2,802 research outputs found
Recent developments in effective field theory
We will give a short introduction to the one-nucleon sector of chiral
perturbation theory and will address the issue of a consistent power counting
and renormalization. We will discuss the infrared regularization and the
extended on-mass-shell scheme. Both allow for the inclusion of further degrees
of freedom beyond pions and nucleons and the application to higher-loop
calculations. As applications we consider the chiral expansion of the nucleon
mass to order O(q^6) and the inclusion of vector and axial-vector mesons in the
calculation of nucleon form factors.Comment: 8 pages, 6 figures, invited talk given at International School of
Nuclear Physics, 29th Course "Quarks in Hadrons and Nuclei", Erice, Sicily,
16 - 24 September 200
A possible experimental determination of from decays
scattering and decays are studied at leading order of
improved chiral perturbation theory. It is shown that high precision experiments at, e.g., DANE should allow for a direct measurement of
the quark mass ratio /.Comment: 9 pages, preprint IPNO-TH 93-17, 2 figures not included, available
upon request, plain Latex, April 199
On the pion cloud of the nucleon
We evaluate the two--pion contribution to the nucleon electromagnetic form
factors by use of dispersion analysis and chiral perturbation theory. After
subtraction of the rho--meson component, we calculate the distributions of
charge and magnetization in coordinate space, which can be interpreted as the
effects of the pion cloud. We find that the charge distribution of this pion
cloud effect peaks at distances of about 0.3 fm. Furthermore, we calculate the
contribution of the pion cloud to the isovector charges and radii of the
nucleon.Comment: 7 pages, latex, 3 ps figures, minor change
Infrared regularization with spin-3/2 fields
We present a Lorentz-invariant formulation of baryon chiral perturbation
theory including spin-3/2 fields. Particular attention is paid to the
projection on the spin-3/2 components of the delta fields. We also discuss the
nucleon mass and the pion-nucleon sigma term.Comment: 9 pp, 1 fi
Structure of the nucleon in chiral perturbation theory
We discuss a renormalization scheme for relativistic baryon chiral
perturbation theory which provides a simple and consistent power counting for
renormalized diagrams. The method involves finite subtractions of dimensionally
regularized diagrams beyond the standard modified minimal subtraction scheme of
chiral perturbation theory to remove contributions violating the power
counting. This is achieved by a suitable renormalization of the parameters of
the most general effective Lagrangian. As applications we discuss the mass of
the nucleon, the term, and the scalar and electromagnetic form
factors.Comment: Invited talk given by S. Scherer at the Fourth International
Conference on Perspectives in Hadronic Physics, Trieste, Italy, 12 -16 May
2003, 8 pages, 7 figure
Chiral Extrapolations and the Covariant Small Scale Expansion
We calculate the nucleon and the delta mass to fourth order in a covariant
formulation of the small scale expansion. We analyze lattice data from the MILC
collaboration and demonstrate that the available lattice data combined with our
knowledge of the physical values for the nucleon and delta masses lead to
consistent chiral extrapolation functions for both observables up to fairly
large pion masses. This holds in particular for very recent data on the delta
mass from the QCDSF collaboration. The resulting pion-nucleon sigma term is
sigma_{piN} = 48.9 MeV. This first quantitative analysis of the quark-mass
dependence of the structure of the Delta(1232) in full QCD within chiral
effective field theory suggests that (the real part of) the nucleon-delta
mass-splitting in the chiral limit, Delta_0 = 0.33 GeV, is slightly larger than
at the physical point. Further analysis of simultaneous fits to nucleon and
delta lattice data are needed for a precision determination of the properties
of the first excited state of the nucleon.Comment: 11 pp, 2 figs, version accepted for publication in Phys. Lett.
Simple-Current Symmetries, Rank-Level Duality, and Linear Skein Relations for Chern-Simons Graphs
A previously proposed two-step algorithm for calculating the expectation
values of Chern-Simons graphs fails to determine certain crucial signs. The
step which involves calculating tetrahedra by solving certain non- linear
equations is repaired by introducing additional linear equations. As a first
step towards a new algorithm for general graphs we find useful linear equations
for those special graphs which support knots and links. Using the improved set
of equations for tetrahedra we examine the symmetries between tetrahedra
generated by arbitrary simple currents. Along the way we uncover the classical
origin of simple-current charges. The improved skein relations also lead to
exact identities between planar tetrahedra in level and level
CS theories, where denotes a classical group. These results are
recast as identities for quantum -symbols and WZW braid matrices. We obtain
the transformation properties of arbitrary graphs and links under simple
current symmetries and rank-level duality. For links with knotted components
this requires precise control of the braid eigenvalue permutation signs, which
we obtain from plethysm and an explicit expression for the (multiplicity free)
signs, valid for all compact gauge groups and all fusion products.Comment: 58 pages, BRX-TH-30
Geometry of WZW Orientifolds
We analyze unoriented Wess-Zumino-Witten models from a geometrical point of
view. We show that the geometric interpretation of simple current crosscap
states is as centre orientifold planes localized on conjugacy classes of the
group manifold. We determine the locations and dimensions of these planes for
arbitrary simply-connected groups and orbifolds thereof. The dimensions of the
O-planes turn out to be given by the dimensions of symmetric coset manifolds
based on regular embeddings. Furthermore, we give a geometrical interpretation
of boundary conjugation in open unoriented WZW models; it yields D-branes
together with their images under the orientifold projection. To find the
agreement between O-planes and crosscap states, we find explicit answers for
lattice extensions of Gaussian sums. These results allow us to express the
modular P-matrix, which is directly related to the crosscap coefficient, in
terms of characters of the horizontal subgroup of the affine Lie algebra. A
corollary of this relation is that there exists a formal linear relation
between the modular P- and the modular S-matrix.Comment: 35 pages LaTeX, 2 tables; Proof added for symmetric space relation;
minor improvements; references adde
Chiral perturbation theory - Success and challenge
Chiral perturbation theory is the effective field theory of the strong
interactions at low energies. We will give a short introduction to chiral
perturbation theory for mesons and will discuss, as an example, the
electromagnetic polarizabilities of the pion. These have recently been
extracted from an experiment on radiative photoproduction from the
proton () at the Mainz Microtron MAMI. Next we will
turn to the one-baryon sector of chiral perturbation theory and will address
the issue of a consistent power counting scheme. As examples of the
heavy-baryon framework we will comment on the extraction of the axial radius
from pion electroproduction and will discuss the generalized polarizabilities
of the proton. Finally, we will discuss two recently proposed manifestly
Lorentz-invariant renormalization schemes and illustrate their application in a
calculation of the nucleon electromagnetic form factors.Comment: 12 pages, 13 figures, invited talk given at the Symposium 20 Years of
Physics at the Mainz Microtron MAMI, 20 - 22 October 2005, Mainz, German
The nucleon and Delta-resonance masses in relativistic chiral effective-field theory
We study the chiral behavior of the nucleon and -isobar masses within
a manifestly covariant chiral effective-field theory, consistent with the
analyticity principle. We compute the and one-loop
contributions to the mass and field-renormalization constant, and find that
they can be described in terms of universal relativistic loop functions,
multiplied by appropriate spin, isospin and coupling constants. We show that
these relativistic one-loop corrections, when properly renormalized, obey the
chiral power-counting and vanish in the chiral limit. The results including
only the -loop corrections compare favorably with the lattice QCD data
for the pion-mass dependence of the nucleon and masses, while
inclusion of the loops tends to spoil this agreement.Comment: 13 pages, 3 figs, 2 table
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