2,407 research outputs found
On the stochastic mechanics of the free relativistic particle
Given a positive energy solution of the Klein-Gordon equation, the motion of
the free, spinless, relativistic particle is described in a fixed Lorentz frame
by a Markov diffusion process with non-constant diffusion coefficient. Proper
time is an increasing stochastic process and we derive a probabilistic
generalization of the equation . A
random time-change transformation provides the bridge between the and the
domain. In the domain, we obtain an \M^4-valued Markov process
with singular and constant diffusion coefficient. The square modulus of the
Klein-Gordon solution is an invariant, non integrable density for this Markov
process. It satisfies a relativistically covariant continuity equation
Willmore minimizers with prescribed isoperimetric ratio
Motivated by a simple model for elastic cell membranes, we minimize the
Willmore functional among two-dimensional spheres embedded in R^3 with
prescribed isoperimetric ratio
Entropy, time irreversibility and Schroedinger equation in a primarily discrete space-time
In this paper we show that the existence of a primarily discrete space-time
may be a fruitful assumption from which we may develop a new approach of
statistical thermodynamics in pre-relativistic conditions. The discreetness of
space-time structure is determined by a condition that mimics the Heisenberg
uncertainty relations and the motion in this space-time model is chosen as
simple as possible. From these two assumptions we define a path-entropy that
measures the number of closed paths associated with a given energy of the
system preparation. This entropy has a dynamical character and depends on the
time interval on which we count the paths. We show that it exists an
like-equilibrium condition for which the path-entropy corresponds exactly to
the usual thermodynamic entropy and, more generally, the usual statistical
thermodynamics is reobtained. This result derived without using the Gibbs
ensemble method shows that the standard thermodynamics is consistent with a
motion that is time-irreversible at a microscopic level. From this change of
paradigm it becomes easy to derive a . A comparison with the
traditional Boltzmann approach is presented. We also show how our approach can
be implemented in order to describe reversible processes. By considering a
process defined simultaneously by initial and final conditions a well defined
stochastic process is introduced and we are able to derive a Schroedinger
equation, an example of time reversible equation.Comment: latex versio
Evolution of Topological Defects During Inflation
Topological defects can be formed during inflation by phase transitions as
well as by quantum nucleation. We study the effect of the expansion of the
Universe on the internal structure of the defects. We look for stationary
solutions to the field equations, i.e. solutions that depend only on the proper
distance from the defect core. In the case of very thin defects, whose core
dimensions are much smaller than the de Sitter horizon, we find that the
solutions are well approximated by the flat space solutions. However, as the
flat space thickness parameter increases we notice a deviation from
this, an effect that becomes dramatic as approaches . Beyond this critical value we find no stationary solutions to the field
equations. We conclude that only defects that have flat space thicknesses less
than the critical value survive, while thicker defects are smeared out by the
expansion.Comment: 14 page
Signal of the pion string at high-energy collisions
We study the possible signals of a pion string associated with the QCD chiral
phase transition in LHC Pb - Pb collision at energy TeV. In
terms of the Kibble-Zurek mechanism we discuss the production and evolution of
the pion string. The pion string is not topologically stable, it decays into
neutral pions and sigma mesons which in turn decay into pions. Our results show
that all the neutral pions from the pion string are distributed at the low
momentum and the ratio of neutral to charged pions from the pion string
violates the isospin symmetry. For the momentum spectra of the total pions, the
signal from the sigma particle decay which is from the pion string will be
affected by the large decay width of the sigma significantly.Comment: 8 pages, 3 figures, one reference added, title changed, version
accepted for publication in Phys. Rev.
L\'evy-Schr\"odinger wave packets
We analyze the time--dependent solutions of the pseudo--differential
L\'evy--Schr\"odinger wave equation in the free case, and we compare them with
the associated L\'evy processes. We list the principal laws used to describe
the time evolutions of both the L\'evy process densities, and the
L\'evy--Schr\"odinger wave packets. To have self--adjoint generators and
unitary evolutions we will consider only absolutely continuous, infinitely
divisible L\'evy noises with laws symmetric under change of sign of the
independent variable. We then show several examples of the characteristic
behavior of the L\'evy--Schr\"odinger wave packets, and in particular of the
bi-modality arising in their evolutions: a feature at variance with the typical
diffusive uni--modality of both the L\'evy process densities, and the usual
Schr\"odinger wave functions.Comment: 41 pages, 13 figures; paper substantially shortened, while keeping
intact examples and results; changed format from "report" to "article";
eliminated Appendices B, C, F (old names); shifted Chapters 4 and 5 (old
numbers) from text to Appendices C, D (new names); introduced connection
between Relativistic q.m. laws and Generalized Hyperbolic law
Dynamical Shakeup of Planetary Systems II. N-body simulations of Solar System terrestrial planet formation induced by secular resonance sweeping
We revisit the "dynamical shakeup" model of Solar System terrestrial planet
formation, wherein the whole process is driven by the sweeping of Jupiter's
secular resonance as the gas disk is removed. Using a large number of 0.5
Gyr-long N-body simulations, we investigate the different outcomes produced by
such a scenario. We confirm that in contrast to existing models, secular
resonance sweeping combined with tidal damping by the disk gas can reproduce
the low eccentricities and inclinations, and high radial mass concentration, of
the Solar System terrestrial planets. At the same time, this also drives the
final assemblage of the planets on a timescale of several tens of millions of
years, an order of magnitude faster than inferred from previous numerical
simulations which neglected these effects, but possibly in better agreement
with timescales inferred from cosmochemical data. In addition, we find that
significant delivery of water-rich material from the outer Asteroid Belt is a
natural byproduct.Comment: To appear in Ap
Effect of pre-existing baryon inhomogeneities on the dynamics of quark-hadron transition
Baryon number inhomogeneities may be generated during the epoch when the
baryon asymmetry of the universe is produced, e.g. at the electroweak phase
transition. The regions with excess baryon number will have a lower temperature
than the background temperature of the universe. Also the value of the quark
hadron transition temperature will be different in these regions as
compared to the background region. Since a first-order quark hadron transition
is very susceptible to small changes in temperature, we investigate the effect
of the presence of such baryonic lumps on the dynamics of quark-hadron
transition. We find that the phase transition is delayed in these lumps for
significant overdensities. Consequently, we argue that baryon concentration in
these regions grows by the end of the transition. We briefly discuss some
models which may give rise to such high overdensities at the onset of the
quark-hadron transition.Comment: 16 pages, no figures, minor changes, version to appear in Phys. Rev.
A novel targeted/untargeted GC-Orbitrap metabolomics methodology applied to Candida albicans and Staphylococcus aureus biofilms
Introduction:
Combined infections from Candida albicans and Staphylococcus aureus are a leading cause of death in the developed world. Evidence suggests that Candida enhances the virulence of Staphylococcus—hyphae penetrate through tissue barriers, while S. aureus tightly associates with the hyphae to obtain entry to the host organism. Indeed, in a biofilm state, C. albicans enhances the antimicrobial resistance characteristics of S. aureus. The association of these microorganisms is also associated with significantly increased morbidity and mortality. Due to this tight association we hypothesised that metabolic effects were also in evidence.
Objectives:
To explore the interaction, we used a novel GC-Orbitrap-based mass spectrometer, the Q Exactive GC, which combines the high peak capacity and chromatographic resolution of gas chromatography with the sub-ppm mass accuracy of an Orbitrap system. This allows the capability to leverage the widely available electron ionisation libraries for untargeted applications, along with expanding accurate mass libraries and targeted matches based around authentic standards.
Methods:
Optimised C. albicans and S. aureus mono- and co-cultured biofilms were analysed using the new instrument in addition to the fresh and spent bacterial growth media.
Results:
The targeted analysis experiment was based around 36 sugars and sugar phosphates, 22 amino acids and five organic acids. Untargeted analysis resulted in the detection of 465 features from fresh and spent medium and 405 from biofilm samples. Three significantly changing compounds that matched to high scoring library fragment patterns were chosen for validation.
Conclusion:
Evaluation of the results demonstrates that the Q Exactive GC is suitable for metabolomics analysis using a targeted/untargeted methodology. Many of the results were as expected: e.g. rapid consumption of glucose and fructose from the medium regardless of the cell type. Modulation of sugar-phosphate levels also suggest that the pentose phosphate pathway could be enhanced in the cells from co-cultured biofilms. Untargeted metabolomics results suggested significant production of cell-wall biosynthesis components and the consumption of non-proteinaceous amino-acids
Resonant Inelastic X-Ray Scattering from Valence Excitations in Insulating Copper-Oxides
We report resonant inelastic x-ray measurements of insulating LaCuO
and SrCuOCl taken with the incident energy tuned near the Cu K
absorption edge. We show that the spectra are well described in a shakeup
picture in 3rd order perturbation theory which exhibits both incoming and
outgoing resonances, and demonstrate how to extract a spectral function from
the raw data. We conclude by showing {\bf q}-dependent measurements of the
charge transfer gap.Comment: minor notational changes, discussion of anderson impurity model
fixed, references added; accepted by PR
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