1,476 research outputs found
The Proton Spin and Flavor Structure in the Chiral Quark Model
After a pedagogical review of the simple constituent quark model and deep
inelastic sum rules, we describe how a quark sea as produced by the emission of
internal Goldstone bosons by the valence quarks can account for the observed
features of proton spin and flavor structures. Some issues concerning the
strange quark content of the nucleon are also discussed.Comment: 59 pages with table of contents, Lecture delivered at the Schladming
Winter School (March 1997), to be published by Springer-Verlag under the
title "Computing Particle Properties" (eds. C. B. Lang and H. Gausterer
Scattering of massive W bosons into gravitinos and tree unitarity in broken supergravity
The WW scattering into gravitino and gaugino is here investigated in the
broken phase, by using both gauge and mass eigenstates. Differently from what
is obtained for unbroken gauge symmetry, we find in the scattering amplitudes
new structures, which can lead to violation of unitarity above a certain scale.
This happens because, in the annihilation diagram, the longitudinal degrees of
freedom in the propagator of the gauge bosons disappear from the amplitude, by
virtue of the SUGRA vertex. We show that the longitudinal polarizations of the
on-shell W become strongly interacting in the high energy limit, and that the
inclusion of diagrams with off-shell scalars of the MSSM does not cancel the
divergences.Comment: 26 pages, 17 figures. Uses JHEP3.cls, epsfig.sty and axodraw.sty.
Some references, together with Ward identities in the basis of mass
eigenstates, have been added. Version accepted for publication in JHE
The AddAB helicase–nuclease catalyses rapid and processive DNA unwinding using a single Superfamily 1A motor domain
The oligomeric state of Superfamily I DNA helicases is the subject of considerable and ongoing debate. While models based on crystal structures imply that a single helicase core domain is sufficient for DNA unwinding activity, biochemical data from several related enzymes suggest that a higher order oligomeric species is required. In this work we characterize the helicase activity of the AddAB helicase–nuclease, which is involved in the repair of double-stranded DNA breaks in Bacillus subtilis. We show that the enzyme is functional as a heterodimer of the AddA and AddB subunits, that it is a rapid and processive DNA helicase, and that it catalyses DNA unwinding using one single-stranded DNA motor of 3′→5′ polarity located in the AddA subunit. The AddB subunit contains a second putative ATP-binding pocket, but this does not contribute to the observed helicase activity and may instead be involved in the recognition of recombination hotspot sequences
Resonant Enhancement of Electronic Raman Scattering
We present an exact solution for electronic Raman scattering in a
single-band, strongly correlated material, including nonresonant, resonant and
mixed contributions. Results are derived for the spinless Falicov-Kimball
model, employing dynamical mean field theory; this system can be tuned through
a Mott metal-insulator transition.Comment: 4 pages, 3 figures, contribution to the SNS'2004 conferenc
Experimental Control and Characterization of Autophagy in Drosophila
Insects such as the fruit fly Drosophila melanogaster, which fundamentally reorganize their body plan during metamorphosis, make extensive use of autophagy for their normal development and physiology. In the fruit fly, the hepatic/adipose organ known as the fat body accumulates nutrient stores during the larval feeding stage. Upon entering metamorphosis, as well as in response to starvation, these nutrients are mobilized through a massive induction of autophagy, providing support to other tissues and organs during periods of nutrient deprivation. High levels of autophagy are also observed in larval tissues destined for elimination, such as the salivary glands and larval gut. Drosophila is emerging as an important system for studying the functions and regulation of autophagy in an in vivo setting. In this chapter we describe reagents and methods for monitoring autophagy in Drosophila, focusing on the larval fat body. We also describe methods for experimentally activating and inhibiting autophagy in this system and discuss the potential for genetic analysis in Drosophila to identify novel genes involved in autophagy
On the origin of the A and B electronic Raman scattering peaks in the superconducting state of YBaCuO
The electronic Raman scattering has been investigated in optimally oxygen
doped YBaCuO single crystals as well as in crystals
with non-magnetic, Zn, and magnetic, Ni, impurities. We found that the
intensity of the A peak is impurity independent and their energy to
ratio is almost constant (). Moreover, the
signal at the B channel is completely smeared out when non-magnetic Zn
impurities are present. These results are qualitatively interpreted in terms of
the Zeyher and Greco's theory that relates the electronic Raman scattering in
the A and B channels to \textit{d}-CDW and superconducting order
parameters fluctuations, respectively.Comment: Submited to Phys. Rev. Let
Net ecosystem fluxes of isoprene over tropical South America inferred from Global Ozone Monitoring Experiment (GOME) observations of HCHO columns
We estimate isoprene emissions over tropical South America during 1997-2001 using column measurements of formaldehyde (HCHO) from the Global Ozone Monitoring Experiment (GOME) satellite instrument, the GEOS-Chem chemistry transport model, and the MEGAN (Model of Emissions of Gases and Aerosols from Nature) bottom-up isoprene inventory. GEOS-Chem is qualitatively consistent with in situ ground-based and aircraft concentration profiles of isoprene and HCHO, and GOME HCHO column data (r = 0.41; bias = +35%), but has less skill in reproducing wet season observations. Observed variability of GOME HCHO columns over South America is determined largely by isoprene and biomass burning. We find that the column contributions from other biogenic volatile organic compounds (VOC) are typically smaller than the column fitting uncertainty. HCHO columns influenced by biomass burning are removed using Along Track Scanning Radiometer (ATSR) firecounts and GOME NO2 columns. We find that South America can be split into eastern and western regions, with fires concentrated over the eastern region. A monthly mean linear transfer function, determined by GEOS-Chem, is used to infer isoprene emissions from observed HCHO columns. The seasonal variation of GOME isoprene emissions over the western region is broadly consistent with MEGAN (r = 0.41; bias = -256%), with largest isoprene emissions during the dry season when the observed variability is consistent with knowledge of temperature dependence. During the wet season other unknown factors play a significant role in determining observed variability. Copyright 2008 by the American Geophysical Union
Close-Packing of Clusters: Application to Al_100
The lowest energy configurations of close-packed clusters up to N=110 atoms
with stacking faults are studied using the Monte Carlo method with Metropolis
algorithm. Two types of contact interactions, a pair-potential and a many-atom
interaction, are used. Enhanced stability is shown for N=12, 26, 38, 50, 59,
61, 68, 75, 79, 86, 100 and 102, of which only the sizes 38, 75, 79, 86, and
102 are pure FCC clusters, the others having stacking faults. A connection
between the model potential and density functional calculations is studied in
the case of Al_100. The density functional calculations are consistent with the
experimental fact that there exist epitaxially grown FCC clusters starting from
relatively small cluster sizes. Calculations also show that several other
close-packed motifs existwith comparable total energies.Comment: 9 pages, 7 figure
Renormalization of the asymptotically expanded Yang-Mills spectral action
We study renormalizability aspects of the spectral action for the Yang-Mills
system on a flat 4-dimensional background manifold, focusing on its asymptotic
expansion. Interpreting the latter as a higher-derivative gauge theory, a
power-counting argument shows that it is superrenormalizable. We determine the
counterterms at one-loop using zeta function regularization in a background
field gauge and establish their gauge invariance. Consequently, the
corresponding field theory can be renormalized by a simple shift of the
spectral function appearing in the spectral action.
This manuscript provides more details than the shorter companion paper, where
we have used a (formal) quantum action principle to arrive at gauge invariance
of the counterterms. Here, we give in addition an explicit expression for the
gauge propagator and compare to recent results in the literature.Comment: 28 pages; revised version. To appear in CMP. arXiv admin note:
substantial text overlap with arXiv:1101.480
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