Submitted to Phys. Rev.
Perceptually Motivated Wavelet Packet Transform for Bioacoustic Signal Enhancement
A significant and often unavoidable problem in bioacoustic signal processing is the presence of background noise due to an adverse recording environment. This paper proposes a new bioacoustic signal enhancement technique which can be used on a wide range of species. The technique is based on a perceptually scaled wavelet packet decomposition using a species-specific Greenwood scale function. Spectral estimation techniques, similar to those used for human speech enhancement, are used for estimation of clean signal wavelet coefficients under an additive noise model. The new approach is compared to several other techniques, including basic bandpass filtering as well as classical speech enhancement methods such as spectral subtraction, Wiener filtering, and Ephraim–Malah filtering. Vocalizations recorded from several species are used for evaluation, including the ortolan bunting (Emberiza hortulana), rhesus monkey (Macaca mulatta), and humpback whale (Megaptera novaeanglia), with both additive white Gaussian noise and environment recording noise added across a range of signal-to-noise ratios (SNRs). Results, measured by both SNR and segmental SNR of the enhanced wave forms, indicate that the proposed method outperforms other approaches for a wide range of noise conditions
Bounds on curved domain walls in 5d gravity
We discuss maximally symmetric curved deformations of the flat domain wall
solutions of five-dimensional dilaton gravity that appeared in a recent
approach to the cosmological constant problem. By analyzing the bulk field
configurations and the boundary conditions at a four-dimensional maximally
symmetric curved domain wall, we obtain constraints on such solutions. For a
special dilaton coupling to the brane tension that appeared in recent works, we
find no curved deformations, confirming and extending slightly a result of
Arkani-Hamed et al which was argued using a -symmetry of the solution. For
more general dilaton-dependent brane tension, we find that the curvature is
bounded by the Kaluza-Klein scale in the fifth dimension.Comment: 9 pages, harvmac big. Version 2: Significant error in quantitative
bounds corrected; bounds are not numerically smaller than Standard Model
physics without further assumptions about the VEV of the dilaton. However,
qualitative bound on curvature scale by bulk Kaluza-Klein scale unchanged
Version 3: Minor change
WW Scattering Parameters via Pseudoscalar Phase Shifts
Using domain-wall lattice simulations, we study pseudoscalar-pseudoscalar
scattering in the maximal isospin channel for an SU(3) gauge theory with two
and six fermion flavors in the fundamental representation. This calculation of
the S-wave scattering length is related to the next-to-leading order
corrections to WW scattering through the low-energy coefficients of the chiral
Lagrangian. While two and six flavor scattering lengths are similar for a fixed
ratio of the pseudoscalar mass to its decay constant, six-flavor scattering
shows a somewhat less repulsive next-to-leading order interaction than its
two-flavor counterpart. Estimates are made for the WW scattering parameters and
the plausibility of detection is discussed.Comment: 8 pages, 6 figure
Approaching Conformality with Ten Flavors
We present first results for lattice simulations, on a single volume, of the
low-lying spectrum of an SU(3) Yang-Mills gauge theory with ten light fermions
in the fundamental representation. Fits to the fermion mass dependence of
various observables are found to be globally consistent with the hypothesis
that this theory is within or just outside the strongly-coupled edge of the
conformal window, with mass anomalous dimension consistent with 1 over the
range of scales simulated. We stress that we cannot rule out the possibility of
spontaneous chiral-symmetry breaking at scales well below our infrared cutoff.
We discuss important systematic effects, including finite-volume corrections,
and consider directions for future improvement.Comment: 7 pages, 3 figures. Submitted to Physical Review Letters. v2:
corrected global fits. v3: corrected estimation of confidence interval
Radio astronomy
The following subject areas are covered: (1) scientific opportunities (millimeter and sub-millimeter wavelength astronomy; meter to hectometer astronomy; the Sun, stars, pulsars, interstellar masers, and extrasolar planets; the planets, asteroids, and comets; radio galaxies, quasars, and cosmology; and challenges for radio astronomy in the 1990's); (2) recommendations for new facilities (the millimeter arrays, medium scale instruments, and small-scale projects); (3) continuing activities and maintenance, upgrading of telescopes and instrumentation; (4) long range programs and technology development; and (5) social, political, and organizational considerations
Antimicrobial treatment improves mycobacterial survival in nonpermissive growth conditions
Antimicrobials targeting cell wall biosynthesis are generally considered inactive against nonreplicating bacteria. Paradoxically, we found that under nonpermissive growth conditions, exposure of Mycobacterium bovis BCG bacilli to such antimicrobials enhanced their survival. We identified a transcriptional regulator, RaaS (for regulator of antimicrobial-assisted survival), encoded by bcg1279 (rv1219c) as being responsible for the observed phenomenon. Induction of this transcriptional regulator resulted in reduced expression of specific ATP-dependent efflux pumps and promoted long-term survival of mycobacteria, while its deletion accelerated bacterial death under nonpermissive growth conditions in vitro and during macrophage or mouse infection. These findings have implications for the design of antimicrobial drug combination therapies for persistent infectious diseases, such as tuberculosis
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