11,256 research outputs found
Resummation of Large Endpoint Corrections to Color-Octet J/psi Photoproduction
An unresolved problem in J/psi phenomenology is a systematic understanding of
the differential photoproduction cross section, dsigma/dz [gamma + p -> J/psi +
X], where z= E_psi/E_gamma in the proton rest frame. In the non-relativistic
QCD (NRQCD) factorization formalism, fixed-order perturbative calculations of
color-octet mechanisms suffer from large perturbative and nonperturbative
corrections that grow rapidly in the endpoint region, z -> 1. In this paper,
NRQCD and soft collinear effective theory are combined to resum these large
corrections to the color-octet photoproduction cross section. We derive a
factorization theorem for the endpoint differential cross section involving the
parton distribution function and the color-octet J/psi shape functions. A one
loop matching calculation explicitly confirms our factorization theorem at
next-to-leading order. Large perturbative corrections are resummed using the
renormalization group. The calculation of the color-octet contribution to
dsigma/dz is in qualitative agreement with data. Quantitative tests of the
universality of color-octet matrix elements require improved knowledge of shape
functions entering these calculations as well as resummation of the
color-singlet contribution which accounts for much of the total cross section
and also peaks near the endpoint.Comment: 30 pages, 6 figure
Exclusive Radiative Decays of Upsilon in SCET
We study exclusive radiative decays of the using soft-collinear
effective theory and non-relativistic QCD. In contrast to inclusive radiative
decays at the endpoint we find that color-octet contributions are power
suppressed in exclusive decays, and can safely be neglected, greatly
simplifying the analysis. We determine the complete set of Lorentz structures
that can appear in the SCET Wilson coefficients and match onto them using
results from a previous calculation. We run these coefficients from the scale
\mups to the scale , thereby summing large
logarithms. Finally we use our results to predict the ratio of branching
fractions , , and the partial rate for .Comment: 17 pages, 2 figures. Updated to reflect published versio
Calculation of the nucleon axial charge in lattice QCD
Protons and neutrons have a rich structure in terms of their constituents,
the quarks and gluons. Understanding this structure requires solving Quantum
Chromodynamics (QCD). However QCD is extremely complicated, so we must
numerically solve the equations of QCD using a method known as lattice QCD.
Here we describe a typical lattice QCD calculation by examining our recent
computation of the nucleon axial charge.Comment: Prepared for Scientific Discovery through Advanced Computing (SciDAC
2006), Denver, Colorado, June 25-29 200
Nucleon structure in the chiral regime with domain wall fermions on an improved staggered sea
Moments of unpolarized, helicity, and transversity distributions,
electromagnetic form factors, and generalized form factors of the nucleon are
presented from a preliminary analysis of lattice results using pion masses down
to 359 MeV. The twist two matrix elements are calculated using a mixed action
of domain wall valence quarks and asqtad staggered sea quarks and are
renormalized perturbatively. Several observables are extrapolated to the
physical limit using chiral perturbation theory. Results are compared with
experimental moments of quark distributions and electromagnetic form factors
and phenomenologically determined generalized form factors, and the
implications on the transverse structure and spin content of the nucleon are
discussed.Comment: Talks of J.W. Negele and D.B. Renner at Lattice 200
Myosin VIIA is required for aminoglycoside accumulation in cochlear hair cells.
Myosin VIIA is expressed by sensory hair cells and has a primary structure predicting a role in membrane trafficking and turnover, processes that may underlie the susceptibility of hair cells to aminoglycoside antibiotics. [3H]Gentamicin accumulation and the effects of aminoglycosides were therefore examined in cochlear cultures of mice with different missense mutations in the myosin VIIA gene, Myo7a, to see whether myosin VIIA plays a role in aminoglycoside ototoxicity. Hair cells from homozygous mutant Myo7a(sh1) mice, with a mutation in a non-conserved region of the myosin VIIA head, respond rapidly to aminoglycoside treatment and accumulate high levels of gentamicin. Hair cells from homozygous mutant Myo7a(6J) mice, with a mutation at a highly conserved residue close to the ATP binding site of the myosin VIIA head, do not accumulate [3H]gentamicin and are protected from aminoglycoside ototoxicity. Hair cells from heterozygotes of both alleles accumulate [3H]gentamicin and respond to aminoglycosides. Although aminoglycoside uptake is thought to be via apical surface-associated endocytosis, coated pit numbers on the apical membrane of heterozygous and homozygous Myo7a(6J) hair cells are similar. Pulse-chase experiments with cationic ferritin confirm that the apical endocytotic pathway is functional in homozygous Myo7a(6J) hair cells. Transduction currents can be recorded from both heterozygous and homozygous Myo7a(6J) hair cells, suggesting it is unlikely that the drug enters via diffusion through the mechanotransducer channel. The results show that myosin VIIA is required for aminoglycoside accumulation in hair cells. Myosin VIIA may transport a putative aminoglycoside receptor to the hair cell surface, indirectly translocate it to sites of membrane retrieval, or retain it in the endocytotic pathway
Effects of surface reflectance and 3D shape on perceived rotation axis
Cataloged from PDF version of article.Surface specularity distorts the optic flow generated by a moving object in a way that provides important cues for identifying surface material properties (Doerschner, Fleming et al., 2011). Here we show that specular flow can also affect the perceived rotation axis of objects. In three experiments, we investigate how threedimensional shape and surface material interact to affect the perceived rotation axis of unfamiliar irregularly shaped and isotropic objects. We analyze observers' patterns of errors in a rotation axis estimation task under four surface material conditions: shiny, matte textured, matte untextured, and silhouette. In addition to the expected large perceptual errors in the silhouette condition, we find that the patterns of errors for the other three material conditions differ from each other and across shape category, yielding the largest differences in error magnitude between shiny and matte, textured isotropic objects. Rotation axis estimation is a crucial implicit computational step to perceive structure from motion; therefore, we test whether a structure from a motion-based model can predict the perceived rotation axis for shiny and matte, textured objects. Our model's predictions closely follow observers' data, even yielding the same reflectance-specific perceptual errors. Unlike previous work (Caudek & Domini, 1998), our model does not rely on the assumption of affine image transformations; however, a limitation of our approach is its reliance on projected correspondence, thus having difficulty in accounting for the perceived rotation axis of smooth shaded objects and silhouettes. In general, our findings are in line with earlier research that demonstrated that shape from motion can be extracted based on several different types of optical deformation (Koenderink & Van Doorn, 1976; Norman & Todd, 1994; Norman, Todd, & Orban, 2004; Pollick, Nishida, Koike, & Kawato, 1994; Todd, 1985). © 2013 Arvo
Non-local quantum correlations and detection processes in QFT
Quantum detection processes in QFT must play a key role in the description of
quantum field correlations, such as the appearance of entanglement, and of
causal effects. We consider the detection in the case of a simple QFT model
with a suitable interaction to exact treatment, consisting of a quantum scalar
field coupled linearly to a classical scalar source. We then evaluate the
response function to the field quanta of two-level point-like quantum model
detectors, and analyze the effects of the approximation adopted in standard
detection theory. We show that the use of the RWA, that characterizes the
Glauber detection model, leads in the detector response to non-local terms
corresponding to an instantaneously spreading of source effects over the whole
space. Other detector models, obtained with non-standard or the no-application
of RWA, give instead local responses to field quanta, apart from source
independent vacuum contribution linked to preexisting correlations of
zero-point field.Comment: 23 page
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