6,948 research outputs found
The Quantum McKay Correspondence for polyhedral singularities
Let G be a polyhedral group, namely a finite subgroup of SO(3). Nakamura's
G-Hilbert scheme provides a preferred Calabi-Yau resolution Y of the polyhedral
singularity C^3/G. The classical McKay correspondence describes the classical
geometry of Y in terms of the representation theory of G. In this paper we
describe the quantum geometry of Y in terms of R, an ADE root system associated
to G. Namely, we give an explicit formula for the Gromov-Witten partition
function of Y as a product over the positive roots of R. In terms of counts of
BPS states (Gopakumar-Vafa invariants), our result can be stated as a
correspondence: each positive root of R corresponds to one half of a genus zero
BPS state. As an application, we use the crepant resolution conjecture to
provide a full prediction for the orbifold Gromov-Witten invariants of [C^3/G].Comment: Introduction rewritten. Issue regarding non-uniqueness of conifold
resolution clarified. Version to appear in Inventione
Gauge Coupling Unification and Non-Equilibrium Thermal Dark Matter
We study a new mechanism for the production of dark matter in the universe
which does not rely on thermal equilibrium. Dark matter is populated from the
thermal bath subsequent to inflationary reheating via a massive mediator whose
mass is above the reheating scale, T_R. To this end, we consider models with an
extra U(1) gauge symmetry broken at some intermediate scale M, of the order of
10^10 -- 10^12 GeV. We show that not only does the model allow for gauge
coupling unification (at a higher scale associated with grand unification) but
can naturally provide a dark matter candidate which is a Standard Model singlet
but charged under the extra U(1). The intermediate scale gauge boson(s) which
are predicted in several E6/SO(10) constructions can be a natural mediator
between dark matter and the thermal bath. We show that the dark matter
abundance, while never having achieved thermal equilibrium, is fixed shortly
after the reheating epoch by the relation T_R^3/M^4. As a consequence, we show
that the unification of gauge couplings which determines M also fixes the
reheating temperature T_R, which can be as high as 10^11 GeV.Comment: 4 pages, 2 figures, 1 tabl
The Lyman Alpha Forest in Hierarchical Cosmologies
The comparison of quasar absorption spectra with numerically simulated
spectra from hierarchical cosmological models of structure formation promises
to be a valuable tool to discriminate among these models. We present simulation
results for the column density, Doppler b parameter, and optical depth
probability distributions for five popular cosmological models.Comment: 4 pages, 3 figures, uses aipproc.sty, to appear in the Proceedings of
the 9th Annual October Astrophysics Conference in Maryland, "After the Dark
Ages: When Galaxies Were Young (the Universe at 2<z<5)", ed. S. S. Holt and
E. P. Smith, October 12-14, 199
Interactions between Octet Baryons in the SU_6 Quark model
The baryon-baryon interactions for the complete baryon octet (B_8) are
investigated in a unified framework of the resonating-group method, in which
the spin-flavor SU_6 quark-model wave functions are employed. Model parameters
are determined to reproduce properties of the nucleon-nucleon system and the
low-energy cross section data for the hyperon-nucleon interaction. We then
proceed to explore B_8 B_8 interactions in the strangeness S=-2, -3 and -4
sectors. The S-wave phase-shift behavior and total cross sections are
systematically understood by 1) the spin-flavor SU_6 symmetry, 2) the special
role of the pion exchange, and 3) the flavor symmetry breaking.Comment: 11 pages, 6 figures, submitted to Phys. Rev. C (Rapid Communication
Simulating Star Formation and Feedback in Galactic Disk Models
We use a high-resolution grid-based hydrodynamics method to simulate the
multi-phase interstellar medium in a Milky Way-size quiescent disk galaxy. The
models are global and three-dimensional, and include a treatment of star
formation and feedback. We examine the formation of gravitational instabilities
and show that a form of the Toomre instability criterion can successfully
predict where star formation will occur. Two common prescriptions for star
formation are investigated. The first is based on cosmological simulations and
has a relatively low threshold for star formation, but also enforces a
comparatively low efficiency. The second only permits star formation above a
number density of 1000 cm^-3 but adopts a high efficiency. We show that both
methods can reproduce the observed slope of the relationship between star
formation and gas surface density (although at too high a rate for our adopted
parameters). A run which includes feedback from type II supernovae is
successful at driving gas out of the plane, most of which falls back onto the
disk. This feedback also substantially reduces the star formation rate.
Finally, we examine the density and pressure distribution of the ISM, and show
that there is a rough pressure equilibrium in the disk, but with a wide range
of pressures at a given location (and even wider for the case including
feedbackComment: 14 pages, 12 figures, accepted to Astrophysical Journa
Cost Benefit Analysis: Applications and Future Opportunities
Cost Benefit Analysis (CBA or Benefit Cost Analysis â BCA) is an evaluation tool that state transportation agencies can use to compare infrastructure project options across transportation modes and gauge if the discounted value of benefits exceed the costs. CBA lets policymakers compare alternative project proposals to a baseline scenario, or status quo case, under which no investments are made. CBA is used to select which proposal is the most sensible investmentâideally, the project that maximizes benefits while minimizing costs should be chosen. As the use of CBA to evaluate projects and grant applications (e.g., TIGER Discretionary Grants) continues to grow, state transportation agencies must gain knowledge of CBA methodologies, especially if they want to remain competitive for federal funding. This study documents historical trends in CBA from literature and past studies, provides background on how CBAâs are performed, and describes the purpose and basic function of CBAs. The researchers investigated CBA applications and programs that the Kentucky Transportation Cabinet (KYTC) could potentially use to prioritize projects and identify other investment opportunities. This report serves as a primer on the basic elements of CBA, describes what costs and benefits are typically incorporated into analysis, and gives an overview of the formulas that are used to derive costs and benefits. In the transportation context, most benefits are derived from travel time savings and crash reductions. Typical costs include construction and subsequent maintenance.Researchers developed a simple workbook application that KYTC personnel can use run a CBA. The workbook calculates present value of costs and benefits and makes both NPV and BCR available to the user
A New Approach for Simulating Galaxy Cluster Properties
We describe a subgrid model for including galaxies into hydrodynamical
cosmological simulations of galaxy cluster evolution. Each galaxy construct- or
galcon- is modeled as a physically extended object within which star formation,
galactic winds, and ram pressure stripping of gas are modeled analytically.
Galcons are initialized at high redshift (z~3) after galaxy dark matter halos
have formed but before the cluster has virialized. Each galcon moves
self-consistently within the evolving cluster potential and injects mass,
metals, and energy into intracluster (IC) gas through a well-resolved spherical
interface layer. We have implemented galcons into the Enzo adaptive mesh
refinement code and carried out a simulation of cluster formation in a
LambdaCDM universe. With our approach, we are able to economically follow the
impact of a large number of galaxies on IC gas. We compare the results of the
galcon simulation with a second, more standard simulation where star formation
and feedback are treated using a popular heuristic prescription. One advantage
of the galcon approach is explicit control over the star formation history of
cluster galaxies. Using a galactic SFR derived from the cosmic star formation
density, we find the galcon simulation produces a lower stellar fraction, a
larger gas core radius, a more isothermal temperature profile, and a flatter
metallicity gradient than the standard simulation, in better agreement with
observations.Comment: 4 pages, 2 figures, submitted for publication in ApJ
AMP-activated protein kinase (AMPK) limits mitochondrial DNA damage in human knee OA chondrocytes by upregulation of SIRT3 and the DNA repair enzyme OGG1
Pulsar Wind Nebulae in EGRET Error Boxes
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET
gamma-ray sources. X-ray and radio imaging studies of unidentified EGRET
sources have resulted in the discovery of at least 6 new pulsar wind nebulae
(PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources
with hard spectra, typical for gamma-ray pulsars. Their toroidal morphologies
can help determine the geometry of the pulsar which is useful for constraining
models of pulsed gamma-ray emission. Rapidly moving PWN (RPWN) with more
cometary morphologies seem to be associated with variable EGRET sources in
regions where the ambient medium is dense compared to what is typical for the
ISM.Comment: 8 pages, 5 figures, to appear in the proceedings of "The
Multiwavelength Approach to Unidentified Sources", ed. G. Romero & K.S. Chen
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