1,613 research outputs found
The planar spectrum in U(N)-invariant quantum mechanics by Fock space methods: I. The bosonic case
Prompted by recent results on Susy-U(N)-invariant quantum mechanics in the
large N limit by Veneziano and Wosiek, we have examined the planar spectrum in
the full Hilbert space of U(N)-invariant states built on the Fock vacuum by
applying any U(N)-invariant combinations of creation-operators. We present
results about 1) the supersymmetric model in the bosonic sector, 2) the
standard quartic Hamiltonian. This latter is useful to check our techniques
against the exact result of Brezin et al. The SuSy case is where Fock space
methods prove to be the most efficient: it turns out that the problem is
separable and the exact planar spectrum can be expressed in terms of the
single-trace spectrum. In the case of the anharmonic oscillator, on the other
hand, the Fock space analysis is quite cumbersome due to the presence of large
off-diagonal O(N) terms coupling subspaces with different number of traces;
these terms should be absorbed before taking the planar limit and recovering
the known planar spectrum. We give analytical and numerical evidence that good
qualitative information on the spectrum can be obtained this way.Comment: 17 pages, 4 figures, uses youngtab.sty. Final versio
Relationship between resistivity and specific heat in a canonical non-magnetic heavy fermion alloy system: UPt_5-xAu_x
UPt_(5-x)Au_x alloys form in a single crystal structure, cubic AuBe_5-type,
over a wide range of concentrations from x = 0 to at least x = 2.5. All
investigated alloys, with an exception for x = 2.5, were non-magnetic. Their
electronic specific heat coefficient varies from about 60 (x = 2) to
about 700 mJ/mol K^2 (x = 1). The electrical resistivity for all alloys has a
Fermi-liquid-like temperature variation, \rho = \rho_o + AT^2, in the limit of
T -> 0 K. The coefficient A is strongly enhanced in the heavy-fermion regime in
comparison with normal and transition metals. It changes from about 0.01 (x =
0) to over 2 micro-ohm cm/K^2 (x = 1). A/\gamma^2, which has been postulated to
have a universal value for heavy-fermions, varies from about 10^-6 (x = 0, 0.5)
to 10^-5 micro-ohm cm (mol K/mJ)^2 (x > 1.1), thus from a value typical of
transition metals to that found for some other heavy-fermion metals. This ratio
is unaffected, or only weakly affected, by chemical or crystallographic
disorder. It correlates with the paramagnetic Curie-Weiss temperature of the
high temperature magnetic susceptibility.Comment: 5 pages, 5 eps figures, RevTe
The Phase Structure of the Polyakov--Quark-Meson Model
The relation between the deconfinement and chiral phase transition is
explored in the framework of an Polyakov-loop-extended two-flavor quark-meson
(PQM) model. In this model the Polyakov loop dynamics is represented by a
background temporal gauge field which also couples to the quarks. As a novelty
an explicit quark chemical potential and N_f-dependence in the Polyakov loop
potential is proposed by using renormalization group arguments. The behavior of
the Polyakov loop as well as the chiral condensate as function of temperature
and quark chemical potential is obtained by minimizing the grand canonical
thermodynamic potential of the system. The effect of the Polyakov loop dynamics
on the chiral phase diagram and on several thermodynamic bulk quantities is
presented.Comment: 13 pages, 12 figures, RevTex4; discussion of mu-dependence extended,
references added, version to be published in PR
Group Field Theory: An overview
We give a brief overview of the properties of a higher dimensional
generalization of matrix model which arises naturally in the context of a
background independent approach to quantum gravity, the so called group field
theory. We show that this theory leads to a natural proposal for the physical
scalar product of quantum gravity. We also show in which sense this theory
provides a third quantization point of view on quantum gravity.Comment: 10 page
Hamiltonian Analysis of non-chiral Plebanski Theory and its Generalizations
We consider non-chiral, full Lorentz group-based Plebanski formulation of
general relativity in its version that utilizes the Lagrange multiplier field
Phi with "internal" indices. The Hamiltonian analysis of this version of the
theory turns out to be simpler than in the previously considered in the
literature version with Phi carrying spacetime indices. We then extend the
Hamiltonian analysis to a more general class of theories whose action contains
scalars invariants constructed from Phi. Such theories have recently been
considered in the context of unification of gravity with other forces. We show
that these more general theories have six additional propagating degrees of
freedom as compared to general relativity, something that has not been
appreciated in the literature treating them as being not much different from
GR.Comment: 10 page
Exact and semiclassical approach to a class of singular integral operators arising in fluid mechanics and quantum field theory
A class of singular integral operators, encompassing two physically relevant
cases arising in perturbative QCD and in classical fluid dynamics, is presented
and analyzed. It is shown that three special values of the parameters allow for
an exact eigenfunction expansion; these can be associated to Riemannian
symmetric spaces of rank one with positive, negative or vanishing curvature.
For all other cases an accurate semiclassical approximation is derived, based
on the identification of the operators with a peculiar Schroedinger-like
operator.Comment: 12 pages, 1 figure, amslatex, bibtex (added missing label eq.11
Plebanski Theory and Covariant Canonical Formulation
We establish an equivalence between the Hamiltonian formulation of the
Plebanski action for general relativity and the covariant canonical formulation
of the Hilbert-Palatini action. This is done by comparing the symplectic
structures of the two theories through the computation of Dirac brackets. We
also construct a shifted connection with simplified Dirac brackets, playing an
important role in the covariant loop quantization program, in the Plebanski
framework. Implications for spin foam models are also discussed.Comment: 18 page
The unexpected survival of an ancient lineage of anseriform birds into the Neogene of Australia: the youngest record of Presbyornithidae
. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.Presbyornithids were the dominant birds in Palaeogene lacustrine assemblages, especially in the Northern Hemisphere, but are thought to have disappeared worldwide by the mid-Eocene. Now classified within Anseriformes (screamers, ducks, swans and geese), their relationships have long been obscured by their strange wader-like skeletal morphology. Reassessment of the late Oligocene South Australian material attributed to Wilaru tedfordi, long considered to be of a stone-curlew (Burhinidae, Charadriiformes), reveals that this taxon represents the first record of a presbyornithid in Australia. We also describe the larger Wilaru prideauxi sp. nov. from the early Miocene of South Australia, showing that presbyornithids survived in Australia at least until ca 22 Ma. Unlike on other continents, where presbyornithids were replaced by aquatic crown-group anatids (ducks, swans and geese), species of Wilaru lived alongside these waterfowl in Australia. The morphology of the tarsometatarsus of these species indicates that, contrary to other presbyornithids, they were predominantly terrestrial birds, which probably contributed to their long-term survival in Australia. The morphological similarity between species of Wilaru and the Eocene South American presbyornithid Telmabates antiquus supports our hypothesis of a Gondwanan radiation during the evolutionary history of the Presbyornithidae. Teviornis gobiensis from the Late Cretaceous of Mongolia is here also reassessed and confirmed as a presbyornithid. These findings underscore the temporal continuance of Australia’s vertebrates and provide a new context in which the phylogeny and evolutionary history of presbyornithids can be examined
Consistently Solving the Simplicity Constraints for Spinfoam Quantum Gravity
We give an independent derivation of the Engle-Pereira-Rovelli spinfoam model
for quantum gravity which recently appeared in [arXiv:0705.2388]. Using the
coherent state techniques introduced earlier in [arXiv:0705.0674], we show that
the EPR model realizes a consistent imposition of the simplicity constraints
implementing general relativity from a topological BF theory.Comment: 6 pages, 2 figures, v2: typos correcte
Revisiting the Simplicity Constraints and Coherent Intertwiners
In the context of loop quantum gravity and spinfoam models, the simplicity
constraints are essential in that they allow to write general relativity as a
constrained topological BF theory. In this work, we apply the recently
developed U(N) framework for SU(2) intertwiners to the issue of imposing the
simplicity constraints to spin network states. More particularly, we focus on
solving them on individual intertwiners in the 4d Euclidean theory. We review
the standard way of solving the simplicity constraints using coherent
intertwiners and we explain how these fit within the U(N) framework. Then we
show how these constraints can be written as a closed u(N) algebra and we
propose a set of U(N) coherent states that solves all the simplicity
constraints weakly for an arbitrary Immirzi parameter.Comment: 28 page
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