15,257 research outputs found
Interference in interacting quantum dots with spin
We study spectral and transport properties of interacting quantum dots with
spin. Two particular model systems are investigated: Lateral multilevel and two
parallel quantum dots. In both cases different paths through the system can
give rise to interference. We demonstrate that this strengthens the multilevel
Kondo effect for which a simple two-stage mechanism is proposed. In parallel
dots we show under which conditions the peak of an interference-induced orbital
Kondo effect can be split.Comment: 8 pages, 8 figure
Of Higgs, Unitarity and other Questions
On the verge of conclusive checks on the Standard Model by the LHC, we
discuss some of the basic assumptions. The reason for this analysis stems from
a recent proposal of an Electroweak Model based on a nonlinearly realized gauge
group SU(2) X U(1), where, in the perturbative approximation, there is no Higgs
boson. The model enjoys the Slavnov-Taylor identities and therefore the
perturbative unitarity. On the other hand, it is commonly believed that the
existence of the Higgs boson is entangled with the property of unitarity, when
high energy processes are considered. The argument is based mostly on the
Froissart bound and on the Equivalence Theorem. In this talk we briefly review
some of our objections on the validity of such arguments. Some open questions
are pointed out, in particular on the limit of zero mass for the vector mesons
and on the fate of the longitudinal polarizations.Comment: 23 pages, 1 figure, presented by Ruggero Ferrari at the International
Conference "Gauge Fields. Yesterday, Today, Tomorrow" in honor of A.A.
Slavnov. Moscow, January 19-24 201
Chern-Simons Field Theories with Non-semisimple Gauge Group of Symmetry
Subject of this work is a class of Chern-Simons field theories with
non-semisimple gauge group, which may well be considered as the most
straightforward generalization of an Abelian Chern-Simons field theory. As a
matter of fact these theories, which are characterized by a non-semisimple
group of gauge symmetry, have cubic interactions like those of non-abelian
Chern-Simons field theories, but are free from radiative corrections. Moreover,
at the tree level in the perturbative expansion,there are only two connected
tree diagrams, corresponding to the propagator and to the three vertex
originating from the cubic interaction terms. For such theories it is derived
here a set of BRST invariant observables, which lead to metric independent
amplitudes. The vacuum expectation values of these observables can be computed
exactly. From their expressions it is possible to isolate the Gauss linking
number and an invariant of the Milnor type, which describes the topological
relations among three or more closed curves.Comment: 16 pages, 1 figure, plain LaTeX + psfig.st
Shock Profiles for the Asymmetric Simple Exclusion Process in One Dimension
The asymmetric simple exclusion process (ASEP) on a one-dimensional lattice
is a system of particles which jump at rates and (here ) to
adjacent empty sites on their right and left respectively. The system is
described on suitable macroscopic spatial and temporal scales by the inviscid
Burgers' equation; the latter has shock solutions with a discontinuous jump
from left density to right density , , which
travel with velocity . In the microscopic system we
may track the shock position by introducing a second class particle, which is
attracted to and travels with the shock. In this paper we obtain the time
invariant measure for this shock solution in the ASEP, as seen from such a
particle. The mean density at lattice site , measured from this particle,
approaches at an exponential rate as , with a
characteristic length which becomes independent of when
. For a special value of the
asymmetry, given by , the measure is
Bernoulli, with density on the left and on the right. In the
weakly asymmetric limit, , the microscopic width of the shock
diverges as . The stationary measure is then essentially a
superposition of Bernoulli measures, corresponding to a convolution of a
density profile described by the viscous Burgers equation with a well-defined
distribution for the location of the second class particle.Comment: 34 pages, LaTeX, 2 figures are included in the LaTeX file. Email:
[email protected], [email protected], [email protected]
Multivalued Fields on the Complex Plane and Conformal Field Theories
In this paper a class of conformal field theories with nonabelian and
discrete group of symmetry is investigated. These theories are realized in
terms of free scalar fields starting from the simple systems and scalar
fields on algebraic curves. The Knizhnik-Zamolodchikov equations for the
conformal blocks can be explicitly solved. Besides of the fact that one obtains
in this way an entire class of theories in which the operators obey a
nonstandard statistics, these systems are interesting in exploring the
connection between statistics and curved space-times, at least in the two
dimensional case.Comment: (revised version), 30 pages + one figure (not included), (requires
harvmac.tex), LMU-TPW 92-1
Colliding axisymmetric pp-waves
An exact solution is found describing the collision of axisymmetric pp-waves
with M=0. They are impulsive in character and their coordinate singularities
become point curvature singularities at the boundaries of the interaction
region. The solution is conformally flat. Concrete examples are given,
involving an ultrarelativistic black hole against a burst of pure radiation or
two colliding beam- like waves.Comment: 6 pages, REVTeX, some misprints are correcte
Bethe Ansatz Solution for a Defect Particle in the Asymmetric Exclusion Process
The asymmetric exclusion process on a ring in one-dimension is considered
with a single defect particle. The steady state has previously been solved by a
matrix product method. Here we use the Bethe ansatz to solve exactly for the
long time limit behaviour of the generating function of the distance travelled
by the defect particle. This allows us to recover steady state properties known
from the matrix approach such as the velocity, and obtain new results such as
the diffusion constant of the defect particle. In the case where the defect
particle is a second class particle we determine the large deviation function
and show that in a certain range the distribution of the distance travelled
about the mean is Gaussian. Moreover the variance (diffusion constant) grows as
L to the power 1/2 where is the system size. This behaviour can be related to
the superdiffusive spreading of excess mass fluctuations on an infinite system.
In the case where the defect particle produces a shock, our expressions for the
velocity and the diffusion constant coincide with those calculated previously
for an infinite system by Ferrari and Fontes.Comment: Latex, 23 page
Fractional Aharonov-Bohm effect in mesoscopic rings
We study the effects of correlations on a one dimensional ring threaded by a
uniform magnetic flux. In order to describe the interaction between particles,
we work in the framework of the U Hubbard and - models. We focus
on the dilute limit. Our results suggest the posibility that the persistent
current has an anomalous periodicity , where is an integer in
the range ( is the number of particles in the ring
and is the flux quantum). We found that this result depends neither
on disorder nor on the detailed form of the interaction, while remains the on
site infinite repulsion.Comment: 14 pages (Revtex), 5 postscript figures. Send e-mail to:
[email protected]
A Massive Yang-Mills Theory based on the Nonlinearly Realized Gauge Group
We propose a subtraction scheme for a massive Yang-Mills theory realized via
a nonlinear representation of the gauge group (here SU(2)). It is based on the
subtraction of the poles in D-4 of the amplitudes, in dimensional
regularization, after a suitable normalization has been performed. Perturbation
theory is in the number of loops and the procedure is stable under iterative
subtraction of the poles. The unphysical Goldstone bosons, the Faddeev-Popov
ghosts and the unphysical mode of the gauge field are expected to cancel out in
the unitarity equation. The spontaneous symmetry breaking parameter is not a
physical variable. We use the tools already tested in the nonlinear sigma
model: hierarchy in the number of Goldstone boson legs and weak power-counting
property (finite number of independent divergent amplitudes at each order). It
is intriguing that the model is naturally based on the symmetry SU(2)_L local
times SU(2)_R global. By construction the physical amplitudes depend on the
mass and on the self-coupling constant of the gauge particle and moreover on
the scale parameter of the radiative corrections. The Feynman rules are in the
Landau gauge.Comment: 44 pages, 1 figure, minor changes, final version accepted by Phys.
Rev.
Quasi-normal modes of charged, dilaton black holes
In this paper we study the perturbations of the charged, dilaton black hole,
described by the solution of the low energy limit of the superstring action
found by Garfinkle, Horowitz and Strominger. We compute the complex frequencies
of the quasi-normal modes of this black hole, and compare the results with
those obtained for a Reissner-Nordstr\"{o}m and a Schwarzschild black hole. The
most remarkable feature which emerges from this study is that the presence of
the dilaton breaks the \emph{isospectrality} of axial and polar perturbations,
which characterizes both Schwarzschild and Reissner-Nordstr\"{o}m black holes.Comment: 15 pages, 5 figure
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