321 research outputs found
Comparison of Some Exact and Perturbative Results for a Supersymmetric SU() Gauge Theory
We consider vectorial, asymptotically free supersymmetric
SU() gauge theories with copies of massless chiral super fields in
various representations and study how perturbative predictions for the lower
boundary of the infrared conformal phase, as a function of , compare with
exact results. We make use of two-loop and three-loop calculations of the beta
function and anomalous dimension of the quadratic chiral super field operator
product for this purpose. The specific chiral superfield contents that we
consider are copies of (i) , (ii) , (iii) ,
and (iv) , where , , , and denote,
respectively, the fundamental, adjoint, and symmetric and antisymmetric rank-2
tensor representations. We find that perturbative results slightly overestimate
the value of relative to the respective exact results for these
representations, i.e., slightly underestimate the interval in for which
the theory has infrared conformal behavior. Our results provide a measure of
how closely perturbative calculations reproduce exact results for these
theories.Comment: 16 pages, 3 figure
Higher-Loop Corrections to the Infrared Evolution of a Gauge Theory with Fermions
We consider a vectorial, asymptotically free gauge theory and analyze the
effect of higher-loop corrections to the beta function on the evolution of the
theory from the ultraviolet to the infrared. We study the case in which the
theory contains copies of a fermion transforming according to the
fundamental representation and several higher-dimensional representations of
the gauge group. We also calculate higher-loop values of the anomalous
dimension of the mass, of at the infrared zero of the
beta function. We find that for a given theory, the values of
calculated to three- and four-loop order, and evaluated at the infrared zero
computed to the same order, tend to be somewhat smaller than the value
calculated to two-loop order. The results are compared with recent lattice
simulations.Comment: 22 pages, latex, matches Phys. Rev. D publicatio
Zeroing in on more photons and gluons
We discuss radiation zeros that are found in gauge tree amplitudes for
processes involving multi-photon emission. Previous results are clarified by
examples and by further elaboration. The conditions under which such amplitude
zeros occur are identical in form to those for the single-photon zeros, and all
radiated photons must travel parallel to each other. Any other neutral particle
likewise must be massless (e.g. gluon) and travel in that common direction. The
relevance to questions like gluon jet identification and computational checks
is considered. We use examples to show how certain multi-photon amplitudes
evade the zeros, and to demonstrate the connection to a more general result,
the decoupling of an external electromagnetic plane wave in the ``null zone".
Brief comments are made about zeros associated with other gauge-boson emission.Comment: 26 page
Are megaquakes clustered?
We study statistical properties of the number of large earthquakes over the
past century. We analyze the cumulative distribution of the number of
earthquakes with magnitude larger than threshold M in time interval T, and
quantify the statistical significance of these results by simulating a large
number of synthetic random catalogs. We find that in general, the earthquake
record cannot be distinguished from a process that is random in time. This
conclusion holds whether aftershocks are removed or not, except at magnitudes
below M = 7.3. At long time intervals (T = 2-5 years), we find that
statistically significant clustering is present in the catalog for lower
magnitude thresholds (M = 7-7.2). However, this clustering is due to a large
number of earthquakes on record in the early part of the 20th century, when
magnitudes are less certain.Comment: 5 pages, 5 figure
The Role of Zero-Modes in the Canonical Quantization of Heavy-Fermion QED in Light-Cone Coordinates
Four-dimensional heavy-fermion QED is studied in light-cone coordinates with
(anti-)periodic field boundary conditions. We carry out a consistent light-cone
canonical quantization of this model using the Dirac algorithm for a system
with first- and second-class constraints. To examine the role of the zero
modes, we consider the quantization procedure in {the }zero-mode {and the
non-zero-mode} sectors separately. In both sectors we obtain the physical
variables and their canonical commutation relations. The physical Hamiltonian
is constructed via a step-by-step exclusion of the unphysical degrees of
freedom. An example using this Hamiltonian in which the zero modes play a role
is the verification of the correct Coulomb potential between two heavy
fermions.Comment: 22 pages, CWRUTH-93-5 (Latex
In Defense of Wireless Carrier Sense
Carrier sense is often used to regulate concurrency in wireless medium access control (MAC) protocols, balancing interference protection and spatial reuse. Carrier sense is known to be imperfect, and many improved techniques have been proposed. Is the search for a replacement justified? This paper presents a theoretical model for average case two-sender carrier sense based on radio propagation theory and Shannon capacity. Analysis using the model shows that carrier sense performance is surprisingly close to optimal for radios with adaptive bitrate. The model suggests that hidden and exposed terminals usually cause modest reductions in throughput rather than dramatic decreases. Finally, it is possible to choose a fixed sense threshold which performs well across a wide range of scenarios, in large part due to the role of the noise floor. Experimental results from an indoor 802.11 testbed support these claims
Approach to Perturbative Results in the N-Delta Transition
We show that constraints from perturbative QCD calculations play a role in
the nucleon to Delta(1232) electromagnetic transition even at moderate momentum
transfer scales. The pQCD constraints, tied to real photoproduction data and
unseparated resonance response functions, lead to explicit forms for the
helicity amplitudes wherein the E2/M1 ratio remains small at moderately large
momentum transfer.Comment: 4 pages, 2 figures, ReVTe
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