4,169 research outputs found
Visibility Fringe Reduction Due to Noise-Induced Effects: Microscopic Approach to Interference Experiments
Decoherence is the main process behind the quantum to classical transition.
It is a purely quantum mechanical effect by which the system looses its ability
to exhibit coherent behavior. The recent experimental observation of
diffraction and interference patterns for large molecules raises some
interesting questions. In this context, we identify possible agents of
decoherence to take into account when modeling these experiments and study
theirs visible (or not) effects on the interference pattern. Thereby, we
present an analysis of matter wave interferometry in the presence of a dynamic
quantum environment and study how much the visibility fringe is reduced and in
which timescale the decoherence effects destroy the interference of massive
objects. Finally, we apply our results to the experimental data reported on
fullerenes and cold neutrons.Comment: 14 pages, 5 figures. Version to appear in Mod. Phys.
One,Two,Zero: Scales of Strong Interactions
We discuss our results on QCD with a number of fundamental fermions ranging
from zero to sixteen. These theories exhibit a wide array of fascinating
phenomena which have been under close scrutiny, especially in recent years,
first and foremost is the approach to conformality. To keep this review
focused, we have chosen scale generation, or lack thereof as a guiding theme,
however the discussion will be set in the general framework of the analysis of
the phases and phase transitions of strong interactions at zero and nonzero
temperature.Comment: 15 pages, prepared for IJMPA Special Issue 'Recent Nonperturbative
Developments in QCD-like Theories
On the particle spectrum and the conformal window
We study the SU(3) gauge theory with twelve flavours of fermions in the
fundamental representation as a prototype of non-Abelian gauge theories inside
the conformal window. Guided by the pattern of underlying symmetries, chiral
and conformal, we analyze the two-point functions theoretically and on the
lattice, and determine the finite size scaling and the infinite volume fermion
mass dependence of the would-be hadron masses. We show that the spectrum in the
Coulomb phase of the system can be described in the context of a universal
scaling analysis and we provide the nonperturbative determination of the
fermion mass anomalous dimension gamma*=0.235(46) at the infrared fixed point.
We comment on the agreement with the four-loop perturbative prediction for this
quantity and we provide a unified description of all existing lattice results
for the spectrum of this system, them being in the Coulomb phase or the
asymptotically free phase. Our results corroborate the view that the fixed
point we are studying is not associated to a physical singularity along the
bare coupling line and estimates of physical observables can be attempted on
either side of the fixed point. Finally, we observe the restoration of the U(1)
axial symmetry in the two-point functions.Comment: 40 pages, 22 figure
Chiral symmetry restoration in QCD with many flavours
We discuss the phases of QCD in the parameter space spanned by the number of
light flavours and the temperature with respect to the realisation of chiral
and conformal symmetries. The intriguing interplay of these symmetries is best
studied by means of lattice simulations, and some selected results from our
recent work are presented here.Comment: 10 pages, proceedings of the 9th International Workshop on Critical
Point and Onset of Deconfinement, 17-21 November, 2014, ZiF, Bielefeld,
German
How Phase Transitions induce classical behaviour
We continue the analysis of the onset of classical behaviour in a scalar
field after a continuous phase transition, in which the system-field, the long
wavelength order parameter of the model, interacts with an environment, of its
own short-wavelength modes and other fields, neutral and charged, with which it
is expected to interact. We compute the decoherence time for the system-field
modes from the master equation and directly from the decoherence functional
(with identical results). In simple circumstances the order parameter field is
classical by the time the transition is complete.Comment: 10 pages, 1 figure: To be published in the International Journal of
Theoretical Physics (2005) as part of the Proceedings of the "Peyresq Physics
9" meeting (2004) on "Micro and Macro structures of spacetime",ed. E.
Verdague
Dense Quarks, and the Fermion Sign Problem, in a SU(N) Matrix Model
We study the effect of dense quarks in a SU(N) matrix model of deconfinement.
For three or more colors, the quark contribution to the loop potential is
complex. After adding the charge conjugate loop, the measure of the matrix
integral is real, but not positive definite. In a matrix model, quarks act like
a background Z(N) field; at nonzero density, the background field also has an
imaginary part, proportional to the imaginary part of the loop. Consequently,
while the expectation values of the loop and its complex conjugate are both
real, they are not equal. These results suggest a possible approach to the
fermion sign problem in lattice QCD.Comment: 9 pages, 3 figure
Metal-insulator crossover in the Boson-Fermion model in infinite dimensions
The Boson-Fermion model, describing a mixture of tightly bound electron pairs
and quasi-free electrons hybridized with each other via a charge exchange term,
is studied in the limit of infinite dimensions, using the Non-Crossing
Approximation within the Dynamical Mean Field Theory. It is shown that a
metal-insulator crossover, driven by strong pair fluctuations, takes place as
the temperature is lowered. It manifests itself in the opening of a pseudogap
in the electron density of states, accompanied by a corresponding effect in the
optical and dc conductivity.Comment: 4 pages, 3 figures, to be published in Phys. Rev. Let
Quantum effects after decoherence in a quenched phase transition
We study a quantum mechanical toy model that mimics some features of a
quenched phase transition. Both by virtue of a time-dependent Hamiltonian or by
changing the temperature of the bath we are able to show that even after
classicalization has been reached, the system may display quantum behaviour
again. We explain this behaviour in terms of simple non-linear analysis and
estimate relevant time scales that match the results of numerical simulations
of the master-equation. This opens new possibilities both in the study of
quantum effects in non-equilibrium phase transitions and in general
time-dependent problems where quantum effects may be relevant even after
decoherence has been completed.Comment: 7 pages, 7 figures, revtex, important revisions made. To be published
in Phys. Rev.
The Newman-Janis Algorithm, Rotating Solutions and Einstein-Born-Infeld Black Holes
A new metric is obtained by applying a complex coordinate trans- formation to
the static metric of the self-gravitating Born-Infeld monopole. The behaviour
of the new metric is typical of a rotating charged source, but this source is
not a spherically symmetric Born-Infeld monopole with rotation. We show that
the structure of the energy-momentum tensor obtained with this new metric does
not correspond to the typical structure of the energy momentum tensor of
Einstein-Born-Infeld theory induced by a rotating spherically symmetric source.
This also show, that the complex coordinate transformations have the
interpretation given by Newman and Janis only in space-time solutions with
linear sources
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