427 research outputs found
Free field representation of Toda field theories
We study the following problem: can a classical Toda field theory be
represented by means of free bosonic oscillators through a Drinfeld--Sokolov
construction? We answer affirmatively in the case of a cylindrical space--time
and for real hyperbolic solutions of the Toda field equations. We establish in
fact a one--to--one correspondence between such solutions and the space of free
left and right bosonic oscillators with coincident zero modes. We discuss the
same problem for real singular solutions with non hyperbolic monodromy.Comment: 29 pages, Latex, SISSA-ISAS 210/92/E
Strings in Yang-Mills-Higgs theory coupled to gravity
Non-Abelian strings for an Einstein-Yang-Mills-Higgs theory are explicitly
constructed. We consider N_f Higgs fields in the fundamental representation of
the U(1)xSU(N_c) gauge group in order to have a color-flavor SU(N_c) group
remaining unbroken. Choosing a suitable ansatz for the metric, Bogomol'nyi-like
first order equations are found and rotationally symmetric solutions are
proposed. In the N_f = N_c case, solutions are local strings and are shown to
be truly non-Abelian by parameterizing them in terms of orientational
collective coordinates. When N_f > N_c, the solutions correspond to semilocal
strings which, beside the orientational degrees of freedom, acquire additional
collective coordinates parameterizing their transverse size. The low-energy
effective theories for the correspondent moduli are found, showing that all
zero modes are normalizable in presence of gravity, even in the semilocal case.Comment: 20 pages, no figure, modified version with new title, abstract and an
additional section completing the study of effective theories. Physical
Review D in pres
Fermion Helicity Flip in Weak Gravitational Fields
The helicity flip of a spin- Dirac particle
interacting gravitationally with a scalar field is analyzed in the context of
linearized quantum gravity. It is shown that massive fermions may have their
helicity flipped by gravity, in opposition to massless fermions which preserve
their helicity.Comment: RevTeX 3.0, 8 pages, 3 figures (available upon request), Preprint
IFT-P.013/9
de Sitter special relativity
A special relativity based on the de Sitter group is introduced, which is the
theory that might hold up in the presence of a non-vanishing cosmological
constant. Like ordinary special relativity, it retains the quotient character
of spacetime, and a notion of homogeneity. As a consequence, the underlying
spacetime will be a de Sitter spacetime, whose associated kinematics will
differ from that of ordinary special relativity. The corresponding modified
notions of energy and momentum are obtained, and the exact relationship between
them, which is invariant under a re-scaling of the involved quantities,
explicitly exhibited. Since the de Sitter group can be considered a particular
deformation of the Poincar\'e group, this theory turns out to be a specific
kind of deformed (or doubly) special relativity. Some experimental
consequences, as well as the causal structure of spacetime--modified by the
presence of the de Sitter horizon--are briefly discussed.Comment: V2: Some presentation changes; a new section introduced, with a
discussion about possible phenomenological consequences; new references
added; version to be published in Classical and Quantum Gravit
Toda Fields on Riemann Surfaces: remarks on the Miura transformation
We point out that the Miura transformation is related to a holomorphic
foliation in a relative flag manifold over a Riemann Surface. Certain
differential operators corresponding to a free field description of
--algebras are thus interpreted as partial connections associated to the
foliation.Comment: AmsLatex 1.1, 10 page
Extensions of Picard 2-Stacks and the cohomology groups Ext^i of length 3 complexes
The aim of this paper is to define and study the 3-category of extensions of
Picard 2-stacks over a site S and to furnish a geometrical description of the
cohomology groups Ext^i of length 3 complexes of abelian sheaves. More
precisely, our main Theorem furnishes
(1) a parametrization of the equivalence classes of objects, 1-arrows,
2-arrows, and 3-arrows of the 3-category of extensions of Picard 2-stacks by
the cohomology groups Ext^i, and
(2) a geometrical description of the cohomology groups Ext^i of length 3
complexes of abelian sheaves via extensions of Picard 2-stacks.
To this end, we use the triequivalence between the 3-category of Picard
2-stacks and the tricategory T^[-2,0](S) of length 3 complexes of abelian
sheaves over S introduced by the second author in arXiv:0906.2393, and we
define the notion of extension in this tricategory T^[-2,0](S), getting a pure
algebraic analogue of the 3-category of extensions of Picard 2-stacks. The
calculus of fractions that we use to define extensions in the tricategory
T^[-2,0](S) plays a central role in the proof of our Main Theorem.Comment: 2 New Appendix: in the first Appendix we compute a long exact
sequence involving the homotopy groups of an extension of Picard 2-stacks,
and in the second Appendix we sketch the proof that the fibered sum of Picard
2-stacks satisfies the universal propert
Retrospective and prospective evaluations of mammography screening narratives: The role of own experience
We investigated the role of previous experience when providing summary judgments of mammography narratives. A total of 807 women who either did or did not have previous experience of a mammogram were presented with a written description of a mammography visit. We manipulated the presentation position of a negative element within the narrative to alter its accessibility in memory and determine whether the latter impacted equally on two types of summary judgments. After the narrative presentation, participants were asked to provide both retrospective and prospective evaluations, that is, summary judgments about the described event and an appraisal of the likelihood of participating in future instances of such event, respectively. A recency effect was observed only for retrospective but not for prospective evaluations. When examined only for the subset of women who had undergone a mammography visit themselves, prospective evaluations were shown to be predicted by the reported quality of the mammography participants experienced themselves. The findings support and extend the accessibility model of emotional self-report and suggest that own experience leaks into evaluations of hypothetical scenarios by selectively impacting on prospective evaluations
Abundances and Physical Conditions in the Interstellar Gas toward HD 192 639
We present a study of the abundances and physical conditions in the
interstellar gas toward the heavily reddened star HD 192639 [E_(B-V) = 0.64],
based on analysis of FUSE and HST/STIS spectra covering the range from 912 to
1361 A. This work constitutes a survey of the analyses that can be performed to
study the interstellar gas when combining data from different instruments.
Low-velocity (-18 to -8 km/s) components are seen primarily for various neutral
and singly ionized species such as C I, O I, S I, Mg II, Cl I, Cl II, Mn II, Fe
II and Cu II. Numerous lines of H2 are present in the FUSE spectra, with a
kinetic temperature for the lowest rotational levels T_(01) = (90 +/- 10) K.
Analysis of the C I fine-structure excitation implies an average local density
of hydrogen n_H = (16 +/- 3) cm^-3. The average electron density, derived from
five neutral/first ion pairs under the assumption of photoionization
equilibrium, is n_e = (0.11 +/- 0.02) cm^-3. The relatively complex component
structure seen in high-resolution spectra of K I and Na I, the relatively low
average density, and the measured depletions all suggest that the line of sight
contains a number of diffuse clouds, rather than a single dense, translucent
cloud. Comparisons of the fractions of Cl in Cl I and of hydrogen in molecular
form suggest a higher molecular fraction, in the region(s) where H2 is present,
than that derived considering the average line of sight. In general, such
comparisons may allow the identification and characterization of translucent
portions of such complex lines of sight. The combined data also show
high-velocity components near -80 km/s for various species which appear to be
predominantly ionized, and may be due to a radiative shock. A brief overview of
the conditions in this gas will be given.Comment: 37 pages, accepted for publication in Ap
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