A review of the basic ideas of chiral perturbation theory. Status of chiral peturbation theory as applied to mesons. Most of the talk will concentrate on recent developments on applying chiral perturbation theory to study nucleon and hyperon properties such as masses, magnetic moments and weak decays.
[BA.02] Experiments Determining the Pion-Nucleon Sigma Term
Gregory Smith (TRIUMF)
The pion-nucleon sigma term is an explicit measure of chiral symmetry breaking in QCD due to the non-zero light quark masses. It has been calculated within the framework of chiral perturbation theory to be (36 +- 7 MeV)/(1-y), where the parameter y accounts for the strange sea quark content of the proton. This parameter is accessible experimentally by exploiting the connection between the sigma term and the pion-nucleon scattering amplitude \barD^+ in the region just below threshold.
A review of the experiments which determine the sigma term will be given, highlighting some recent work in this area performed at TRIUMF. In addition, an outline of TRIUMF experiments planned for the near future which bear on this issue will be presented. The connection to the Goldberger-Triemann relation and Goldberger-Miyazawa-Oehme sum rule will also be discussed, along with an analysis of the current pion-nucleon database. Finally, the impact of the new experiments on the sigma term will be given explicitly, along with the corresponding determination of the strange quark content of the proton.
[BA.03] Threshold Pion Photoproduction
Norm Kolb (Saskatchewan Accelerator Laboratory/University of Saskatchewan)
Threshold photoproduction of pions off nucleons is one of the few low-energy phenomena for which QCD-based effective field theories can be formulated and tested. Model-independent low-energy theorems (LETs) have been used to predict the leading order terms of the s-wave electric dipole amplitude (E_0+) at threshold for these reactions. Chiral Perturbation Theory (CHPT) has also been used to obtain the E_0+ amplitudes. The new physics included in these calculations are the pion-loop diagrams. These contributions are quite large for the neutral-pion production channels and lead to good agreement with recent experimental measurements of the \gamma p\rightarrow p\pi^0 reaction near threshold. This is in contrast to the older LETs which significantly overpredict the amplitude for this channel. The contribution of the pion-loop diagrams to the charged-pion channels is small, allowing precision measurements to place constraints on the fundamental \piNN coupling constant. The CHPT prediction for the \gamma n\rightarrow n\pi^0 E_0+ amplitude is actually larger than the amplitude for neutral-pion production from the proton. Measurements of neutral-pion production from the deuteron may soon provide pioneering tests of CHPT in a nuclear system.
[BA.04] Threshold Pion Electroproduction
Henk P. Blok (Vrije Universiteit, Amsterdam, The Netherlands)
Electroproduction of pions on nucleons near threshold is one of the experiments for which Chiral Perturbation Theory can make clear predictions. Especially \pi^0 production represents a very sensitive test of various ingredients of the calculations, since the model-independent term due to the charge of the pion is absent.
Compared to photoproduction electroproduction adds several important features: the charge multipole L_0+, the Q^2 dependence of the multipoles (especially of E_0+ and L_0+), and the possibility to determine four structure functions, which all represent different combinations of the contributing multipoles.
Data on the proton have been taken at NIKHEF(Amsterdam) and at MAMI (Mainz). In both cases the scattered electron and the residual proton were detected in two high-resolution magnetic spectrometers. Since near threshold the proton is emitted in a narrow cone around the momentum transfer vector, the full \pi^0 angular distribution can be measured in just a few settings of the spectrometer.
The experiments yielded accurate values for the E_0+ and L_0+ multipoles and for the p-wave multipoles, for which ChPT calculations converge much more rapidly than for the s-wave multipoles. Comparisons will be made with recent ChPT calculations.