3,289 research outputs found
A quantum jump description for the non-Markovian dynamics of the spin-boson model
We derive a time-convolutionless master equation for the spin-boson model in
the weak coupling limit. The temporarily negative decay rates in the master
equation indicate short time memory effects in the dynamics which is explicitly
revealed when the dynamics is studied using the non-Markovian jump description.
The approach gives new insight into the memory effects influencing the spin
dynamics and demonstrates, how for the spin-boson model the the co-operative
action of different channels complicates the detection of memory effects in the
dynamics.Comment: 9 pages, 6 figures, submitted to Proceedings of CEWQO200
Determination of the weak Hamiltonian in the SU(4) chiral limit through topological zero-mode wave functions
A new method to determine the low-energy couplings of the weak
Hamiltonian is presented. It relies on a matching of the topological poles in
of three-point correlators of two pseudoscalar densities and a
four-fermion operator, measured in lattice QCD, to the same observables
computed in the -regime of chiral perturbation theory. We test this
method in a theory with a light charm quark, i.e. with an SU(4) flavour
symmetry. Quenched numerical measurements are performed in a 2 fm box, and
chiral perturbation theory predictions are worked out up to next-to-leading
order. The matching of the two sides allows to determine the weak low-energy
couplings in the SU(4) limit. We compare the results with a previous
determination, based on three-point correlators containing two left-handed
currents, and discuss the merits and drawbacks of the two procedures.Comment: 38 pages, 9 figure
Weak low-energy couplings from topological zero-mode wavefunctions
We discuss a new method to determine the low-energy couplings of the weak Hamiltonian in the -regime. It relies on a matching of the
topological poles in of three-point functions of two pseudoscalar
densities and a four-fermion operator computed in lattice QCD, to the same
observables in the Chiral Effective Theory. We present the results of a NLO
computation in chiral perturbation theory of these correlation functions
together with some preliminary numerical results.Comment: 7 pages. Contribution to Lattice 200
CO Distribution and Kinematics Along the Bar in the Strongly Barred Spiral NGC 7479
We report on the 2.5 arcsec (400 pc) resolution CO (J = 1 -> 0) observations
covering the whole length of the bar in the strongly barred late-type spiral
galaxy NGC 7479. CO emission is detected only along a dust lane that traverses
the whole length of the bar, including the nucleus. The emission is strongest
in the nucleus. The distribution of emission is clumpy along the bar outside
the nucleus, and consists of gas complexes that are unlikely to be
gravitationally bound. The CO kinematics within the bar consist of two separate
components. A kinematically distinct circumnuclear disk, < 500 pc in diameter,
is undergoing predominantly circular motion with a maximum rotational velocity
of 245 km/s at a radius of 1 arcsec (160 pc). The CO-emitting gas in the bar
outside the circumnuclear disk has substantial noncircular motions which are
consistent with a large radial velocity component, directed inwards. The CO
emission has a large velocity gradient across the bar dust lane, ranging from
0.5 to 1.9 km/s/pc after correcting for inclination, and the projected velocity
change across the dust lane is as high as 200 km/s. This sharp velocity
gradient is consistent with a shock front at the location of the bar dust lane.
A comparison of H-alpha and CO kinematics across the dust lane shows that
although the H-alpha emission is often observed both upstream and downstream
from the dust lane, the CO emission is observed only where the velocity
gradient is large. We also compare the observations with hydrodynamic models
and discuss star formation along the bar.Comment: 16 pages, including 10 figures. Accepted for publication in Ap
Multivariate texture discrimination based on geodesics to class centroids on a generalized Gaussian Manifold
A texture discrimination scheme is proposed wherein probability distributions are deployed on a probabilistic manifold for modeling the wavelet statistics of images. We consider the Rao geodesic distance (GD) to the class centroid for texture discrimination in various classification experiments. We compare the performance of GD to class centroid with the Euclidean distance in a similar context, both in terms of accuracy and computational complexity. Also, we compare our proposed classification scheme with the k-nearest neighbor algorithm. Univariate and multivariate Gaussian and Laplace distributions, as well as generalized Gaussian distributions with variable shape parameter are each evaluated as a statistical model for the wavelet coefficients. The GD to the centroid outperforms the Euclidean distance and yields superior discrimination compared to the k-nearest neighbor approach
Quantification of uncertainty in aerosol optical thickness retrieval arising from aerosol microphysical model and other sources, applied to Ozone Monitoring Instrument (OMI) measurements
Satellite instruments are nowadays successfully utilised for measuring
atmospheric aerosol in many applications as well as in research. Therefore,
there is a growing need for rigorous error characterisation of the
measurements. Here, we introduce a methodology for quantifying the
uncertainty in the retrieval of aerosol optical thickness (AOT). In
particular, we concentrate on two aspects: uncertainty due to aerosol
microphysical model selection and uncertainty due to imperfect forward
modelling. We apply the introduced methodology for aerosol optical thickness
retrieval of the Ozone Monitoring Instrument (OMI) on board NASA's Earth
Observing System (EOS) Aura satellite, launched in 2004. We apply statistical
methodologies that improve the uncertainty estimates of the aerosol optical
thickness retrieval by propagating aerosol microphysical model selection and
forward model error more realistically. For the microphysical model selection
problem, we utilise Bayesian model selection and model averaging methods.
Gaussian processes are utilised to characterise the smooth systematic
discrepancies between the measured and modelled reflectances (i.e.
residuals). The spectral correlation is composed empirically by exploring a
set of residuals. The operational OMI multi-wavelength aerosol retrieval
algorithm OMAERO is used for cloud-free, over-land pixels of the OMI
instrument with the additional Bayesian model selection and model discrepancy
techniques introduced here. The method and improved uncertainty
characterisation is demonstrated by several examples with different aerosol
properties: weakly absorbing aerosols, forest fires over Greece and Russia,
and Sahara desert dust. The statistical methodology presented is general; it
is not restricted to this particular satellite retrieval application
Quarkonium in Hot Medium
I review recent progress in studying quarkonium properties in hot medium as
well as possible consequences for quarkonium production in heavy ion
collisions.Comment: Invited talk at SQM 2009, Buzios, Brazil, Sep. 27 -Oct. 2 2009,
LaTeX, 8 pages,3 figures; typos corrected, references adde
Phenomenological memory-kernel master equations and time-dependent Markovian processes
Do phenomenological master equations with memory kernel always describe a
non-Markovian quantum dynamics characterized by reverse flow of information? Is
the integration over the past states of the system an unmistakable signature of
non-Markovianity? We show by a counterexample that this is not always the case.
We consider two commonly used phenomenological integro-differential master
equations describing the dynamics of a spin 1/2 in a thermal bath. By using a
recently introduced measure to quantify non-Markovianity [H.-P. Breuer, E.-M.
Laine, and J. Piilo, Phys. Rev. Lett. 103, 210401 (2009)] we demonstrate that
as far as the equations retain their physical sense, the key feature of
non-Markovian behavior does not appear in the considered memory kernel master
equations. Namely, there is no reverse flow of information from the environment
to the open system. Therefore, the assumption that the integration over a
memory kernel always leads to a non-Markovian dynamics turns out to be
vulnerable to phenomenological approximations. Instead, the considered
phenomenological equations are able to describe time-dependent and
uni-directional information flow from the system to the reservoir associated to
time-dependent Markovian processes.Comment: 5 pages, no figure
Real-time static potential in hot QCD
We derive a static potential for a heavy quark-antiquark pair propagating in
Minkowski time at finite temperature, by defining a suitable gauge-invariant
Green's function and computing it to first non-trivial order in Hard Thermal
Loop resummed perturbation theory. The resulting Debye-screened potential could
be used in models that attempt to describe the ``melting'' of heavy quarkonium
at high temperatures. We show, in particular, that the potential develops an
imaginary part, implying that thermal effects generate a finite width for the
quarkonium peak in the dilepton production rate. For quarkonium with a very
heavy constituent mass M, the width can be ignored for T \lsim g^2 M/12\pi,
where g^2 is the strong gauge coupling; for a physical case like bottomonium,
it could become important at temperatures as low as 250 MeV. Finally, we point
out that the physics related to the finite width originates from the
Landau-damping of low-frequency gauge fields, and could be studied
non-perturbatively by making use of the classical approximation.Comment: 20 pages. v2: a number of clarifications and a few references added;
published versio
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