10,692 research outputs found
Primordial Nucleosynthesis and the Abundances of Beryllium and Boron
The ability to now make measurements of Be and B as well as put constraints
on \lisix\ abundances in metal-poor stars has led to a detailed reexamination
of Big Bang Nucleosynthesis in the A\groughly6 regime. The nuclear reaction
network has been significantly expanded with many new rates added. It is
demonstrated that although a number of reaction rates are poorly
determined, even with extreme values chosen, the standard homogeneous model is
unable to produce significant yields (Be/H and B/H when
abundances fit) above and the \liseven/\lisix\ ratio always exceeds 500.
We also preliminarily explore inhomogeneous models, such as those inspired by a
first order quark-hadron phase transition, where regions with high
neutron/proton ratios can allow some leakage up to . However models that
fit the abundances still seem to have difficulty in obtaining
significant yields.Comment: Plain TeX, 28 pages, 8 figures (not included, but available from
authors). UMN-TH-1020/9
Large Scale Baryon Isocurvature Inhomogeneities
Big bang nucleosynthesis constraints on baryon isocurvature perturbations are
determined. A simple model ignoring the effects of the scale of the
perturbations is first reviewed. This model is then extended to test the claim
that large amplitude perturbations will collapse, forming compact objects and
preventing their baryons from contributing to the observed baryon density. It
is found that baryon isocurvature perturbations are constrained to provide only
a slight increase in the density of baryons in the universe over the standard
homogeneous model. In particular it is found that models which rely on power
laws and the random phase approximation for the power spectrum are incompatible
with big bang nucleosynthesis unless an {\em ad hoc}, small scale cutoff is
included.Comment: 11pages + 8figures, LaTeX (2.09), postscript figures available via
anonymous ftp from oddjob.uchicago.edu:/ftp/ibbn/fig?.ps where ?=1-8 or via
email from [email protected], Fermilab-Pub-94/???-A and UMN-TH-1307/9
Modularity and 4D-2D spectral equivalences for large-N gauge theories with adjoint matter
In recent work, we demonstrated that the confined-phase spectrum of
non-supersymmetric pure Yang-Mills theory coincides with the spectrum of the
chiral sector of a two-dimensional conformal field theory in the large-
limit. This was done within the tractable setting in which the gauge theory is
compactified on a three-sphere whose radius is small compared to the strong
length scale. In this paper, we generalize these observations by demonstrating
that similar results continue to hold even when massless adjoint matter fields
are introduced. These results hold for both thermal and -twisted
partition functions, and collectively suggest that the spectra of large-
confining gauge theories are organized by the symmetries of two-dimensional
conformal field theories.Comment: 51 pages, LaTeX, 3 figure
Microwave Dielectric Heating of Drops in Microfluidic Devices
We present a technique to locally and rapidly heat water drops in
microfluidic devices with microwave dielectric heating. Water absorbs microwave
power more efficiently than polymers, glass, and oils due to its permanent
molecular dipole moment that has a large dielectric loss at GHz frequencies.
The relevant heat capacity of the system is a single thermally isolated
picoliter drop of water and this enables very fast thermal cycling. We
demonstrate microwave dielectric heating in a microfluidic device that
integrates a flow-focusing drop maker, drop splitters, and metal electrodes to
locally deliver microwave power from an inexpensive, commercially available 3.0
GHz source and amplifier. The temperature of the drops is measured by observing
the temperature dependent fluorescence intensity of cadmium selenide
nanocrystals suspended in the water drops. We demonstrate characteristic
heating times as short as 15 ms to steady-state temperatures as large as 30
degrees C above the base temperature of the microfluidic device. Many common
biological and chemical applications require rapid and local control of
temperature, such as PCR amplification of DNA, and can benefit from this new
technique.Comment: 6 pages, 4 figure
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