26,488 research outputs found
Linguistic Constraints in LFG-DOP
LFG-DOP (Bod and Kaplan, 1998, 2003) provides an appealing answer to the question of how probabilistic methods can be incorporated into linguistic theory. However, despite its attractions, the standard model of LFG-DOP suffers from serious problems of overgeneration, because (a) it is unable to define fragments of the right level of generality, and (b) it has no way of capturing the effect of anything except simple positive constraints. We show how the model can be extended to overcome these problems. The question of how probabilistic methods should be incorporated into linguistic theory is important from both a practical, grammar engineering, perspective, and from the perspective of ‘pure ’ linguistic theory. From a practical point of view such techniques are essential if a system is to achieve a useful breadth of coverag
High temperature color conductivity at next-to-leading log order
The non-Abelian analog of electrical conductivity at high temperature has
previously been known only at leading logarithmic order: that is, neglecting
effects suppressed only by an inverse logarithm of the gauge coupling. We
calculate the first sub-leading correction. This has immediate application to
improving, to next-to-leading log order, both effective theories of
non-perturbative color dynamics, and calculations of the hot electroweak baryon
number violation rate.Comment: 47 pages, 6+2 figure
Nontwist non-Hamiltonian systems
We show that the nontwist phenomena previously observed in Hamiltonian
systems exist also in time-reversible non-Hamiltonian systems. In particular,
we study the two standard collision/reconnection scenarios and we compute the
parameter space breakup diagram of the shearless torus. Besides the Hamiltonian
routes, the breakup may occur due to the onset of attractors. We study these
phenomena in coupled phase oscillators and in non-area-preserving maps.Comment: 7 pages, 5 figure
On Koopman-von Neumann Waves II
In this paper we continue the study, started in [1], of the operatorial
formulation of classical mechanics given by Koopman and von Neumann (KvN) in
the Thirties. In particular we show that the introduction of the KvN Hilbert
space of complex and square integrable "wave functions" requires an enlargement
of the set of the observables of ordinary classical mechanics. The possible
role and the meaning of these extra observables is briefly indicated in this
work. We also analyze the similarities and differences between non selective
measurements and two-slit experiments in classical and quantum mechanics.Comment: 18+1 pages, 1 figure, misprints fixe
Weld-brazing - a new joining process
A joining process designated weld brazing which combines resistance spot welding and brazing has been developed. Resistance spot welding is used to position and align the parts as well as to establish a suitable faying surface gap for brazing. Fabrication is then completed by capillary flow of the braze alloy into the joint. The process has been used successfully to fabricate Ti-6Al-4V titanium alloy joints using 3003 aluminum braze alloy. Test results obtained on single overlap and hat-stiffened structural specimens show that weld brazed joints are superior in tensile shear, stress rupture, fatigue, and buckling than joint fabricated by spotwelding or brazing. Another attractive feature of the process is that the brazed joints is hermetically sealed by the braze material
Two-dimensional topological gravity and equivariant cohomology
In this paper, we examine the analogy between topological string theory and
equivariant cohomology. We also show that the equivariant cohomology of a
topological conformal field theory carries a certain algebraic structure, which
we call a gravity algebra. (Error on page 9 corrected: BRS current contains
total derivatives.)Comment: 18 page
Generalized Boltzmann equations for on-shell particle production in a hot plasma
A novel refinement of the conventional treatment of Kadanoff--Baym equations
is suggested. Besides the Boltzmann equation another differential equation is
used for calculating the evolution of the non-equilibrium two-point function.
Although it was usually interpreted as a constraint on the solution of the
Boltzmann equation, we argue that its dynamics is relevant to the determination
and resummation of the particle production cut contributions. The differential
equation for this new contribution is illustrated in the example of the cubic
scalar model. The analogue of the relaxation time approximation is suggested.
It results in the shift of the threshold location and in smearing out of the
non-analytic threshold behaviour of the spectral function. Possible
consequences for the dilepton production are discussed.Comment: 22 pages, latex, 2 ps figure
Effective Kinetic Theory for High Temperature Gauge Theories
Quasiparticle dynamics in relativistic plasmas associated with hot,
weakly-coupled gauge theories (such as QCD at asymptotically high temperature
) can be described by an effective kinetic theory, valid on sufficiently
large time and distance scales. The appropriate Boltzmann equations depend on
effective scattering rates for various types of collisions that can occur in
the plasma. The resulting effective kinetic theory may be used to evaluate
observables which are dominantly sensitive to the dynamics of typical
ultrarelativistic excitations. This includes transport coefficients
(viscosities and diffusion constants) and energy loss rates. We show how to
formulate effective Boltzmann equations which will be adequate to compute such
observables to leading order in the running coupling of high-temperature
gauge theories [and all orders in ]. As previously proposed
in the literature, a leading-order treatment requires including both
particle scattering processes as well as effective ``'' collinear
splitting processes in the Boltzmann equations. The latter account for nearly
collinear bremsstrahlung and pair production/annihilation processes which take
place in the presence of fluctuations in the background gauge field. Our
effective kinetic theory is applicable not only to near-equilibrium systems
(relevant for the calculation of transport coefficients), but also to highly
non-equilibrium situations, provided some simple conditions on distribution
functions are satisfied.Comment: 40 pages, new subsection on soft gauge field instabilities adde
Non-perturbative dynamics of hot non-Abelian gauge fields: beyond leading log approximation
Many aspects of high-temperature gauge theories, such as the electroweak
baryon number violation rate, color conductivity, and the hard gluon damping
rate, have previously been understood only at leading logarithmic order (that
is, neglecting effects suppressed only by an inverse logarithm of the gauge
coupling). We discuss how to systematically go beyond leading logarithmic order
in the analysis of physical quantities. Specifically, we extend to
next-to-leading-log order (NLLO) the simple leading-log effective theory due to
Bodeker that describes non-perturbative color physics in hot non-Abelian
plasmas. A suitable scaling analysis is used to show that no new operators
enter the effective theory at next-to-leading-log order. However, a NLLO
calculation of the color conductivity is required, and we report the resulting
value. Our NLLO result for the color conductivity can be trivially combined
with previous numerical work by G. Moore to yield a NLLO result for the hot
electroweak baryon number violation rate.Comment: 20 pages, 1 figur
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