18,961 research outputs found
Aspects of Nucleon Compton Scattering
We consider the spin-averaged nucleon forward Compton scattering amplitude in
heavy baryon chiral perturbation theory including all terms to order . The chiral prediction for the spin-averaged forward Compton scattering
amplitude is in good agreement with the data for photon energies MeV. We also evaluate the nucleon electric and magnetic Compton
polarizabilities to this order and discuss the uncertainties of the various
counter terms entering the chiral expansion of these quantities.Comment: 17 pp, TeX, 7 figures available from the authors, preprint CRN-93/5
Topics in Chiral Perturbation Theory
I consider some selected topics in chiral perturbation theory (CHPT). For the
meson sector, emphasis is put on processes involving pions in the isospin zero
S-wave which require multi-loop calculations. The advantages and shortcomings
of heavy baryon CHPT are discussed. Some recent results on the structure of the
baryons are also presented.Comment: 30 pp, TeX, Review talk, Third Workshop on High Energy Particle
Physics (WHEPP III), Madras, India, January 1994. 7 figures available upon
request. CRN--94/0
Threshold Photo/Electro Pion Production - Working Group Summary
We summarize the pertinent experimental and theoretical developments in the
field of pion photo- and electroproduction in the threshold region. We discuss
which experiments and which calculations should be done/performed in the
future.Comment: plain TeX (macro included), 6pp, summary talk presented at the
workshop on "Chiral Dynamics: Theory and Experiments", MIT, July 25-29, 199
The pion charge radius from charged pion electroproduction
We analyze a low-energy theorem of threshold pion electroproduction which
allows one to determine the charge radius of the pion. We show that at the same
order where the radius appears, pion loops induce a correction to the momentum
dependence of the longitudinal dipole amplitude . This
model-independent correction amounts to an increase of the pion charge radius
squared from the electroproduction data by about 0.26~fm. It sheds light on
the apparent discrepancy between the recent determination of the pion radius
from electroproduction data and the one based on pion-electron scattering.Comment: 3 pp, REVTeX, uses eps
Momentum Dependent Vertices , and : The NJL Scalar Hidden by Chiral Symmetry
We calculate the momentum dependence of three particle vertices , and in the context of a
Nambu Jona Lasinio type model. We show how they influence the processes , and and how chiral symmetry
shadows the presence of the .Comment: 9 pages (latex), 5 figures available from the authors, preprint
Coimbra 940506, IJS-TP-94/10, accepted for publication in Zeit. f. Physik
Tests of non-standard electroweak couplings of right-handed quarks
The standard model can be interpreted as the leading order of a Low-Energy
Effective Theory (LEET) invariant under a higher non linearly realized symmetry
equipped with a systematic power
counting. Within the minimal version of this ``not quite decoupling'' LEET, the
dominant non-standard effect appears at next-to-leading order (NLO) and is a
modification of the couplings of fermions to W and Z. In particular, the
coupling of right-handed quarks to Z is modified and a direct coupling of
right-handed quarks to W emerges. Charged right-handed lepton currents are
forbidden by an additional discrete symmetry in the lepton sector originally
designed to suppress Dirac neutrino masses. A complete NLO analysis of
experimental constraints on these modified couplings is presented. Concerning
couplings of light quarks, the interface of the electroweak tests with QCD
aspects is discussed in detail.Comment: 56 pages, 14 figures, v2: references added, minor modifications in
the text, accepted for publication in JHE
Spin structure of the nucleon at low energies
The spin structure of the nucleon is analyzed in the framework of a
Lorentz-invariant formulation of baryon chiral perturbation theory. The
structure functions of doubly virtual Compton scattering are calculated to
one-loop accuracy (fourth order in the chiral expansion). We discuss the
generalization of the Gerasimov-Drell-Hearn sum rule, the Burkhardt-Cottingham
sum rule and moments of these. We give predictions for the forward and the
longitudinal-transverse spin polarizabilities of the proton and the neutron at
zero and finite photon virtuality. A detailed comparison to results obtained in
heavy baryon chiral perturbation theory is also given.Comment: 29 pp, 14 fig
Unitarized pion-nucleon scattering within Heavy Baryon Chiral Perturbation Theory
By means of the Inverse Amplitude Method we unitarize the elastic
pion-nucleon scattering amplitudes obtained from Heavy Baryon Chiral
Perturbation Theory to O(q^3). Within this approach we can enlarge their
applicability range and generate the Delta(1232) resonance. We can find a
reasonable description of the pion nucleon phase shifts with (q^2) parameters
in agreement with the resonance saturation hypothesis. However, the
uncertainties in the analysis of the low energy data as well as the large
number of chiral parameters, which can have strong correlations, allow us to
obtain very good fits with rather different sets of chiral constants.Comment: Shortened version to appear in Phys. Rev. D. Brief Report
Is Heavy Baryon Approach Necessary?
It is demonstrated that using an appropriately chosen renormalization
condition one can respect power counting within the relativistic baryon chiral
perturbation theory without appealing to the technique of the heavy baryon
approach. Explicit calculations are performed for diagrams including two-loops.
It is argued that the introduction of the heavy baryon chiral perturbation
theory was useful but not necessary.Comment: 9 pages, 2 figures, minor changes, references adde
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