920 research outputs found
Nucleation versus Spinodal decomposition in a first order quark hadron phase transition
We investigate the scenario of homogeneous nucleation for a first order
quark-hadron phase transition in a rapidly expanding background of quark gluon
plasma. Using an improved preexponential factor for homogeneous nucleation
rate, we solve a set of coupled equations to study the hadronization and the
hydrodynamical evolution of the matter. It is found that significant
supercooling is possible before hadronization begins. This study also suggests
that spinodal decomposition competes with nucleation and may provide an
alternative mechanism for phase conversion particularly if the transition is
strong enough and the medium is nonviscous. For weak enough transition, the
phase conversion may still proceed via homogeneous nucleation.Comment: LaTeX, 10 pages with 7 Postscript figures, more discussions and
referencese added, typos correcte
Two-loop corrections to the decay rate of parapositronium
Order corrections to the decay rate of parapositronium are
calculated. A QED scattering calculation of the amplitude for electron-positron
annihilation into two photons at threshold is combined with the technique of
effective field theory to determine an NRQED Hamiltonian, which is then used in
a bound state calculation to determine the decay rate. Our result for the
two-loop correction is in units of times the
lowest order rate. This is consistent with but more precise than the result
of a previous calculation.Comment: 26 pages, 7 figure
A comparative study of the electrochemical properties of vitamin B-6 related compounds at physiological pH
A comparative study of vitamin B6 group and related compounds in buffered solutions using electrochemical techniques has been performed at neutral pH. Irreversible bi- or tetra-electronic processes are observed for these substances, and the electron transfer coefficient (αn) calculated. It was concluded that either the first or second electron transfer were the rate determining step of the electrode process. The diffusion coefficient of these substances was calculated and the values given follow an inverse tendency to the molecular size. For aldehydes the values obtained were corrected of the hydration reaction.
It is important to remark that catalytic waves were reported for the first time for these compounds. Using a model involving the nitrogen of the basic structure the kinetic constants were calculated for most of them
Influence of Impact Parameter on Thermal Description of Relativistic Heavy Ion Collisions at GSI/SIS
Attention is drawn to the role played by the size of the system in the
thermodynamic analysis of particle yields in relativistic heavy ion collisions
at SIS energies. This manifests itself in the non-linear dependence of K+ and
K- yields in collisions at 1 -- 2 A.GeV on the number of participants. It
is shown that this dependence can be quantitatively well described in terms of
a thermal model with a canonical strangeness conservation. The measured
particle multiplicity ratios (pi+/p, pi-/pi+, d/p, K+/pi+ and K+/K- but not
eta/pi0) in central Au-Au and Ni-Ni collisions at 0.8 -- 2.0 A.GeV are also
explained in the context of a thermal model with a common freeze-out
temperature and chemical potential. Including the concept of collective flow a
consistent picture of particle energy distributions is derived with the flow
velocity being strongly impact-parameter dependent.Comment: revtex, 20 figure
Colloquium: Mechanical formalisms for tissue dynamics
The understanding of morphogenesis in living organisms has been renewed by
tremendous progressin experimental techniques that provide access to
cell-scale, quantitative information both on theshapes of cells within tissues
and on the genes being expressed. This information suggests that
ourunderstanding of the respective contributions of gene expression and
mechanics, and of their crucialentanglement, will soon leap forward.
Biomechanics increasingly benefits from models, which assistthe design and
interpretation of experiments, point out the main ingredients and assumptions,
andultimately lead to predictions. The newly accessible local information thus
calls for a reflectionon how to select suitable classes of mechanical models.
We review both mechanical ingredientssuggested by the current knowledge of
tissue behaviour, and modelling methods that can helpgenerate a rheological
diagram or a constitutive equation. We distinguish cell scale ("intra-cell")and
tissue scale ("inter-cell") contributions. We recall the mathematical framework
developpedfor continuum materials and explain how to transform a constitutive
equation into a set of partialdifferential equations amenable to numerical
resolution. We show that when plastic behaviour isrelevant, the dissipation
function formalism appears appropriate to generate constitutive equations;its
variational nature facilitates numerical implementation, and we discuss
adaptations needed in thecase of large deformations. The present article
gathers theoretical methods that can readily enhancethe significance of the
data to be extracted from recent or future high throughput
biomechanicalexperiments.Comment: 33 pages, 20 figures. This version (26 Sept. 2015) contains a few
corrections to the published version, all in Appendix D.2 devoted to large
deformation
Measurements of the Production, Decay and Properties of the Top Quark: A Review
With the full Tevatron Run II and early LHC data samples, the opportunity for
furthering our understanding of the properties of the top quark has never been
more promising. Although the current knowledge of the top quark comes largely
from Tevatron measurements, the experiments at the LHC are poised to probe
top-quark production and decay in unprecedented regimes. Although no current
top quark measurements conclusively contradict predictions from the standard
model, the precision of most measurements remains statistically limited.
Additionally, some measurements, most notably the forward-backward asymmetry in
top quark pair production, show tantalizing hints of beyond-the-Standard-Model
dynamics. The top quark sample is growing rapidly at the LHC, with initial
results now public. This review examines the current status of top quark
measurements in the particular light of searching for evidence of new physics,
either through direct searches for beyond the standard model phenomena or
indirectly via precise measurements of standard model top quark properties
Ecological Invasion, Roughened Fronts, and a Competitor's Extreme Advance: Integrating Stochastic Spatial-Growth Models
Both community ecology and conservation biology seek further understanding of
factors governing the advance of an invasive species. We model biological
invasion as an individual-based, stochastic process on a two-dimensional
landscape. An ecologically superior invader and a resident species compete for
space preemptively. Our general model includes the basic contact process and a
variant of the Eden model as special cases. We employ the concept of a
"roughened" front to quantify effects of discreteness and stochasticity on
invasion; we emphasize the probability distribution of the front-runner's
relative position. That is, we analyze the location of the most advanced
invader as the extreme deviation about the front's mean position. We find that
a class of models with different assumptions about neighborhood interactions
exhibit universal characteristics. That is, key features of the invasion
dynamics span a class of models, independently of locally detailed demographic
rules. Our results integrate theories of invasive spatial growth and generate
novel hypotheses linking habitat or landscape size (length of the invading
front) to invasion velocity, and to the relative position of the most advanced
invader.Comment: The original publication is available at
www.springerlink.com/content/8528v8563r7u2742
The beam energy measurement system for the Beijing electron-positron collider
The beam energy measurement system (BEMS) for the upgraded Beijing
electron-positron collider BEPC-II is described. The system is based on
measuring the energies of Compton back-scattered photons. The relative
systematic uncertainty of the electron and positron beam energy determination
is estimated as 2 \cdot 10^{-5}. The relative uncertainty of the beam's energy
spread is about 6 %
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
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