905 research outputs found
Self Consistent Expansion In The Presence Of Electroweak Interactions
In the conventional approach to the expansion, electroweak
interactions are switched off and large QCD is treated in isolation. We
study the self-consistency of taking the large limit in the presence of
electroweak interaction. If the electroweak coupling constants are held
constant, the large counting rules are violated by processes involving
internal photon or weak boson lines. Anomaly cancellations, however, fix the
ratio of electric charges of different fermions. This allows a self-consistent
way to scale down the electronic charge in the large limit and hence
restoring the validity of the large counting rules.Comment: 9 pages in REVTeX, no figure
PR-box correlations have no classical limit
One of Yakir Aharonov's endlessly captivating physics ideas is the conjecture
that two axioms, namely relativistic causality ("no superluminal signalling")
and nonlocality, so nearly contradict each other that a unique theory - quantum
mechanics - reconciles them. But superquantum (or "PR-box") correlations imply
that quantum mechanics is not the most nonlocal theory (in the sense of
nonlocal correlations) consistent with relativistic causality. Let us consider
supplementing these two axioms with a minimal third axiom: there exists a
classical limit in which macroscopic observables commute. That is, just as
quantum mechanics has a classical limit, so must any generalization of quantum
mechanics. In this classical limit, PR-box correlations violate relativistic
causality. Generalized to all stronger-than-quantum bipartite correlations,
this result is a derivation of Tsirelson's bound without assuming quantum
mechanics.Comment: for a video of this talk at the Aharonov-80 Conference in 2012 at
Chapman University, see quantum.chapman.edu/talk-10, published in Quantum
Theory: A Two-Time Success Story (Yakir Aharonov Festschrift), eds. D. C.
Struppa and J. M. Tollaksen (New York: Springer), 2013, pp. 205-21
Self-dual noncommutative \phi^4-theory in four dimensions is a non-perturbatively solvable and non-trivial quantum field theory
We study quartic matrix models with partition function Z[E,J]=\int dM
\exp(trace(JM-EM^2-(\lambda/4)M^4)). The integral is over the space of
Hermitean NxN-matrices, the external matrix E encodes the dynamics, \lambda>0
is a scalar coupling constant and the matrix J is used to generate correlation
functions. For E not a multiple of the identity matrix, we prove a universal
algebraic recursion formula which gives all higher correlation functions in
terms of the 2-point function and the distinct eigenvalues of E. The 2-point
function itself satisfies a closed non-linear equation which must be solved
case by case for given E. These results imply that if the 2-point function of a
quartic matrix model is renormalisable by mass and wavefunction
renormalisation, then the entire model is renormalisable and has vanishing
\beta-function.
As main application we prove that Euclidean \phi^4-quantum field theory on
four-dimensional Moyal space with harmonic propagation, taken at its
self-duality point and in the infinite volume limit, is exactly solvable and
non-trivial. This model is a quartic matrix model, where E has for N->\infty
the same spectrum as the Laplace operator in 4 dimensions. Using the theory of
singular integral equations of Carleman type we compute (for N->\infty and
after renormalisation of E,\lambda) the free energy density
(1/volume)\log(Z[E,J]/Z[E,0]) exactly in terms of the solution of a non-linear
integral equation. Existence of a solution is proved via the Schauder fixed
point theorem.
The derivation of the non-linear integral equation relies on an assumption
which we verified numerically for coupling constants 0<\lambda\leq (1/\pi).Comment: LaTeX, 64 pages, xypic figures. v4: We prove that recursion formulae
and vanishing of \beta-function hold for general quartic matrix models. v3:
We add the existence proof for a solution of the non-linear integral
equation. A rescaling of matrix indices was necessary. v2: We provide
Schwinger-Dyson equations for all correlation functions and prove an
algebraic recursion formula for their solutio
A Systematic Extended Iterative Solution for QCD
An outline is given of an extended perturbative solution of Euclidean QCD
which systematically accounts for a class of nonperturbative effects, while
allowing renormalization by the perturbative counterterms. Proper vertices
Gamma are approximated by a double sequence Gamma[r,p], with r the degree of
rational approximation w.r.t. the QCD mass scale Lambda, nonanalytic in the
coupling g, and p the order of perturbative corrections in g-squared,
calculated from Gamma[r,0] - rather than from the perturbative Feynman rules
Gamma(0)(pert) - as a starting point. The mechanism allowing the
nonperturbative terms to reproduce themselves in the Dyson-Schwinger equations
preserves perturbative renormalizability and is tied to the divergence
structure of the theory. As a result, it restricts the self-consistency problem
for the Gamma[r,0] rigorously - i.e. without decoupling approximations - to the
superficially divergent vertices. An interesting aspect of the scheme is that
rational-function sequences for the propagators allow subsequences describing
short-lived excitations. The method is calculational, in that it allows known
techniques of loop computation to be used while dealing with integrands of
truly nonperturbative content.Comment: 48 pages (figures included). Scope of replacement: correction of a
technical defect; no changes in conten
Gauge symmetry and the EMC spin effect
We emphasise the EMC spin effect as a problem of symmetry and discuss the
renormalisation of the axial tensor operators. This involves the
generalisation of the Adler-Bell-Jackiw anomaly to each of these operators. We
find that the contribution of the axial anomaly to the spin dependent structure
function scales at . This means that the anomaly
can be a large effect in . Finally we discuss the jet signature of the
anomaly.Comment: 17 pages, Latex, Cavendish preprint HEP 93/
A model for spin-polarized transport in perovskite manganite bi-crystal grain boundaries
We have studied the temperature dependence of low-field magnetoresistance and
current-voltage characteristics of a low-angle bi-crystal grain boundary
junction in perovskite manganite La_{2/3}Sr_{1/3}MnO_3 thin film. By gradually
trimming the junction we have been able to reveal the non-linear behavior of
the latter. With the use of the relation M_{GB} \propto M_{bulk}\sqrt{MR^*} we
have extracted the grain boundary magnetization. Further, we demonstrate that
the built-in potential barrier of the grain boundary can be modelled by
V_{bi}\propto M_{bulk}^2 - M_{GB}^2. Thus our model connects the
magnetoresistance with the potential barrier at the grain boundary region. The
results indicate that the band-bending at the grain boundary interface has a
magnetic origin.Comment: 9 pages, 5 figure
Wave functions and properties of massive states in three-dimensional supersymmetric Yang-Mills theory
We apply supersymmetric discrete light-cone quantization (SDLCQ) to the study
of supersymmetric Yang-Mills theory on R x S^1 x S^1. One of the compact
directions is chosen to be light-like and the other to be space-like. Since the
SDLCQ regularization explicitly preserves supersymmetry, this theory is totally
finite, and thus we can solve for bound-state wave functions and masses
numerically without renormalizing. We present an overview of all the massive
states of this theory, and we see that the spectrum divides into two distinct
and disjoint sectors. In one sector the SDLCQ approximation is only valid up to
intermediate coupling. There we find a well defined and well behaved set of
states, and we present a detailed analysis of these states and their
properties. In the other sector, which contains a completely different set of
states, we present a much more limited analysis for strong coupling only. We
find that, while these state have a well defined spectrum, their masses grow
with the transverse momentum cutoff. We present an overview of these states and
their properties.Comment: RevTeX, 25 pages, 16 figure
Solution of coupled vertex and propagator Dyson-Schwinger equations in the scalar Munczek-Nemirovsky model
In a scalar model, we exactly solve the vertex and
propagator Dyson-Schwinger equations under the assumption of a spatially
constant (Munczek-Nemirovsky) propagator for the field. Various
truncation schemes are also considered.Comment: 7 pages,4 figures, minor changes, reference added for published
versio
Cancellation of Global Anomalies in Spontaneously Broken Gauge Theories
We discuss the generalization to global gauge anomalies of the familiar
procedure for the cancellation of local gauge anomalies in effective theories
of spontaneously broken symmetries. We illustrate this mechanism in a recently
proposed six-dimensional extension of the standard model.Comment: 5 pages; v2: version to appear in Phys. Rev.
Spin fluctuations in nearly magnetic metals from ab-initio dynamical spin susceptibility calculations:application to Pd and Cr95V5
We describe our theoretical formalism and computational scheme for making
ab-initio calculations of the dynamic paramagnetic spin susceptibilities of
metals and alloys at finite temperatures. Its basis is Time-Dependent Density
Functional Theory within an electronic multiple scattering, imaginary time
Green function formalism. Results receive a natural interpretation in terms of
overdamped oscillator systems making them suitable for incorporation into spin
fluctuation theories. For illustration we apply our method to the nearly
ferromagnetic metal Pd and the nearly antiferromagnetic chromium alloy Cr95V5.
We compare and contrast the spin dynamics of these two metals and in each case
identify those fluctuations with relaxation times much longer than typical
electronic `hopping times'Comment: 21 pages, 9 figures. To appear in Physical Review B (July 2000
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