4,643 research outputs found
The Mandelstam--Leibbrandt Prescription and the Discretized Light Front Quantization
It is shown that the quantization of the unphysical degrees of freedom, which
leads to the Mandelstam--Leibbrandt prescription for the infrared spurious
singularities in the continuum light cone gauge, does indeed suggest some quite
natural recipe to treat the zero modes in the Discretized Light Front
Quantization of gauge theories.Comment: 5 pages, LaTeX, Talk given at the Fourth International Workshop on
Light Cone Quantization and Non--Perturbative Dynamics, Polana Zgorzelisk
Turbulent breakage of ductile aggregates
In this paper we study breakage rate statistics of small colloidal aggregates
in non-homogeneous anisotropic turbulence. We use pseudo-spectral direct
numerical simulation of turbulent channel flow and Lagrangian tracking to
follow the motion of the aggregates, modelled as sub-Kolmogorov massless
particles. We focus specifically on the effects produced by ductile rupture:
This rupture is initially activated when fluctuating hydrodynamic stresses
exceed a critical value, , and is brought to completion
when the energy absorbed by the aggregate meets the critical breakage value. We
show that ductile rupture breakage rates are significantly reduced with respect
to the case of instantaneous brittle rupture (i.e. breakage occurs as soon as
). These discrepancies are due to the different energy
values at play as well as to the statistical features of energy distribution in
the anisotropic turbulence case examined.Comment: Accepted for publication in Phys. Rev. E (April 2015
The Unruh effect revisited
A new and exact derivation of the Bogoliubov coefficients is obtained for the
simplest case of a spinless, neutral, massive field in a uniformly accelerated
frame with a constant acceleration. The method can be suitably generalized in a
straightforward manner to any field with spin and charges.Comment: 5 page
Neutral Massive Spin 1/2 Particles Emission in a Rindler Spacetime
The Unruh effect for the rate of emission and absorption of neutral massive
Majorana spinor particles -- the most plausible consituents of Dark Matter --
in a Rindler spacetime is thoroughly investigated. The corresponding Bogolyubov
coefficients are explicitly calculated and the consistency with Fermi-Dirac
statistics and the Pauli principle is actually verified.Comment: v2: clarifications added to Introduction and Conclusions, added
appendices; v3 removed review part, published versio
Time persistency of floating particle clusters in free-surface turbulence
We study the dispersion of light particles floating on a flat shear-free
surface of an open channel in which the flow is turbulent. This configuration
mimics the motion of buoyant matter (e.g. phytoplankton, pollutants or
nutrients) in water bodies when surface waves and ripples are smooth or absent.
We perform direct numerical simulation of turbulence coupled with Lagrangian
particle tracking, considering different values of the shear Reynolds number
(Re{\tau} = 171 and 509) and of the Stokes number (0.06 < St < 1 in viscous
units). Results show that particle buoyancy induces clusters that evolve
towards a long-term fractal distribution in a time much longer than the
Lagrangian integral fluid time scale, indicating that such clusters over-live
the surface turbulent structures which produced them. We quantify cluster
dynamics, crucial when modeling dispersion in free-surface flow turbulence, via
the time evolution of the cluster correlation dimension
Axial Anomaly in the Presence of the Aharonov-Bohm Gauge Field
We investigate on the plane the axial anomaly for euclidean Dirac fermions in
the presence of a background Aharonov--Bohm gauge potential. The non
perturbative analysis depends on the self--adjoint extensions of the Dirac
operator and the result is shown to be influenced by the actual way of
understanding the local axial current. The role of the quantum mechanical
parameters involved in the expression for the axial anomaly is discussed. A
derivation of the effective action by means of the stereographic projection is
also considered.Comment: 15 pages, Plain.TeX, Preprint DFUB/94 - 1
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