364 research outputs found
New Predictions for generalized spin polarizabilities from heavy baryon chiral perturbation theory
We extract the next-to-next-to-leading order results for spin-flip
generalized polarizabilities (GPs) of the nucleon from the spin-dependent
amplitudes for virtual Compton scattering (VCS) at in heavy
baryon chiral perturbation theory. At this order, no unknown low energy
constants enter the theory, allowing us to make absolute predictions for all
spin-flip GPs. Furthermore, by using constraint equations between the GPs due
to nucleon crossing combined with charge conjugation symmetry of the VCS
amplitudes, we get a next-to-next-to-next-to-leading order prediction for one
of the GPs. We provide estimates for forthcoming double polarization
experiments which allow to access these spin-flip GPs of the nucleon.Comment: 15 pages, 3 figure
Light-by-light scattering sum rules constraining meson transition form factors
Relating the forward light-by-light scattering to energy weighted integrals
of the \gamma* \gamma -fusion cross sections, with one real photon (\gamma) and
one virtual photon (\gamma*), we find two new exact super-convergence
relations. They complement the known super-convergence relation based on the
extension of the GDH sum rule to the light-light system. We also find a set of
sum rules for the low-energy photon-photon interaction. All of the new
relations are verified here exactly at leading order in scalar and spinor QED.
The super-convergence relations, applied to the \gamma* \gamma -production of
mesons, lead to intricate relations between the \gamma \gamma -decay widths or
the \gamma* \gamma -transition form factors for (pseudo-) scalar, axial-vector
and tensor mesons. We discuss the phenomenological implications of these
results for mesons in both the light-quark sector and the charm-quark sector.Comment: 32 pages, 7 figure
Generalized Parton Distributions and Nucleon Resonances
We discuss the relations between Generalized Parton Distributions (GPDs) and
nucleon resonances. We first briefly introduce the concept of "transition"
GPDs. Then we discuss a straightforward application to the modelization of the
N- magnetic transition form factor. Finally, we discuss the
experimental aspects of the subject and present first preliminary experimental
investigations in this field.Comment: 6 pages, 4 figures, Nstar02 conference proceedin
How to reconcile the Rosenbluth and the polarization transfer method in the measurement of the proton form factors
The apparent discrepancy between the Rosenbluth and the polarization transfer
method for the ratio of the electric to magnetic proton form factors can be
explained by a two-photon exchange correction which does not destroy the
linearity of the Rosenbluth plot. Though intrinsically small, of the order of a
few percent of the cross section, this correction is kinematically enhanced in
the Rosenbluth method while it is small for the polarization transfer method,
at least in the range of (Q^2) where it has been used until now.Comment: 4 pages, 4 figures. Version accepted for publication in Phys. Rev.
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Olfactory receptors are displayed on dog mature sperm cells
Olfactory receptors constitute a huge family of structurally related G protein-coupled receptors, with up to a thousand members expected. We have shown previously that genes belonging to this family were expressed in the male germ line from both dog and human. The functional significance of this unexpected site of expression was further investigated in the present study. We demonstrate that a few dog genes representative of various subfamilies of olfactory receptors are expressed essentially in testis, with little or no expression in olfactory mucosa. Other randomly selected members of the family show the expected site of expression, restricted to the olfactory system. Antibodies were generated against the deduced amino acid sequence of the most abundantly expressed olfactory receptor gene in dog testis. The purified serum was able to detect the gene product (DTMT receptor) in late round and elongated spermatids, as well as in the cytoplasmic droplet that characterizes the maturation of dog sperm cells, and on the tail midpiece of mature spermatozoa. Western blotting further confirmed the presence of a 40-kD immunoreactive protein in the membrane of mature sperm cells. Altogether , these results demonstrate that the main expression site of a subset of the large olfactory receptor gene family is not olfactory mucosa but testis. This expression correlates with the presence of the corresponding protein during sperm cell maturation, and on mature sperm cells. The pattern of expression is consistent with a role as sensor for unidentified chemicals possibly involved in the control of mammalian sperm maturation, migration, and/or fertilizatio
Serological profiles in nursery piglets colonized with Staphylococcus aureus
At present, the immune response of pigs in relation to Staphylococcus aureus carriage is poorly understood. This study aimed at investigating the dynamics of the anti-staphylococcal humoral immune response in methicillin-susceptible S. aureus (MSSA)-positive piglets and at assessing the effect of the experimental introduction of a methicillin-resistant S. aureus (MRSA) Sequence Type (ST) 398 strain. Therefore, serum samples were collected at different times from 31 weaned piglets originating from four different sows. Twenty-four out of the 31 piglets were challenged with MRSA ST398. The serum samples were analysed for IgG antibodies to 39 S. aureus antigens, using a multiplex bead-based assay (xMAP technology, Luminex Corporation). Though antibody responses showed broad inter-individual variability, serological results appeared to be clustered by litter of origin. For most antigens, an age-related response was observed with an apparent increase in antibody titres directed against staphylococcal microbial surface components recognizing adhesive matrix molecules (MSCRAMMs), which have been shown to play a role in S. aureus colonization. In most animals, antibody titres directed against staphylococcal toxins or immune-modulating proteins decreased with age, possibly reflecting absence of bacterial invasion. The introduction of MRSA ST398 did not elicit a significant humoral immune reaction.
This study describes, for the first time, the humoral immune response in weaned pigs colonized with S. aureus
Soft spectator scattering in the nucleon form factors at large within the SCET approach
The proton form factors at large momentum transfer are dominated by two
contributions which are associated with the hard and soft rescattering
respectively. Motivated by a very active experimental form factor program at
intermediate values of momentum transfers, ,
where an understanding in terms of only a hard rescattering mechanism cannot
yet be expected, we investigate in this work the soft rescattering contribution
using soft collinear effective theory (SCET). Within such description, the form
factor is characterized, besides the hard scale , by a semi-hard scale , which arises due to presence of soft spectators, with virtuality
( GeV), such that . We show that in this case a two-step factorization can be
successfully carried out using the SCET approach. In a first step (SCET),
we perform the leading order matching of the QCD electromagnetic current onto
the relevant SCET operators and perform a resummation of large logarithms
using renormalization group equations. We then discuss the further matching
onto a SCET framework, and propose the complete factorization formula
for the Dirac form factor, accounting for both hard and soft contributions. We
also present a qualitative discussion of the phenomenological consequences of
this new framework.Comment: 33 pages, 19 figures; typos corrected, text improved. Version to
appear in Phys.Rev.
Generalized polarizabilities of the nucleon studied in the linear sigma model (II)
In a previous paper virtual Compton scattering off the nucleon has been
investigated in the one-loop approximation of the linear sigma model in order
to determine the 3 scalar generalized polarizabilities. We have now extended
this work and calculated the 7 vector polarizabilities showing up in the
spin-dependent amplitude of virtual Compton scattering. The results fulfill 3
model-independent constraints recently derived. Compared to the constituent
quark model there exist enormous differences for some of the vector
polarizabilities. At vanishing three-momentum of the virtual photon, the
analytical results of the sigma model and of chiral perturbation theory can be
related. The influence of the exchange in the channel has been
discussed in some detail. Besides, the vector polarizabilities determine 2
linear combinations of the third order spin-polarizabilities appearing in real
Compton scattering.Comment: 17 pages, 4 figures, latex2e (Revtex), submitted to Z. Phys.
Dispersion relation formalism for virtual Compton scattering and the generalized polarizabilities of the nucleon
A dispersion relation formalism for the virtual Compton scattering (VCS)
reaction on the proton is presented, which for the first time allows a
dispersive evaluation of 4 generalized polarizabilities at a four-momentum
transfer 0.5 GeV. The dispersive integrals are calculated using
a state-of-the-art pion photo- and electroproduction analysis. The dispersion
formalism provides a new tool to analyze VCS experiments above pion threshold,
thus increasing the sensitivity to the generalized polarizabilities of the
nucleon.Comment: 4 pages, 2 figure
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