Abstract:
We propose to measure the two--photon capture mode of pionic hydrogen using the RMC pair spectrometer at the TRIUMF cyclotron. Currently, only an experimental upper limit of B.R.<= 5.5E-4 is available for the two-gamma capture mode. Our new data will critically test the theoretical calculations and the intriguing predicted dominance of the pi pi --> gamma gamma annihilation graph. More speculatively, since crossing symmetry relates pi pi --> gamma gamma to gamma pi --> gamma pi, this threshold reaction may be a novel probe of the pion's electric polarizability. We show results of our 1997 engineering run and Monte Carlo studies for the two photon capture mode of pionic hydrogen and carbon. We also present our preliminary upper limit for the pi- p --> gamma gamma n B.R. <= 2.8E-4.

Abstract:
We report the first observation of the doubly-radiative decay mode of pionic hydrogen (pi- p --> gamma gamma n) and and deuterium (pi- d --> gamma gamma X) using the RMC pair spectrometer at TRIUMF. The process is interesting in the context of the pi-Compton scattering and the chiPT predictions of the pion polarizability We present our preliminary values for the B.R.(pi- p --> gamma gamma n)=3.8x10^-5 and B.R.(pi- d --> gamma gamma X)=1.6x10^-5. Our pi- p data indicates the dominance of pi pi --> gamma gamma mechanism. The pi- d data shows no evidence for the d1* dibaryon.

Abstract:
An extensive study of three-nucleon force effects in the entire phase space of the nucleon-deuteron breakup process, for energies from above the deuteron breakup threshold up to 200 MeV, has been performed. 3N Faddeev equations have been solved rigorously using the modern high precision nucleon-nucleon potentials AV18, CD Bonn, Nijm I, II and Nijm 93, and also adding 3N forces. We compare predictions for cross sections and various polarization observables when NN forces are used alone or when the two pion-exchange Tucson-Melbourne 3NF was combined with each of them. In addition AV18 was combined with the Urbana IX 3NF and CD Bonn with the TM' 3NF, which is a modified version of the TM 3NF, more consistent with chiral symmetry. Large but generally model dependent 3NF effects have been found in certain breakup configurations, especially at the higher energies, both for cross sections and spin observables. These results demonstrate the usefulness of the kinematically complete breakup reaction in testing the proper structure of 3N forces.

Abstract:
Selected Nd breakup data over a wide energy range are compared to solutions of Faddeev equations based on modern high precision NN interactions alone and adding current three-nucleon force models. Unfortunately currently available data probe phase space regions for the final three nucleon momenta which are rather insensitive to 3NF effects as predicted by current models. Overall there is good to fair agreement between present day theory and experiment but also some cases exist with striking discrepancies. Regions in the phase space are suggested where large 3NF effects can be expected.

Abstract:
Some present day results for few-nucleon bound state, two-nucleon correlation functions and scattering observables are briefly reviewed. The old idea of the Coulomb sum rule as a way to extract the pp correlation function is reconsidered. It appears to be an ideal case to probe properties of the density operator and the ground state wave functions without disturbances of final state interactions (FSI) in exclusive electron scattering on 3He. Also exclusive photodisintegration of 3He is regarded which appears to be promising to identify 3N force effects.

Abstract:
We report a search for d^* dibaryon production by double-radiative capture on pionic deuterium. The experiment was conducted at the TRIUMF cyclotron using the RMC cylindrical pair spectrometer, and detected gamma-ray coincidences following pion stops in liquid deuterium. We found no evidence for narrow dibaryons, and obtained a branching ratio upper limit, BR < 6.7 times 10^{-6} (90% C.L.), for narrow d^* production in the mass range from 1920 to 1980 MeV. Replaced with Physics Letter B accepted version and corrected normalization.

Abstract:
Faddeev equations for elastic Nd scattering have been solved using modern NN forces combined with the Tucson-Melbourne two-pion exchange three-nucleon force, with a modification thereof closer to chiral symmetry and the Urbana IX three-nucleon force. Theoretical predictions for the differential cross section and several spin observables using NN forces only and NN forces combined with three-nucleon force models are compared to each other and to the existing data. A wide range of energies from 3 to 200 MeV is covered. Especially at the higher energies striking three-nucleon force effects are found, some of which are supported by the still rare set of data, some are in conflict with data and thus very likely point to defects in those three-nucleon force models.

Abstract:
A systematic method is proposed for analyzing the renormalization scheme uncertainties in the next-next-to-leading order QCD predicitions, based on a condition which eliminates schemes that give rise to large cancellations in the expression for the characteristic scheme invariant combination of the expansion coefficients. Using this method it is shown that the QCD corrections to the tau lepton decay are rather stable with respect to change of the scheme, provided that an improved formula is used, which involves numerical evaluation of the contour integral in the complex energy plane with the Adler function under the integral. Optimized predictions for the tau decay corrections are given. It is shown that also in the case of the of QCD corrections to $e^{+}e^{-}$ annihilation into hadrons the conventional expansion has sizable scheme dependence, even at large energies. However, a considerable improvement is obtained when the QCD corrections are expressed as a contour integral, with the Adler function under the integral, resumming in this way the large $\pi^{2}$ contributions. In the case of the corrections to the Bjorken sum rule for polarized structure functions it is found that for $n_{f}=4$ they are insensitive to change of the scheme. However, the $n_{f}=3$ expression is found to be strongly scheme dependent at lower energies.

Abstract:
The QCD corrections to the R_{\tau}^{12} moment of the invariant mass distribution in hadronic decays of the tau lepton are discussed. The next-to-next-to-leading order prediction is shown to be stable with respect to change of the renormalization scheme, provided that the contour integral expression is used. The optimized predictions are obtained using the principle of minimal sensitivity to select the preferred renormalization scheme. The optimized predictions for R_{\tau}^{12} and R_{\tau} are used in a simplified fit to the experimental data to determine the strong coupling constant and the parameter characterizing the nonperturbative contribution.

Abstract:
Renormalization scheme uncertainties in the next-next-to-leading order QCD predictions are discussed. To obtain an estimate of these uncertainties it is proposed to compare predictions in all schemes that do not have unnaturally large expansion coefficients. A concrete prescription for eliminating the unnatural schemes is given, based on the requirement that large cancellations in the expression for the characteristic renormalization scheme invariant should be avoided. As an example the QCD corrections to the Bjorken sum rule are considered. The importance of the next-next-to-leading order corrections for a proper evaluation of perturbative QCD predictions is emphasized.