8,875 research outputs found
Antigen presentation by hapten-specific B lymphocytes. I. Role of surface immunoglobulin receptors.
The present study examines the ability of hapten-specific murine splenic B lymphocytes to present hapten-proteins to carrier-specific T cell hybridomas. BALB/cB cells specific for 2,4,6-trinitrophenyl (TNP) were isolated from spleens of immune mice by elution from TNP-gelatin-coated dishes. Such cells presented the TNP-modified terpolymer, GL phi, at concentrations as low as 0.1 microgram/ml, to a GL phi-specific, I-Ed-restricted, interleukin 2-producing T cell hybridoma. In contrast, the same B lymphocytes required 1,000-fold higher concentrations of unmodified GL phi to stimulate the same T cell hybridoma. The presentation of low concentrations of TNP-GL phi by TNP-specific B lymphocytes was significantly or completely blocked by anti-Ig antibody or TNP-proteins, indicating that surface Ig receptors were critically involved in this phenomenon. Finally, binding of TNP-proteins did not alter the ability of the B cells to present unrelated, unhaptenated proteins or to stimulate alloreactive T cells. These results suggest that surface Ig receptors serve to focus antigens onto specific B lymphocytes and that such cells are highly efficient at presenting linked antigenic determinants to T cells. The implications of these findings for the mechanisms of physiologic, histocompatibility-restricted T-B collaboration are discussed
Determination of the strong coupling from hadronic tau decays using renormalization group summed perturbation theory
We determine the strong coupling constant \alpha_s from the \tau hadroni
width using a renormalization group summed (RGS) expansion of the QCD Adler
function. The main theoretical uncertainty in the extraction of \alpha_s is due
to the manner in which renormalization group invariance is implemented, and the
as yet uncalculated higher order terms in the QCD perturbative series. We show
that new expansion exhibits good renormalization group improvement and the
behaviour of the series is similar to that of the standard CIPT expansion. The
value of the strong coupling in {\bar{\rm MS}} scheme obtained with the RGS
expansion is \alpha_s(M_\tau^2)= 0.338 \pm 0.010. The convergence properties of
the new expansion can be improved by Borel transformation and analytic
continuation in the Borel plane. This is discussed elsewhere in these
proceedings.Comment: Contribution to the proceedings of the workshop "Determination of the
Fundamental Parameters of QCD", Nanyang Technological University, Singapore,
18-22 March 2013, to be published in Mod. Phys. Lett. A, version 2 contains
an extra footnote and a reference compared to version
New constraints on the Pion EM form factor using Pi'(-Q^2)
We study the constraints arising on the expansion parameters c and d of the
Pion electromagnetic form factor from the inclusion of pure space-like data and
the phase of time-like data along with one space-like datum, using as input the
first derivative of the QCD polarization amplitude Pi'(-Q^2). These constraints
when combined with other analyses, provide a valuable check on a determination
of c due to Guo et al. and on our previous work where pionic contribution to
the (g-2) of the muon was used as the input. This work further illustrates the
power of analyticity techniques in form factor analysis.Comment: 8 pages latex, uses EPJA style files, contains 12 figures; replaced
with version accepted for publication in EPJA, minor typos corrected,
discussion improved, reference adde
Perturbative expansion of the QCD Adler function improved by renormalization-group summation and analytic continuation in the Borel plane
We examine the large-order behaviour of a recently proposed
renormalization-group-improved expansion of the Adler function in perturbative
QCD, which sums in an analytically closed form the leading logarithms
accessible from renormalization-group invariance. The expansion is first
written as aneffective series in powers of the one-loop coupling, and its
leading singularities in the Borel plane are shown to be identical to those of
the standard "contour-improved" expansion. Applying the technique of conformal
mappings for the analytic continuation in the Borel plane, we define a class of
improved expansions, which implement both the renormalization-group invariance
and the knowledge about the large-order behaviour of the series. Detailed
numerical studies of specific models for the Adler function indicate that the
new expansions have remarkable convergence properties up to high orders. Using
these expansions for the determination of the strong coupling from the the
hadronic width of the lepton we obtain, with a conservative estimate of
the uncertainty due to the nonperturbative corrections, , which translates to .Comment: 15 pages latex using revtex, 4 figures; v2 corresponds to PRD
version; compared to v1, power-correction estimates have been enlarged
resulting in somewhat larger errors for alpha_S, relevant discussion has been
provided, a reference has been added, minor typographical errors have been
remove
Expansions of hadronic spectral function moments in a nonpower QCD perturbation theory with tamed large order behavior
The moments of the hadronic spectral functions are of interest for the
extraction of the strong coupling and other QCD parameters from the
hadronic decays of the lepton. Motivated by the recent analyses of a
large class of moments in the standard fixed-order and contour-improved
perturbation theories, we consider the perturbative behavior of these moments
in the framework of a QCD nonpower perturbation theory, defined by the
technique of series acceleration by conformal mappings, which simultaneously
implements renormalization-group summation and has a tame large-order behavior.
Two recently proposed models of the Adler function are employed to generate the
higher order coefficients of the perturbation series and to predict the exact
values of the moments, required for testing the properties of the perturbative
expansions. We show that the contour-improved nonpower perturbation theories
and the renormalization-group-summed nonpower perturbation theories have very
good convergence properties for a large class of moments of the so-called
"reference model", including moments that are poorly described by the standard
expansions. The results provide additional support for the plausibility of the
description of the Adler function in terms of a small number of dominant
renormalons.Comment: 15 pages, latex using revtex, 4 figures; compared to v1, slightly
improved figures and discussion, version to appear in PR
Linear Relationship Statistics in Diffusion Limited Aggregation
We show that various surface parameters in two-dimensional diffusion limited
aggregation (DLA) grow linearly with the number of particles. We find the ratio
of the average length of the perimeter and the accessible perimeter of a DLA
cluster together with its external perimeters to the cluster size, and define a
microscopic schematic procedure for attachment of an incident new particle to
the cluster. We measure the fractal dimension of the red sites (i.e., the sites
upon cutting each of them splits the cluster) equal to that of the DLA cluster.
It is also shown that the average number of the dead sites and the average
number of the red sites have linear relationships with the cluster size.Comment: 4 pages, 5 figure
The form factors from Analyticity and Unitarity
Analyticity and unitarity techniques are employed to obtain bounds on the
shape parameters of the scalar and vector form factors of semileptonic
decays. For this purpose we use vector and scalar correlators evaluated in
pQCD, a low energy theorem for scalar form factor, lattice results for the
ratio of kaon and pion decay constants, chiral perturbation theory calculations
for the scalar form factor at the Callan-Treiman point and experimental
information on the phase and modulus of form factors up to an energy
\tin=1 {\rm GeV}^2. We further derive regions on the real axis and in the
complex-energy plane where the form factors cannot have zeros.Comment: 6 pages, 5 figures; Seminar given at DAE-BRNS Workshop on Hadron
Physics Bhabha Atomic Research Centre, Mumbai, India October 31-November 4,
2011, submitted to Proceeding
Constraining Form Factors with the Method of Unitarity Bounds
The availability of a reliable bound on an integral involving the square of
the modulus of a form factor on the unitarity cut allows one to constrain the
form factor at points inside the analyticity domain and its shape parameters,
and also to isolate domains on the real axis and in the complex energy plane
where zeros are excluded. In this lecture note, we review the mathematical
techniques of this formalism in its standard form, known as the method of
unitarity bounds, and recent developments which allow us to include information
on the phase and modulus along a part of the unitarity cut. We also provide a
brief summary of some results that we have obtained in the recent past, which
demonstrate the usefulness of the method for precision predictions on the form
factors.Comment: 12 pages, 2 figures; Lecture given at the DAE-BRNS Workshop on Hadron
Physics, Bhabha Atomic Research Centre, Mumbai, India, October 31-November 4,
2011, submitted to Proceeding
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