377 research outputs found
A novel synthesis of 2'-hydroxy-1',3'-xylyl crown ethers
Six novel 2' - hydroxy - 1',3' - xylyl crown ethers (8a–e and 13)1 have been synthesized utilizing the allyl group to protect the OH function during the cyclization reaction. The macrocycles 6a-e were formed in yields of 26 to 52%, by intermolecular reaction of 4 - chloro - 2,6 - bis(bromomethyl) - 1 - (2 - propenyloxy)benzene (5) with polyethylene glycols; 6a was also obtained by an intramolecular cyclization reaction of monotosylate 14.\ud
A 30-membered ring with a 2' - hydroxy - 1',3' - xylyl sub-unit was obtained in 87% yield by reaction of ditosylate 9 with bis [2 - (o - hydroxyphenoxy)ethyl]ether (11) in the presence of cesium fluoride. The synthesis of crown ethers with a 2' - hydroxy - 1',3' - xylyl sub-unit (1c–e, H for CH3) by demethylation of the corresponding 2'-methoxy crown ethers 1c–e with lithium iodide were unsuccessful; it would appear that the demethylation reaction is restricted to 15- and 18-membered rings. One of the 2' - hydroxy - 1',3' - xylyl crown ethers 8d forms a crystalline 1:1-complex with water
Statistical Theory for Incoherent Light Propagation in Nonlinear Media
A novel statistical approach based on the Wigner transform is proposed for
the description of partially incoherent optical wave dynamics in nonlinear
media. An evolution equation for the Wigner transform is derived from a
nonlinear Schrodinger equation with arbitrary nonlinearity. It is shown that
random phase fluctuations of an incoherent plane wave lead to a Landau-like
damping effect, which can stabilize the modulational instability. In the limit
of the geometrical optics approximation, incoherent, localized, and stationary
wave-fields are shown to exist for a wide class of nonlinear media.Comment: 4 pages, REVTeX4. Submitted to Physical Review E. Revised manuscrip
New features of modulational instability of partially coherent light; importance of the incoherence spectrum
It is shown that the properties of the modulational instability of partially
coherent waves propagating in a nonlinear Kerr medium depend crucially on the
profile of the incoherent field spectrum. Under certain conditions, the
incoherence may even enhance, rather than suppress, the instability. In
particular, it is found that the range of modulationally unstable wave numbers
does not necessarily decrease monotonously with increasing degree of
incoherence and that the modulational instability may still exist even when
long wavelength perturbations are stable.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Statistical Effects in the Multistream Model for Quantum Plasmas
A statistical multistream description of quantum plasmas is formulated, using
the Wigner-Poisson system as dynamical equations. A linear stability analysis
of this system is carried out, and it is shown that a Landau-like damping of
plane wave perturbations occurs due to the broadening of the background Wigner
function that arises as a consequence of statistical variations of the wave
function phase. The Landau-like damping is shown to suppress instabilities of
the one- and two-stream type.Comment: 5 page
Many-body aspects of positron annihilation in the electron gas
We investigate positron annihilation in electron liquid as a case study for
many-body theory, in particular the optimized Fermi Hypernetted Chain (FHNC-EL)
method. We examine several approximation schemes and show that one has to go up
to the most sophisticated implementation of the theory available at the moment
in order to get annihilation rates that agree reasonably well with experimental
data. Even though there is basically just one number to look at, the
electron-positron pair distribution function at zero distance, it is exactly
this number that dictates how the full pair distribution behaves: In most
cases, it falls off monotonously towards unity as the distance increases. Cases
where the electron-positron pair distribution exhibits a dip are precursors to
the formation of bound electron--positron pairs. The formation of
electron-positron pairs is indicated by a divergence of the FHNC-EL equations,
from this we can estimate the density regime where positrons must be localized.
This occurs in our calculations in the range 9.4 <= r_s <=10, where r_s is the
dimensionless density parameter of the electron liquid.Comment: To appear in Phys. Rev. B (2003
A Planck-scale axion and SU(2) Yang-Mills dynamics: Present acceleration and the fate of the photon
From the time of CMB decoupling onwards we investigate cosmological evolution
subject to a strongly interacting SU(2) gauge theory of Yang-Mills scale
eV (masquerading as the factor of the SM at
present). The viability of this postulate is discussed in view of cosmological
and (astro)particle physics bounds. The gauge theory is coupled to a spatially
homogeneous and ultra-light (Planck-scale) axion field. As first pointed out by
Frieman et al., such an axion is a viable candidate for quintessence, i.e.
dynamical dark energy, being associated with today's cosmological acceleration.
A prediction of an upper limit for the duration of the
epoch stretching from the present to the point where the photon starts to be
Meissner massive is obtained: billion years.Comment: v3: consequences of an error in evolution equation for coupling
rectified, only a minimal change in physics results, two refs. adde
On the dissipative non-minimal braneworld inflation
We study the effects of the non-minimal coupling on the dissipative dynamics
of the warm inflation in a braneworld setup, where the inflaton field is
non-minimally coupled to induced gravity on the warped DGP brane. We study with
details the effects of the non-minimal coupling and dissipation on the
inflationary dynamics on the normal DGP branch of this scenario in the
high-dissipation and high-energy regime. We show that incorporation of the
non-minimal coupling in this setup decreases the number of e-folds relative to
the minimal case. We also compare our model parameters with recent
observational data.Comment: 32 pages, 6 figures. arXiv admin note: substantial text overlap with
arXiv:1001.044
Focus Point SUSY at the LHC Revisited
The estimation of the backgrounds for gluino signals in focus point
supersymmetry is extended by including the backgrounds from the production of
four third generation quarks in the analysis. We find that these backgrounds
are negligible if one uses the strong selection criteria proposed in the
literature (including this analysis) for heavy gluino searches. Softer
selection criteria often recommended for lighter gluino searches yield
backgrounds which are small but numerically significant. We have also repeated
the more conventional background calculations and compared our results with the
other groups. We find that the size of the total residual background estimated
by different groups using different event generators and hard kinematical cuts
agree approximately. In view of the theoretical uncertainties in the leading
order signal and background cross sections mainly due to the choice of the QCD
scale, the gluino mass reach at the LHC cannot be pinpointed. However,
requiring a signal with tagged b-jets (instead of the standard
choice of ) it is shown that gluino masses close to 2 TeV can be
probed at the LHC for a range of reasonable choices of the QCD scale for an
integrated luminosity of 300 fb.Comment: 17 pages, 4 figures, minor typos correctio
Unitarity bounds on low scale quantum gravity
We study the unitarity of models with low scale quantum gravity both in four
dimensions and in models with a large extra-dimensional volume. We find that
models with low scale quantum gravity have problems with unitarity below the
scale at which gravity becomes strong. An important consequence of our work is
that their first signal at the Large Hadron Collider would not be of a
gravitational nature such as graviton emission or small black holes, but rather
linked to the mechanism which fixes the unitarity problem. We also study models
with scalar fields with non minimal couplings to the Ricci scalar. We consider
the strength of gravity in these models and study the consequences for
inflation models with non-minimally coupled scalar fields. We show that a
single scalar field with a large non-minimal coupling can lower the Planck mass
in the TeV region. In that model, it is possible to lower the scale at which
gravity becomes strong down to 14 TeV without violating unitarity below that
scale.Comment: 15 page
Quintessence from Shape Moduli
We show that shape moduli in sub-millimeter extra dimensional scenarios,
addressing the gauge hierarchy problem, can dominate the energy density of the
universe today. In our scenario, the volume of the extra dimensions is
stabilized at a sufficiently high scale to avoid conflicts with nucleosynthesis
and solar-system precision gravity experiments, while the shape moduli remain
light but couple extremely weakly to brane-localized matter and easily avoid
these bounds. Nonlocal effects in the bulk of the extra dimension generate a
potential for the shape moduli. The potential has the right form and order of
magnitude to account for the present day cosmic acceleration, in a way
analogous to models of quintessence as a pseudo Nambu-Goldstone boson.Comment: 8 pages, 1 figur
- …