40,172 research outputs found
Modeling the Void H I Column Density Spectrum
The equivalent width distribution function (EWDF) of \hone absorbers specific
to the void environment has been recently derived (Manning 2002), revealing a
large line density of clouds (dN/dz ~500 per unit z for Log (N_HI)> 12.4). I
show that the void absorbers cannot be diffuse (or so-called filamentary)
clouds, expanding with the Hubble flow, as suggested by N-body/hydro
simulations. Absorbers are here modeled as the baryonic remnants of
sub-galactic perturbations that have expanded away from their dark halos in
response to reionization at z ~ 6.5. A 1-D Lagrangian hydro/gravity code is
used to follow the dynamic evolution and ionization structure of the baryonic
clouds for a range of halo circular velocities. The simulation products at z=0
can be combined according to various models of the halo velocity distribution
function to form a column density spectrum that can be compared with the
observed. I find that such clouds may explain the observed EWDF if the halo
velocity distribution function is as steep as that advanced by Klypin (1999),
and the halo mass distribution is closer to isothermal than to NFW.Comment: 21 pages, 15 figures. Paper in press; ApJ 591, n
The Exact MSSM Spectrum from String Theory
We show the existence of realistic vacua in string theory whose observable
sector has exactly the matter content of the MSSM. This is achieved by
compactifying the E_8 x E_8 heterotic superstring on a smooth Calabi-Yau
threefold with an SU(4) gauge instanton and a Z_3 x Z_3 Wilson line.
Specifically, the observable sector is N=1 supersymmetric with gauge group
SU(3)_C x SU(2)_L x U(1)_Y x U(1)_{B-L}, three families of quarks and leptons,
each family with a right-handed neutrino, and one Higgs-Higgs conjugate pair.
Importantly, there are no extra vector-like pairs and no exotic matter in the
zero mode spectrum. There are, in addition, 6 geometric moduli and 13 gauge
instanton moduli in the observable sector. The holomorphic SU(4) vector bundle
of the observable sector is slope-stable.Comment: 15 pages, LaTeX; v2: Hidden sector is unstable, symbol typesetting
error corrected, clarifications and references added; v3: New discussion of
hidden secto
The B-L/Electroweak Hierarchy in Smooth Heterotic Compactifications
E8 X E8 heterotic string and M-theory, when appropriately compactified, can
give rise to realistic, N=1 supersymmetric particle physics. In particular, the
exact matter spectrum of the MSSM, including three right-handed neutrino
supermultiplets, one per family, and one pair of Higgs-Higgs conjugate
superfields is obtained by compactifying on Calabi-Yau manifolds admitting
specific SU(4) vector bundles. These "heterotic standard models" have the
SU(3)_{C} X SU(2)_{L} X U(1)_{Y} gauge group of the standard model augmented by
an additional gauged U(1)_{B-L}. Their minimal content requires that the B-L
gauge symmetry be spontaneously broken by a vacuum expectation value of at
least one right-handed sneutrino. In a previous paper, we presented the results
of a renormalization group analysis showing that B-L gauge symmetry is indeed
radiatively broken with a B-L/electroweak hierarchy of O(10) to O(10^{2}). In
this paper, we present the details of that analysis, extending the results to
include higher order terms in tan[beta]^{-1} and the explicit spectrum of all
squarks and sleptons.Comment: 60 pages, 6 figure
Magnetic fields in nearby galaxies
We describe a recent full-polarization radio continuum survey, performed
using the Westerbork Synthesis Radio Telescope (WSRT), of several nearby
galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample. The
WSRT-SINGS survey has been utilized to study the polarized emission and Faraday
rotation measures (RMs) in the targets, and reveals an important new
observational trend. The azimuthal distribution of polarized flux seems to be
intimately related to the kinematic orientation of galaxies, such that in
face-on galaxies the lowest level of polarized flux is detected along the
kinematic major axis. In highly inclined galaxies, the polarized flux is
minimized on both ends of the major axis, and peaks near the minor axis. Using
models of various three-dimensional magnetic field geometries, and including
the effects of turbulent depolarization in the midplane, we are able to
reproduce the qualitative distribution of polarized flux in the target
galaxies, its variation with inclination, and the distribution of RMs, thereby
constraining the global magnetic field structure in galaxies. Future radio
telescope facilities, now being planned and constructed, will have properties
making them extremely well-suited to perform vastly larger surveys of this
type, and are thereby poised to significantly increase our understanding of the
global structure of galactic magnetic fields. We discuss progress that can be
made using surveys which will be realized with these new facilities, focusing
in particular on the Aperture Tile in Focus (APERTIF) and Australian Square
Kilometre Array Pathfinder (ASKAP) telescopes, both based on Focal Plane Array
(FPA) designs, which are expected to be particularly useful for wide-field
polarization applications.Comment: In proceedings of "Panoramic Radio Astronomy" conference held 2-5
June 2009, Groningen, the Netherlands. 6 pages, 2 figure
Yukawa Couplings in Heterotic Standard Models
In this paper, we present a formalism for computing the Yukawa couplings in
heterotic standard models. This is accomplished by calculating the relevant
triple products of cohomology groups, leading to terms proportional to Q*H*u,
Q*Hbar*d, L*H*nu and L*Hbar*e in the low energy superpotential. These
interactions are subject to two very restrictive selection rules arising from
the geometry of the Calabi-Yau manifold. We apply our formalism to the
"minimal" heterotic standard model whose observable sector matter spectrum is
exactly that of the MSSM. The non-vanishing Yukawa interactions are explicitly
computed in this context. These interactions exhibit a texture rendering one
out of the three quark/lepton families naturally light.Comment: 21 pages, LaTe
Stability of the Minimal Heterotic Standard Model Bundle
The observable sector of the "minimal heterotic standard model" has precisely
the matter spectrum of the MSSM: three families of quarks and leptons, each
with a right-handed neutrino, and one Higgs-Higgs conjugate pair. In this
paper, it is explicitly proven that the SU(4) holomorphic vector bundle leading
to the MSSM spectrum in the observable sector is slope-stable.Comment: LaTeX, 19 page
Three Generations on the Quintic Quotient
A three-generation SU(5) GUT, that is 3x(10+5bar) and a single 5-5bar pair,
is constructed by compactification of the E_8 heterotic string. The base
manifold is the Z_5 x Z_5-quotient of the quintic, and the vector bundle is the
quotient of a positive monad. The group action on the monad and its
bundle-valued cohomology is discussed in detail, including topological
restrictions on the existence of equivariant structures. This model and a
single Z_5 quotient are the complete list of three generation quotients of
positive monads on the quintic.Comment: 19 pages, LaTeX. v2: section on anomaly cancellation adde
Another attempt to quantify the benefits of reducing inflation
This article estimates the benefits of reducing U.S. inflation below its current level when the government simultaneously raises another distortionary tax. Other researchers have suggested that reducing inflation would have fairly large benefitsâfrom 1 to 3 percent of gross domestic product. But that result depends on the unrealistic assumption that the government would replace inflation with a lump-sum tax, one which does not affect people's incentives. If, instead, inflation is replaced with an increase in the labor income tax, then the welfare gains that can be expected from reducing inflation below its current level are much smallerâfrom one-third to one-half of 1 percent of gross domestic product.Inflation (Finance)
The Kinematic and Spatial Deployment of Compact, Isolated High-Velocity Clouds
We have identified a class of high-velocity clouds which are compact and
apparently isolated. The clouds are compact in that they have angular sizes
less than 2 degrees FWHM. They are isolated in that they are separated from
neighboring emission by expanses where no emission is seen to the detection
limit of the available data. Candidates for inclusion in this class were
extracted from the Leiden/Dwingeloo HI survey of Hartmann & Burton and from the
Wakker & van Woerden catalogue of high-velocity clouds. The candidates were
subject to independent confirmation using either the 25-meter telescope in
Dwingeloo or the 140-foot telescope in Green Bank. We argue that the resulting
list, even if incomplete, is sufficiently representative of the ensemble of
compact, isolated HVCs - CHVCs - that the characteristics of their disposition
on the sky, and of their kinematics, are revealing of some physical aspects of
the class. The CHVCs are in fact distributed quite uniformly across the sky. A
global search for the reference frame which minimizes the velocity dispersion
of the ensemble returns the Local Group Standard of Rest with high confidence.
The CHVCs are not stationary with respect to this reference frame but have a
mean infall velocity of 100 km/s. These properties are strongly suggestive of a
population which has as yet had little interaction with the more massive Local
Group members. At a typical distance of about 1 Mpc these objects would have
sizes of about 15 kpc and gas masses, M_HI, of a few times 10^7 M_Sun,
corresponding to those of (sub-)dwarf galaxies. (abridged)Comment: 13 page LaTeX, requires aa.cls and rotate.sty, 5 GIF figures.
Accepted for publication in A&
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