744 research outputs found
Collective Excitations and Ground State Correlations
A generalized RPA formalism is presented which treats pp and ph correlations
on an equal footing. The effect of these correlations on the single-particle
Green function is discussed and it is demonstrated that a self-consistent
treatment of the single-particle Green function is required to obtain stable
solutions. A simple approximation scheme is presented which incorporates for
this self-consistency requirement and conserves the number of particles.
Results of numerical calculations are given for O using a G-matrix
interaction derived from a realistic One-Boson-Exchange potential.Comment: 16 Pages + 2 Figures (included at the end as uuencoded ps-files),
TU-18089
On the reheating stage after inflation
We point out that inflaton decay products acquire plasma masses during the
reheating phase following inflation. The plasma masses may render inflaton
decay kinematicaly forbidden, causing the temperature to remain frozen for a
period at a plateau value. We show that the final reheating temperature may be
uniquely determined by the inflaton mass, and may not depend on its coupling.
Our findings have important implications for the thermal production of
dangerous relics during reheating (e.g., gravitinos), for extracting bounds on
particle physics models of inflation from Cosmic Microwave Background
anisotropy data, for the production of massive dark matter candidates during
reheating, and for models of baryogenesis or leptogensis where massive
particles are produced during reheating.Comment: 8 pages, 2 figures. Submitted for publication in Phys. Rev.
A Comparative Study of the Formation of Aromatics in Rich Methane Flames Doped by Unsaturated Compounds
For a better modeling of the importance of the different channels leading to
the first aromatic ring, we have compared the structures of laminar rich
premixed methane flames doped with several unsaturated hydrocarbons: allene and
propyne, because they are precursors of propargyl radicals which are well known
as having an important role in forming benzene, 1,3-butadiene to put in
evidence a possible production of benzene due to reactions of C4 compounds,
and, finally, cyclopentene which is a source of cyclopentadienylmethylene
radicals which in turn are expected to easily isomerizes to give benzene. These
flames have been stabilized on a burner at a pressure of 6.7 kPa (50 Torr)
using argon as dilutant, for equivalence ratios (?) from 1.55 to 1.79. A unique
mechanism, including the formation and decomposition of benzene and toluene,
has been used to model the oxidation of allene, propyne, 1,3 butadiene and
cyclopentene. The main reaction pathways of aromatics formation have been
derived from reaction rate and sensitivity analyses and have been compared for
the three types of additives. These combined analyses and comparisons can only
been performed when a unique mechanism is available for all the studied
additives
Real-time nonequilibrium dynamics in hot QED plasmas: dynamical renormalization group approach
We study the real-time nonequilibrium dynamics in hot QED plasmas
implementing a dynamical renormalization group and using the hard thermal loop
(HTL) approximation. The focus is on the study of the relaxation of gauge and
fermionic mean fields and on the quantum kinetics of the photon and fermion
distribution functions. For semihard photons of momentum eT << k << T we find
to leading order in the HTL that the gauge mean field relaxes in time with a
power law as a result of infrared enhancement of the spectral density near the
Landau damping threshold. The dynamical renormalization group reveals the
emergence of detailed balance for microscopic time scales larger than 1/k while
the rates are still varying with time. The quantum kinetic equation for the
photon distribution function allows us to study photon production from a
thermalized quark-gluon plasma (QGP) by off-shell effects. We find that for a
QGP at temperature T ~ 200 MeV and of lifetime 10 < t < 50 fm/c the hard (k ~
T) photon production from off-shell bremsstrahlung (q -> q \gamma and \bar{q}
-> \bar{q}\gamma) at O(\alpha) grows logarithmically in time and is comparable
to that produced from on-shell Compton scattering and pair annihilation at
O(\alpha \alpha_s). Fermion mean fields relax as e^{-\alpha T t ln(\omega_P t)}
with \omega_P=eT/3 the plasma frequency, as a consequence of the emission and
absorption of soft magnetic photons. A quantum kinetic equation for hard
fermions is obtained directly in real time from a field theoretical approach
improved by the dynamical renormalization group. The collision kernel is
time-dependent and infrared finite.Comment: RevTeX, 46 pages, including 5 EPS figures, published versio
Rich methane laminar flames doped with light unsaturated hydrocarbons. Part II: 1,3butadiene
In line with the study presented in the part I of this paper, the structure
of a laminar rich premixed methane flame doped with 1,3-butadiene has been
investigated. The flame contains 20.7% (molar) of methane, 31.4% of oxygen and
3.3% of 1,3-butadiene, corresponding to an equivalence ratio of 1.8, and a
ratio C4H6 / CH4 of 16 %. The flame has been stabilized on a burner at a
pressure of 6.7 kPa using argon as dilutant, with a gas velocity at the burner
of 36 cm/s at 333 K. The temperature ranged from 600 K close to the burner up
to 2150 K. Quantified species included usual methane C0-C2 combustion products
and 1,3-butadiene, but also propyne, allene, propene, propane, 1,2-butadiene,
butynes, vinylacetylene, diacetylene, 1,3-pentadiene, 2-methyl-1,3-butadiene
(isoprene), 1-pentene, 3-methyl-1-butene, benzene and toluene. In order to
model these new results, some improvements have been made to a mechanism
previously developed in our laboratory for the reactions of C3-C4 unsaturated
hydrocarbons. The main reaction pathways of consumption of 1,3-butadiene and of
formation of C6 aromatic species have been derived from flow rate analyses. In
this case, the C4 route to benzene formation plays an important role in
comparison to the C3 pathway
Loop quantum gravity and light propagation
Within loop quantum gravity we construct a coarse-grained approximation for
the Einstein-Maxwell theory that yields effective Maxwell equations in flat
spacetime comprising Planck scale corrections.
The corresponding Hamiltonian is defined as the expectation value of the
electromagnetic term in the Einstein-Maxwell Hamiltonian constraint,
regularized a la Thiemann, with respect to a would-be semiclassical state. The
resulting energy dispersion relations entail Planck scale corrections to those
in flat spacetime. Both the helicity dependent contribution of Gambini and
Pullin [GP] and, for a value of a parameter of our approximation, that of Ellis
et. al. [ELLISETAL] are recovered. The electric/magnetic asymmetry in the
regularization procedure yields nonlinearities only in the magnetic sector
which are briefly discussed. Observations of cosmological Gamma Ray Bursts
might eventually lead to the needed accuracy to study some of these quantum
gravity effects.Comment: Latex, 45 pages, shorter abstract, additional reference
On Traversable Lorentzian Wormholes in the Vacuum Low Energy Effective String Theory in Einstein and Jordan Frames
Three new classes (II-IV) of solutions of the vacuum low energy effective
string theory in four dimensions are derived. Wormhole solutions are
investigated in those solutions including the class I case both in the Einstein
and in the Jordan (string) frame. It turns out that, of the eight classes of
solutions investigated (four in the Einstein frame and four in the
corresponding string frame), massive Lorentzian traversable wormholes exist in
five classes. Nontrivial massless limit exists only in class I Einstein frame
solution while none at all exists in the string frame. An investigation of test
scalar charge motion in the class I solution in the two frames is carried out
by using the Plebanski-Sawicki theorem. A curious consequence is that the
motion around the extremal zero (Keplerian) mass configuration leads, as a
result of scalar-scalar interaction, to a new hypothetical "mass" that confines
test scalar charges in bound orbits, but does not interact with neutral test
particles.Comment: 18 page
Thermal rates for baryon and anti-baryon production
We use a form of the fluctuation-dissipation theorem to derive formulas
giving the rate of production of spin-1/2 baryons in terms of the fluctuations
of either meson or quark fields. The most general formulas do not assume
thermal or chemical equilibrium. When evaluated in a thermal ensemble we find
equilibration times on the order of 10 fm/c near the critical temperature in
QCD.Comment: 22 pages, 4 tables and 2 figures, REVTe
Transcriptional regulation of the urokinase receptor (u-PAR) - A central molecule of invasion and metastasis
The phenomenon of tumor-associated proteolysis has been acknowledged as a decisive step in the progression of cancer. This short review focuses on the urokinase receptor (u-PAR), a central molecule involved in tumor-associated invasion and metastasis, and summarizes the transcriptional regulation of u-PAR. The urokinase receptor (u-PAR) is a heavily glycosylated cell surface protein and binds the serine protease urokinase specifically and with high affinity. It consists of three similar cysteine-rich repeats and is anchored to the cell membrane via a GPI-anchor. The u-PAR gene comprises 7 exons and is located on chromosome 19q13. Transcriptional activation of the u-PAR promoter region can be induced by binding of transcription factors (Sp1, AP-1, AP-2, NF-kappaB). One current study gives an example for transcriptional downregulation of u-PAR through a PEA3/ets transcriptional silencing element. Knowledge of the molecular regulation of this molecule in tumor cells could be very important for diagnosis and therapy in the near future
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