14,660 research outputs found
A simple shower and matching algorithm
We present a simple formalism for parton-shower Markov chains. As a first
step towards more complete uncertainty bands, we incorporate a comprehensive
exploration of the ambiguities inherent in such calculations. To reduce this
uncertainty, we then introduce a matching formalism which allows a generated
event sample to simultaneously reproduce any infrared safe distribution
calculated at leading or next-to-leading order in perturbation theory, up to
sub-leading corrections. To enable a more universal definition of perturbative
calculations, we also propose a more general definition of the hadronization
cutoff. Finally, we present an implementation of some of these ideas for
final-state gluon showers, in a code dubbed VINCIA.Comment: 32 pages, 6 figure
Frustrated multiband superconductivity
We show that a clean multiband superconductor may display one or several
phase transitions with increasing temperature from or to frustrated
configurations of the relative phases of the superconducting order parameters.
These transitions may occur when more than two bands are involved in the
formation of the superconducting phase and when the number of repulsive
interband interactions is odd. These transitions are signalled by slope changes
in the temperature dependence of the superconducting gaps.Comment: 5 pages, 3 figure
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ELAV links paused Pol II to alternative polyadenylation in the Drosophila nervious system
Alternative polyadenylation (APA) has been implicated in a variety of developmental and disease processes. A particularly dramatic form of APA occurs in the developing nervous system of flies and mammals, whereby various developmental genes undergo coordinate 3' UTR extension. In Drosophila, the RNA-binding protein ELAV inhibits RNA processing at proximal polyadenylation sites, thereby fostering the formation of exceptionally long 3' UTRs. Here, we present evidence that paused Pol II promotes recruitment of ELAV to extended genes. Replacing promoters of extended genes with heterologous promoters blocks normal 3' extension in the nervous system, while extension-associated promoters can induce 3' extension in ectopic tissues expressing ELAV. Computational analyses suggest that promoter regions of extended genes tend to contain paused Pol II and associated cis-regulatory elements such as GAGA. ChIP-seq assays identify ELAV in the promoter regions of extended genes. Our study provides evidence for a regulatory link between promoter-proximal pausing and APA
Observing two dark accelerators around the Galactic Centre with Fermi Large Area Telescope
We report the results from a detailed ray investigation in the field
of two "dark accelerators", HESS J1745-303 and HESS J1741-302, with years
of data obtained by the Fermi Large Area Telescope. For HESS J1745-303, we
found that its MeV-GeV emission is mainly originated from the "Region A" of the
TeV feature. Its ray spectrum can be modeled with a single power-law
with a photon index of from few hundreds MeV to TeV. Moreover,
an elongated feature, which extends from "Region A" toward northwest for
, is discovered for the first time. The orientation of this
feature is similar to that of a large scale atomic/molecular gas distribution.
For HESS J1741-302, our analysis does not yield any MeV-GeV counterpart for
this unidentified TeV source. On the other hand, we have detected a new point
source, Fermi J1740.1-3013, serendipitously. Its spectrum is apparently curved
which resembles that of a ray pulsar. This makes it possibly
associated with PSR B1737-20 or PSR J1739-3023.Comment: 11 pages, 7 figures, 2 tables, accepted for publication in MNRA
Spin-S bilayer Heisenberg models: Mean-field arguments and numerical calculations
Spin-S bilayer Heisenberg models (nearest-neighbor square lattice
antiferromagnets in each layer, with antiferromagnetic interlayer couplings)
are treated using dimer mean-field theory for general S and high-order
expansions about the dimer limit for S=1, 3/2,...,4. We suggest that the
transition between the dimer phase at weak intraplane coupling and the Neel
phase at strong intraplane coupling is continuous for all S, contrary to a
recent suggestion based on Schwinger boson mean-field theory. We also present
results for S=1 layers based on expansions about the Ising limit: In every
respect the S=1 bilayers appear to behave like S=1/2 bilayers, further
supporting our picture for the nature of the order-disorder phase transition.Comment: 6 pages, Revtex 3.0, 8 figures (not embedded in text
Transport through a finite Hubbard chain connected to reservoirs
The dc conductance through a finite Hubbard chain of size N coupled to two
noninteracting leads is studied at T = 0 in an electron-hole symmetric case.
Assuming that the perturbation expansion in U is valid for small N (=1,2,3,...)
owing to the presence of the noninteracting leads, we obtain the self-energy at
\omega = 0 analytically in the real space within the second order in U. Then,
we calculate the inter-site Green's function which connects the two boundaries
of the chain, G_{N1}, solving the Dyson equation. The conductance can be
obtained through G_{N1}, and the result shows an oscillatory behavior as a
function of N. For odd N, a perfect transmission occurs independent of U. This
is due to the inversion and electron-hole symmetries, and is attributed to a
Kondo resonance appearing at the Fermi level. On the other hand, for even N,
the conductance is a decreasing function of N and U.Comment: 11 pages, RevTeX, 6 figures, to be published in Phys. Rev. B 59
(1999
An Integrated Circuit for Signal Processing of the AMS RICH Photmultipliers Tubes
An analog integrated circuit has been designed, in a BiCMOS 0.8 micron
technology, for the feasability study of the signal processing of the AMS RICH
photomultiplier tubes. This low power, three channel gated integrator includes
its own gate and no external analog delay is requiered. It processes PMT pulses
over a dynamic range of more than 100. A logic output that indicates whether
the analog charge has to be considered is provided. This gated integrator is
used with a compact DSP based acquisition system in a 132 channels RICH
prototype. The charge calibration of each channel is carried out using a LED.
The pedestal measurement is performed on activation of a dedicated input. The
noise contribution study of the input RC network and amplifiers is presented.Comment: IEEE symp. on Nucl. Sci. and Med. Imaging, Toront
Detecting the Kondo screening cloud around a quantum dot
A fundamental prediction of scaling theories of the Kondo effect is the
screening of an impurity spin by a cloud of electrons spread out over a
mesoscopic distance. This cloud has never been observed experimentally.
Recently, aspects of the Kondo effect have been observed in experiments on
quantum dots embedded in quantum wires. Since the length of the wire may be of
order the size of the screening cloud, such systems provide an ideal
opportunity to observe it. We point out that persistent current measurements in
a closed ring provide a conceptually simple way of detecting this fundamental
length scale.Comment: 4 pages, RevTex, 1 postscript figur
Effect of the Coulomb repulsion on the {\it ac} transport through a quantum dot
We calculate in a linear response the admittance of a quantum dot out of
equilibrium. The interaction between two electrons with opposite spins
simultaneously residing on the resonant level is modeled by an Anderson
Hamiltonian. The electron correlations lead to the appearence of a new feature
in the frequency dependence of the conductance. For certain parameter values
there are two crossover frequencies between a capacitive and an inductive
behavior of the imaginary part of the admittance. The experimental implications
of the obtained results are briefly discussed.Comment: 13 pages, REVTEX 3.0, 2 .ps figures from [email protected],
NUB-308
Inflation and nonequilibrium renormalization group
We study de spectrum of primordial fluctuations and the scale dependence of
the inflaton spectral index due to self-interactions of the field. We compute
the spectrum of fluctuations by applying nonequilibrium renormalization group
techniques.Comment: 6 pages, 1 figure, submitted to J. Phys.
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