942 research outputs found
Spin dependent D-brane interactions and scattering amplitudes in matrix theory
Spin interactions beteween two moving Dp-branes are analyzed using the
Green-Schwarz formalism of boundary states. This approach turns out to be
extremely efficient to compute all the spin effects related by supersymmetry to
the leading v^4/r^7-p term. All these terms are shown to be scale invariant,
supporting a matrix model description of supergravity interactions. By
employing the LSZ reduction formula for matrix theory and the mentioned
supersymmetric effective potential for D0-branes, we compute the t-pole of
graviton-graviton and three form-three form scattering in matrix theory. The
results are found to be in complete agreement with tree level supergravity in
the corresponding kinematical regime and provide, moreover, an explicit map
between these degrees of freedom in both theories.Comment: 8 pages, no figures, talk presented at the conference "Quantum
aspects of gauge theories, supergravity and unification", Corfu, Greece, to
appear in the proceeding
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Problems encountered during impact calculations using analytic equations of state
During modeling of the impact of a projectile on a target or other calculations that bring materials together at high velocities, computer simulations of materials being shocked to high pressure and then releasing to low pressure are performed. Depending on the circumstances, the release to low pressure is often accompanied by release to a very low density. Numerical problems leading to very large sound speeds (and thus small time steps) or to negative Lagrangian volumes have been encountered during MESA-2D calculations of this nature. These problems can be traced to the behavior of the equation of state (EOS) in the limit as the density becomes much less than the normal or reference density. Although analytic solutions for expansion isentropes may show acceptable behavior in the low-density limit, numerical solutions can show undesirable behavior. Examples of this undesirable behavior in the low-density regime are given for some simple, analytic equations of state that have closed-form solutions for isentropes. The behavior of three analytic EOSs that are frequently used in MESA-2D calculations are then discussed. These EOSs are the Los Alamos EOS, the MESA polynomial EOS, and a Mie-Gruneisen EOS based on a linear relation between shock and particle velocity. The problems in the low-density region can be corrected for the Los Alamos EOS and the MESA polynomial EOS by the proper choice of EOS coefficients in the expansion region (density less than the reference density). Problems with the Mie-Gruneisen EOS can be corrected if the functional relationship between the Gruneisen parameter ({Tau}) and density differs above and below the reference density
Bound States of Type I D-Strings
We study the infra-red limit of the O(N) gauge theory that describes the low
energy modes of a system of type I D-strings and provide some support to
the conjecture that, in this limit, the theory flows to an orbifold conformal
theory. We compute the elliptic genus of the orbifold theory and argue that its
longest string sector describes the bound states of D-strings. We show that, as
a result, the masses and multiplicities of the bound states are in agreement
with the predictions of heterotic-type I duality in 9 dimensions, for all the
BPS charges in the lattice .Comment: 14 pages, (AMS-)LaTex file using amstex.st
Localized modes at a D-brane--O-plane intersection and heterotic Alice strings
We study a system of -branes intersecting -branes and
-planes in 1+1-dimensions. We use anomaly cancellation and string dualities
to argue that there must be chiral fermion zero-modes on the -branes which
are localized near the -planes. Away from the orientifold limit we verify
this by using index theory as well as explicit construction of the zero-modes.
This system is related to F-theory on K3 and heterotic matrix string theory,
and the heterotic strings are related to Alice string defects in
Super-Yang-Mills. In the limit of large we find an
dual of the heterotic matrix string CFT.Comment: 44 pages, typos corrected, version published in JHE
Counting BPS states on the Enriques Calabi-Yau
We study topological string amplitudes for the FHSV model using various
techniques. This model has a type II realization involving a Calabi-Yau
threefold with Enriques fibres, which we call the Enriques Calabi-Yau. By
applying heterotic/type IIA duality, we compute the topological amplitudes in
the fibre to all genera. It turns out that there are two different ways to do
the computation that lead to topological couplings with different BPS content.
One of them leads to the standard D0-D2 counting amplitudes, and from the other
one we obtain information about bound states of D0-D4-D2 branes on the Enriques
fibre. We also study the model using mirror symmetry and the holomorphic
anomaly equations. We verify in this way the heterotic results for the D0-D2
generating functional for low genera and find closed expressions for the
topological amplitudes on the total space in terms of modular forms, and up to
genus four. This model turns out to be much simpler than the generic B-model
and might be exactly solvable.Comment: 62 pages, v3: some results at genus 3 corrected, more typos correcte
Geometric K-Homology of Flat D-Branes
We use the Baum-Douglas construction of K-homology to explicitly describe
various aspects of D-branes in Type II superstring theory in the absence of
background supergravity form fields. We rigorously derive various stability
criteria for states of D-branes and show how standard bound state constructions
are naturally realized directly in terms of topological K-cycles. We formulate
the mechanism of flux stabilization in terms of the K-homology of non-trivial
fibre bundles. Along the way we derive a number of new mathematical results in
topological K-homology of independent interest.Comment: 45 pages; v2: References added; v3: Some substantial revision and
corrections, main results unchanged but presentation improved, references
added; to be published in Communications in Mathematical Physic
Predicting impacts of chemicals from organisms to ecosystem service delivery: A case study of endocrine disruptor effects on trout
We demonstrate how mechanistic modeling can be used to predict whether and how biological responses to chemicals at (sub)organismal levels in model species (i.e., what we typically measure) translate into impacts on ecosystem service delivery (i.e., what we care about). We consider a hypothetical case study of two species of trout, brown trout (Salmo trutta; BT) and greenback cutthroat trout (Oncorhynchus clarkii stomias; GCT). These hypothetical populations live in a high-altitude river system and are exposed to human-derived estrogen (17α‑ethinyl estradiol, EE2), which is the bioactive estrogen in many contraceptives. We use the individual based model in STREAM to explore how seasonally varying concentrations of EE2 could influence male spawning and sperm quality. Resulting impacts on trout recruitment and the consequences of such for anglers and for the continued viability of populations of GCT (the state fish of Colorado) are explored. in STREAM incorporates seasonally varying river flow and temperature, fishing pressure, the influence of EE2 on species-specific demography, and inter-specific competition. The model facilitates quantitative exploration of the relative importance of endocrine disruption and inter-species competition on trout population dynamics. Simulations predicted constant EE2 loading to have more impacts on GCT than BT. However, increasing removal of BT by anglers can enhance the persistence of GCT and offset some of the negative effects of EE2. We demonstrate how models that quantitatively link impacts of chemicals and other stressors on individual survival, growth, and reproduction to consequences for populations and ecosystem service delivery, can be coupled with ecosystem service valuation. The approach facilitates interpretation of toxicity data in an ecological context and gives beneficiaries of ecosystem services amore explicit role in management decisions. Although challenges remain, this type of approach may be particularly helpful for site-specific risk assessments and those in which trade offs and synergies among ecosystem services need to be considered
The luminosities of protostars in the spitzer c2d and gould belt legacy clouds
Journal ArticlePublished version available online at the Astronomical Journal, Volume 145, Number 4, Article 94; doi: doi: 10.1088/0004-6256/145/4/94Motivated by the long-standing "luminosity problem" in low-mass star formation whereby protostars are underluminous compared to theoretical expectations, we identify 230 protostars in 18 molecular clouds observed by two Spitzer Space Telescope Legacy surveys of nearby star-forming regions. We compile complete spectral energy distributions, calculate L bol for each source, and study the protostellar luminosity distribution. This distribution extends over three orders of magnitude, from 0.01 L ȯ to 69 L ȯ, and has a mean and median of 4.3 L ȯ and 1.3 L ȯ, respectively. The distributions are very similar for Class 0 and Class I sources except for an excess of low luminosity (L bol ≲ 0.5 L) Class I sources compared to Class 0. 100 out of the 230 protostars (43%) lack any available data in the far-infrared and submillimeter (70 μm <λ < 850 μm) and have L bol underestimated by factors of 2.5 on average, and up to factors of 8-10 in extreme cases. Correcting these underestimates for each source individually once additional data becomes available will likely increase both the mean and median of the sample by 35%-40%. We discuss and compare our results to several recent theoretical studies of protostellar luminosities and show that our new results do not invalidate the conclusions of any of these studies. As these studies demonstrate that there is more than one plausible accretion scenario that can match observations, future attention is clearly needed. The better statistics provided by our increased data set should aid such future work. © 2013. The American Astronomical Society. All rights reserved..National Science FoundationNational Aeronautics and Space AdministrationJet Propulsion Laboratory, California Institute of Technolog
NN Core Interactions and Differential Cross Sections from One Gluon Exchange
We derive nonstrange baryon-baryon scattering amplitudes in the
nonrelativistic quark model using the ``quark Born diagram" formalism. This
approach describes the scattering as a single interaction, here the
one-gluon-exchange (OGE) spin-spin term followed by constituent interchange,
with external nonrelativistic baryon wavefunctions attached to the scattering
diagrams to incorporate higher-twist wavefunction effects. The short-range
repulsive core in the NN interaction has previously been attributed to this
spin-spin interaction in the literature; we find that these perturbative
constituent-interchange diagrams do indeed predict repulsive interactions in
all I,S channels of the nucleon-nucleon system, and we compare our results for
the equivalent short-range potentials to the core potentials found by other
authors using nonperturbative methods. We also apply our perturbative
techniques to the N and systems: Some
channels are found to have attractive core potentials and may accommodate
``molecular" bound states near threshold. Finally we use our Born formalism to
calculate the NN differential cross section, which we compare with experimental
results for unpolarised proton-proton elastic scattering. We find that several
familiar features of the experimental differential cross section are reproduced
by our Born-order result.Comment: 27 pages, figures available from the authors, revtex, CEBAF-TH-93-04,
MIT-CTP-2187, ORNL-CCIP-93-0
Ramond-Ramond Fields, Fractional Branes and Orbifold Differential K-Theory
We study D-branes and Ramond-Ramond fields on global orbifolds of Type II
string theory with vanishing H-flux using methods of equivariant K-theory and
K-homology. We illustrate how Bredon equivariant cohomology naturally realizes
stringy orbifold cohomology. We emphasize its role as the correct cohomological
tool which captures known features of the low-energy effective field theory,
and which provides new consistency conditions for fractional D-branes and
Ramond-Ramond fields on orbifolds. We use an equivariant Chern character from
equivariant K-theory to Bredon cohomology to define new Ramond-Ramond couplings
of D-branes which generalize previous examples. We propose a definition for
groups of differential characters associated to equivariant K-theory. We derive
a Dirac quantization rule for Ramond-Ramond fluxes, and study flat
Ramond-Ramond potentials on orbifolds.Comment: 46 pages; v2: typos correcte
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