197 research outputs found
Induced scalar potentials for hypermultiplets
Charged BPS hypermultiplets can develop a non-trivial self-interaction in the
Coulomb branch of an N=2 supersymmetric gauge theory, whereas neutral BPS
hypermultiplets in the Higgs branch may also have a non-trivial
self-interaction in the presence of Fayet-Iliopoulos terms. The exact
hypermultiplet low-energy effective action (LEEA) takes the form of the
non-linear sigma-model (NLSM) with a hyper-K"ahler metric. A non-trivial scalar
potential is also quantum-mechanically generated at non-vanishing central
charges, either perturbatively (Coulomb branch), or non-perturbatively (Higgs
branch). We calculate the effective scalar potentials for (i) a single charged
hypermultiplet in the Coulomb branch and (ii) a single neutral hypermultiplet
in the Higgs branch. The first case corresponds to the NLSM with the Taub-NUT
(or KK-monopole) metric for the kinetic LEEA, whereas the second one is
attached to the NLSM having the Eguchi-Hanson instanton metric.Comment: 12 pages, LaTeX, macros include
Generalized polarizabilities of the pion in chiral perturbation theory
We present a calculation of the virtual Compton scattering amplitude for at . We explicitly derive expressions for generalized electromagnetic polarizabilities and discuss alternative definitions of these quantities
Virtual Compton Scattering off the Pseudoscalar Meson Octet
We present a calculation of the virtual Compton scattering amplitude for the
pseudoscalar meson octet in the framework of chiral perturbation theory at
. We calculate the electromagnetic generalized polarizabilities
and compare the results in the real Compton scattering limit to available
experimental values. Finally, we give predictions for the differential cross
section of electron-meson bremsstrahlung.Comment: 9 pages, Latex, uses cjp3.sty (included), 4 eps figures, to be
published in the proceedings of the 13th Indian-Summer School "Understanding
the Structure of Hadrons," August 28 - September 1, 2000, Prague, Czech
Republi
Radiative corrections to real and virtual muon Compton scattering revisited
We calculate in closed analytical form the one-photon loop radiative
corrections to muon Compton scattering .
Ultraviolet and infrared divergencies are both treated in dimensional
regularization. Infrared finiteness of the (virtual) radiative corrections is
achieved (in the standard way) by including soft photon radiation below an
energy cut-off . We find that the anomalous magnetic moment
provides only a very small portion of the full radiative
corrections. Furthermore, we extend our calculation of radiative corrections to
the muon-nucleus bremsstrahlung process (or virtual muon Compton scattering
). These results are particularly relevant
for analyzing the COMPASS experiment at CERN in which muon-nucleus
bremsstrahlung serves to calibrate the Primakoff scattering of high-energy
pions off a heavy nucleus with the aim of measuring the pion electric and
magnetic polarizabilities. We find agreement with an earlier calculation of
these radiative corrections based on a different method.Comment: 20 pages, 10 figure, to be published in Nuclear Physics
Applicability of Perturbative QCD to Pion Virtual Compton Scattering
We study explicitly the applicability of perturbative QCD (pQCD) to the pion
virtual Compton scattering. It is found that there are central-region
singularities introduced by the QCD running coupling constant, in addition to
the end-point singularities generally existed in other exclusive processes such
as the pion form factor. We introduce a simple technique to evaluate the
contributions from these singularities, so that we can arrive at a judgement
that these contributions will be unharmful to the applicability of pQCD at
certain energy scale, i.e., the ``work point'' which is defined to determine
when pQCD is applicable to exclusive processes. The applicability of pQCD for
different pion distribution amplitudes are explored in detail. We show that
pQCD begins to work at 10 . If we relax our constraint to a weak
sense, the work point may be as low as 4 .Comment: 13 Latex pages, 10 figures, to appear in PL
Sphingosine kinase 1/S1P receptor signaling axis is essential for cellular uptake of Neisseria meningitidis in brain endothelial cells
Invasion of brain endothelial cells (BECs) is central to the pathogenicity of Neisseria meningitidis infection. Here, we established a key role for the bioactive sphingolipid sphingosine-1-phosphate (S1P) and S1P receptor (S1PR) 2 in the uptake process. Quantitative sphingolipidome analyses of BECs infected with N. meningitidis revealed elevated S1P levels, which could be attributed to enhanced expression of the enzyme sphingosine kinase 1 and its activity. Increased activity was dependent on the interaction of meningococcal type IV pilus with the endothelial receptor CD147. Concurrently, infection led to increased expression of the S1PR2. Blocking S1PR2 signaling impaired epidermal growth factor receptor (EGFR) phosphorylation, which has been shown to be involved in cytoskeletal remodeling and bacterial endocytosis. Strikingly, targeting S1PR1 or S1PR3 also interfered with bacterial uptake. Collectively, our data support a critical role of the SphK/S1P/S1PR axis in the invasion of N. meningitidis into BECs, defining a potential target for adjuvant therapy
Final-sate radiation in electron-positron annihilation into a pion pair
The process of annihilation into a pair with radiation
of a photon is considered. The amplitude of the reaction
consists of the model independent initial-state
radiation (ISR) and model-dependent final-state radiation (FSR). The general
structure of the FSR tensor is constructed from Lorentz covariance, gauge
invariance and discrete symmetries in terms of the three invariant functions.
To calculate these functions we apply Chiral Perturbation Theory (ChPT) with
vector and axial-vector mesons. The contribution of
process to the muon anomalous magnetic moment is evaluated, and results are
compared with the dominant contribution in the framework of a hybrid model,
consisting of VMD and point-like scalar eletrodynamics. The developed approach
allows us also to calculate the charge asymmetry.Comment: 21 pages, 8 figure
Pion Generalized Dipole Polarizabilities by Virtual Compton Scattering
We present a calculation of the cross section and the event generator of the
reaction . This reaction is sensitive to the pion
generalized dipole polarizabilities, namely, the longitudinal electric
, the transverse electric , and the magnetic
which, in the real-photon limit, reduce to the ordinary electric
and magnetic polarizabilities and , respectively.
The calculation of the cross section is done in the framework of chiral
perturbation theory at . A pion VCS event generator has been
written which is ready for implementation in GEANT simulation codes or for
independent use.Comment: 33 pages, Revtex, 15 figure
- …