933 research outputs found
A Construction of Solutions to Reflection Equations for Interaction-Round-a-Face Models
We present a procedure in which known solutions to reflection equations for
interaction-round-a-face lattice models are used to construct new solutions.
The procedure is particularly well-suited to models which have a known fusion
hierarchy and which are based on graphs containing a node of valency . Among
such models are the Andrews-Baxter-Forrester models, for which we construct
reflection equation solutions for fixed and free boundary conditions.Comment: 9 pages, LaTe
Gauges and Cosmological Backreaction
We present a formalism for spatial averaging in cosmology applicable to
general spacetimes and coordinates, and allowing the easy incorporation of a
wide variety of matter sources. We apply this formalism to a
Friedmann-LeMaitre-Robertson-Walker universe perturbed to second-order and
present the corrections to the background in an unfixed gauge. We then present
the corrections that arise in uniform curvature and conformal Newtonian gauges.Comment: 13 pages. Updated: reference added, typos corrected, exposition
clarified. Version 3: Replaced with version published by JCA
Curve counting via stable pairs in the derived category
For a nonsingular projective 3-fold , we define integer invariants
virtually enumerating pairs where is an embedded curve and
is a divisor. A virtual class is constructed on the associated
moduli space by viewing a pair as an object in the derived category of . The
resulting invariants are conjecturally equivalent, after universal
transformations, to both the Gromov-Witten and DT theories of . For
Calabi-Yau 3-folds, the latter equivalence should be viewed as a wall-crossing
formula in the derived category.
Several calculations of the new invariants are carried out. In the Fano case,
the local contributions of nonsingular embedded curves are found. In the local
toric Calabi-Yau case, a completely new form of the topological vertex is
described.
The virtual enumeration of pairs is closely related to the geometry
underlying the BPS state counts of Gopakumar and Vafa. We prove that our
integrality predictions for Gromov-Witten invariants agree with the BPS
integrality. Conversely, the BPS geometry imposes strong conditions on the
enumeration of pairs.Comment: Corrected typos and duality error in Proposition 4.6. 47 page
Cosmological Backreaction from Perturbations
We reformulate the averaged Einstein equations in a form suitable for use
with Newtonian gauge linear perturbation theory and track the size of the
modifications to standard Robertson-Walker evolution on the largest scales as a
function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In
both cases the effective energy density arising from linear perturbations is of
the order of 10^-5 the matter density, as would be expected, with an effective
equation of state w ~ -1/19. Employing a modified Halofit code to extend our
results to quasilinear scales, we find that, while larger, the deviations from
Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA
The charges of a twisted brane
The charges of the twisted D-branes of certain WZW models are determined. The
twisted D-branes are labelled by twisted representations of the affine algebra,
and their charge is simply the ground state multiplicity of the twisted
representation. It is shown that the resulting charge group is isomorphic to
the charge group of the untwisted branes, as had been anticipated from a
K-theory calculation. Our arguments rely on a number of non-trivial Lie
theoretic identities.Comment: 27 pages, 1 figure, harvmac (b
Conformal Field Theories, Graphs and Quantum Algebras
This article reviews some recent progress in our understanding of the
structure of Rational Conformal Field Theories, based on ideas that originate
for a large part in the work of A. Ocneanu. The consistency conditions that
generalize modular invariance for a given RCFT in the presence of various types
of boundary conditions --open, twisted-- are encoded in a system of integer
multiplicities that form matrix representations of fusion-like algebras. These
multiplicities are also the combinatorial data that enable one to construct an
abstract ``quantum'' algebra, whose - and -symbols contain essential
information on the Operator Product Algebra of the RCFT and are part of a cell
system, subject to pentagonal identities. It looks quite plausible that the
classification of a wide class of RCFT amounts to a classification of ``Weak
- Hopf algebras''.Comment: 23 pages, 12 figures, LateX. To appear in MATHPHYS ODYSSEY 2001
--Integrable Models and Beyond, ed. M. Kashiwara and T. Miwa, Progress in
Math., Birkhauser. References and comments adde
Averaging Robertson-Walker Cosmologies
The cosmological backreaction arises when one directly averages the Einstein
equations to recover an effective Robertson-Walker cosmology, rather than
assuming a background a priori. While usually discussed in the context of dark
energy, strictly speaking any cosmological model should be recovered from such
a procedure. We apply the Buchert averaging formalism to linear
Robertson-Walker universes containing matter, radiation and dark energy and
evaluate numerically the discrepancies between the assumed and the averaged
behaviour, finding the largest deviations for an Einstein-de Sitter universe,
increasing rapidly with Hubble rate to a 0.01% effect for h=0.701. For the LCDM
concordance model, the backreaction is of the order of Omega_eff~4x10^-6, with
those for dark energy models being within a factor of two or three. The impacts
at recombination are of the order of 10^-8 and those in deep radiation
domination asymptote to a constant value. While the effective equations of
state of the backreactions in Einstein-de Sitter, concordance and quintessence
models are generally dust-like, a backreaction with an equation of state
w_eff<-1/3 can be found for strongly phantom models.Comment: 18 pages, 11 figures, ReVTeX. Updated to version accepted by JCA
Exact solution and surface critical behaviour of open cyclic SOS lattice models
We consider the -state cyclic solid-on-solid lattice models under a class
of open boundary conditions. The integrable boundary face weights are obtained
by solving the reflection equations. Functional relations for the fused
transfer matrices are presented for both periodic and open boundary conditions.
The eigen-spectra of the unfused transfer matrix is obtained from the
functional relations using the analytic Bethe ansatz. For a special case of
crossing parameter , the finite-size corrections to the
eigen-spectra of the critical models are obtained, from which the corresponding
conformal dimensions follow. The calculation of the surface free energy away
from criticality yields two surface specific heat exponents,
and , where
coprime to . These results are in agreement with the scaling relations
and .Comment: 13 pages, LaTeX, to appear in J. Phys.
Theoretical study of incoherent phi photoproduction on a deuteron target
We study the photoproduction of phi mesons in deuteron, paying attention to
the modification of the cross section from bound protons to the free ones with
the aim of comparing with recent results at LEPS. For this purpose we take into
account Fermi motion in single scattering and rescattering of the phi to
account for phi absorption on a second nucleon as well as the rescattering of
the proton. We find that the contribution of the double scattering is much
smaller than the typical cross section of gamma p to phi p in free space, which
implies a very small screening of the phi production in deuteron. The
contribution from the proton rescattering, on the other hand, is found to be
not negligible compared to the cross section of gamma p to phi p in free space,
and leads to a moderate reduction of the phi photoproduction cross section on a
deuteron at forward angles if LEPS set up is taken into account. The Fermi
motion allows contribution of the single scattering in regions forbidden by
phase space in the free case. In particular, we find that for momentum
transferred squared close to the maximum value, the Fermi motion changes
drastically the shape of d sigma / dt, to the point that the ratio of this
cross section to the free one becomes very sensitive to the precise value of t
chosen, or the size of the bin used in an experimental analysis. Hence, this
particular region of t does not seem the most indicated to find effects of a
possible phi absorption in the deuteron. This reaction is studied theoretically
as a function of t and the effect of the experimental angular cuts at LEPS is
also discussed, providing guidelines for future experimental analyses of the
reaction.Comment: 17 pages, 16 figure
Orientifolds, Unoriented Instantons and Localization
We consider world-sheet instanton effects in N=1 string orientifolds of
noncompact toric Calabi-Yau threefolds. We show that unoriented closed string
topological amplitudes can be exactly computed using localization techniques
for holomorphic maps with involution. Our results are in precise agreement with
mirror symmetry and large N duality predictions.Comment: 25 pages, 10 figures, published version; v4: typos correcte
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