755 research outputs found
The Fubini-Furlan-Rossetti Sum Rule Revisited
The Fubini-Furlan-Rossetti sum rule for pion photoproduction on the nucleon
is evaluated by dispersion relations at constant t, and the corrections to the
sum rule due to the finite pion mass are calculated. Near threshold these
corrections turn out to be large due to pion-loop effects, whereas the sum rule
value is closely approached if the dispersion integrals are evaluated for
sub-threshold kinematics. This extension to the unphysical region provides a
unique framework to determine the low-energy constants of chiral perturbation
theory by global properties of the excitation spectrum.Comment: 12 pages, 7 postscript figures, EPJ style files include
The Orbifolds of Permutation-Type as Physical String Systems at Multiples of c=26 IV. Orientation Orbifolds Include Orientifolds
In this fourth paper of the series, I clarify the somewhat mysterious
relation between the large class of {\it orientation orbifolds} (with twisted
open-string CFT's at ) and {\it orientifolds} (with untwisted open
strings at ), both of which have been associated to division by
world-sheet orientation-reversing automorphisms. In particular -- following a
spectral clue in the previous paper -- I show that, even as an {\it interacting
string system}, a certain half-integer-moded orientation orbifold-string system
is in fact equivalent to the archetypal orientifold. The subtitle of this
paper, that orientation orbifolds include and generalize standard orientifolds,
then follows because there are many other orientation orbifold-string systems
-- with higher fractional modeing -- which are not equivalent to untwisted
string systems.Comment: 22 pages, typos correcte
Affine Lie Algebras in Massive Field Theory and Form-Factors from Vertex Operators
We present a new application of affine Lie algebras to massive quantum field
theory in 2 dimensions, by investigating the limit of the q-deformed
affine symmetry of the sine-Gordon theory, this limit occurring
at the free fermion point. Working in radial quantization leads to a
quasi-chiral factorization of the space of fields. The conserved charges which
generate the affine Lie algebra split into two independent affine algebras on
this factorized space, each with level 1 in the anti-periodic sector, and level
in the periodic sector. The space of fields in the anti-periodic sector can
be organized using level- highest weight representations, if one supplements
the \slh algebra with the usual local integrals of motion. Introducing a
particle-field duality leads to a new way of computing form-factors in radial
quantization. Using the integrals of motion, a momentum space bosonization
involving vertex operators is formulated. Form-factors are computed as vacuum
expectation values in momentum space. (Based on talks given at the Berkeley
Strings 93 conference, May 1993, and the III International Conference on
Mathematical Physics, String Theory, and Quantum Gravity, Alushta, Ukraine,
June 1993.)Comment: 13 pages, CLNS 93/125
Assessment of the potential respiratory hazard of volcanic ash from future Icelandic eruptions: A study of archived basaltic to rhyolitic ash samples
Relation between concurrence and Berry phase of an entangled state of two spin 1/2 particles
We have studied here the influence of the Berry phase generated due to a
cyclic evolution of an entangled state of two spin 1/2 particles. It is shown
that the measure of formation of entanglement is related to the cyclic
geometric phase of the individual spins. \\Comment: 6 pages. Accepted in Europhys. Letters (likely to be published in vol
73, pp1-6 (2006)
Effective boost and "point-form" approach
Triangle Feynman diagrams can be considered as describing form factors of
states bound by a zero-range interaction. These form factors are calculated for
scalar particles and compared to point-form and non-relativistic results. By
examining the expressions of the complete calculation in different frames, we
obtain an effective boost transformation which can be compared to the
relativistic kinematical one underlying the present point-form calculations, as
well as to the Galilean boost. The analytic expressions obtained in this simple
model allow a qualitative check of certain results obtained in similar studies.
In particular, a mismatch is pointed out between recent practical applications
of the point-form approach and the one originally proposed by Dirac.Comment: revised version as accepted for publicatio
Two-spin entanglement distribution near factorized states
We study the two-spin entanglement distribution along the infinite
chain described by the XY model in a transverse field; closed analytical
expressions are derived for the one-tangle and the concurrences ,
being the distance between the two possibly entangled spins, for values of the
Hamiltonian parameters close to those corresponding to factorized ground
states. The total amount of entanglement, the fraction of such entanglement
which is stored in pairwise entanglement, and the way such fraction distributes
along the chain is discussed, with attention focused on the dependence on the
anisotropy of the exchange interaction. Near factorization a characteristic
length-scale naturally emerges in the system, which is specifically related
with entanglement properties and diverges at the critical point of the fully
isotropic model. In general, we find that anisotropy rule a complex behavior of
the entanglement properties, which results in the fact that more isotropic
models, despite being characterized by a larger amount of total entanglement,
present a smaller fraction of pairwise entanglement: the latter, in turn, is
more evenly distributed along the chain, to the extent that, in the fully
isotropic model at the critical field, the concurrences do not depend on .Comment: 14 pages, 6 figures. Final versio
Adiabatic multicritical quantum quenches: Continuously varying exponents depending on the direction of quenching
We study adiabatic quantum quenches across a quantum multicritical point
(MCP) using a quenching scheme that enables the system to hit the MCP along
different paths. We show that the power-law scaling of the defect density with
the rate of driving depends non-trivially on the path, i.e., the exponent
varies continuously with the parameter that defines the path, up to a
critical value ; on the other hand for , the scaling exponent saturates to a constant value. We show that
dynamically generated and {\it path()-dependent} effective critical
exponents associated with the quasicritical points lying close to the MCP (on
the ferromagnetic side), where the energy-gap is minimum, lead to this
continuously varying exponent. The scaling relations are established using the
integrable transverse XY spin chain and generalized to a MCP associated with a
-dimensional quantum many-body systems (not reducible to two-level systems)
using adiabatic perturbation theory. We also calculate the effective {\it
path-dependent} dimensional shift (or the shift in center of the
impulse region) that appears in the scaling relation for special paths lying
entirely in the paramagnetic phase. Numerically obtained results are in good
agreement with analytical predictions.Comment: 5 pages, 4 figure
Dynamics of an inhomogeneous quantum phase transition
We argue that in a second order quantum phase transition driven by an
inhomogeneous quench density of quasiparticle excitations is suppressed when
velocity at which a critical point propagates across a system falls below a
threshold velocity equal to the Kibble-Zurek correlation length times the
energy gap at freeze-out divided by . This general prediction is
supported by an analytic solution in the quantum Ising chain. Our results
suggest, in particular, that adiabatic quantum computers can be made more
adiabatic when operated in an "inhomogeneous" way.Comment: 7 pages; version to appear in a special issue of New J. Phy
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