We present the most accurate and complete data set for the analyzing power Ay(θ) in neutron–proton scattering. The experimental data were corrected for the effects of multiple scattering, both in the center detector and in the neutron detectors. The final data at En=12.0 MeV deviate considerably from the predictions of nucleon–nucleon phase-shift analyses and potential models. The impact of the new data on the value of the charged pion–nucleon coupling constant is discussed in a model study.

The methods employed and the results obtained from measurements and calculations of the detection efficiency for the neutron detectors used at Triangle Universities Nuclear Laboratory (TUNL) in the simultaneous determination of the 1S0 neutron–neutron and neutron–proton scattering lengths ann and anp, respectively, are described. Typical values for the detector efficiency were 0.3. Very good agreement between the different experimental methods and between data and calculation has been obtained in the neutron energy range below En=13MeV.

We report on results of a kinematically complete neutron-deuteron breakup experiment performed at Triangle Universities Nuclear Laboratory using an E-n = 13MeV incident neutron beam. The S-1(0) neutron-neutron scattering length ann has been determined for four production angles of the neutron-neutron final-state interaction configuration. The absolute cross-section data were analyzed with rigorous three-nucleon calculations. Our average value of a(nn) = -18.7 +/- 0.7 fm is in excellent agreement with a(nn) = -18.6 +/- 0.4 fm obtained from capture experiments of negative pions on deuterons. We also performed a shape analysis of the final-state interaction cross-section enhancements by allowing the normalization of the data to float. From these relative data, we obtained an average value of a(nn) = -18.8 +/- 0.5 fm, in agreement with the result obtained from the absolute cross-section measurements. Our result deviates from the world average of a(nn) = -16.7 +/- 0.5 fm determined from previous kinematically complete neutron-deuteron breakup experiments, including the most recent one carried out at Bonn. However, this low value for ann is at variance with theoretical expectation and other experimental information about the sign of charge-symmetry breaking of the nucleon-nucleon interaction. In agreement with theoretical predictions, no evidence was found of significant three-nucleon force effects on the neutron-neutron final-state interaction cross sections.

Cross-section measurements of seven exit-channel configurations in the neutron-deuteron breakup at 13.0 MeV are reported and compared to rigorous calculations. Our data are consistent with those of previous measurements in four of six configurations. The present data for five configurations are in good agreement with theoretical predictions. The cross-section data for the space-star and another out-of-plane configuration are larger than the theoretical predictions by more than three standard deviations. The previously observed 20% discrepancy between theory and data for the space-star configuration is confirmed in the present work. The inclusion of the Tucson-Melbourne 2 pi-exchange three-nucleon force changes the predicted cross section by only 2% and in the wrong direction needed to bring theory into agreement with data.

The analyzing power ${A}_{y}(\ensuremath{\theta})$ for neutron elastic scattering from $^{12}\mathrm{C}$ has been measured for 33 neutron energies between ${E}_{n}=2.2$ and 8.5 MeV in the angular range from 25${}^{\ifmmode^\circ\else\textdegree\fi{}}$ to 145${}^{\ifmmode^\circ\else\textdegree\fi{}}$ in the laboratory system. The primary motivation for these measurements is the need for an accurate knowledge of ${A}_{y}(\ensuremath{\theta})$ for $^{12}\mathrm{C}$$(n,n)$$^{12}\mathrm{C}$ elastic scattering to enable corrections to high-precision neutron-proton and neutron-deuteron ${A}_{y}(\ensuremath{\theta})$ data in the neutron-energy range below ${E}_{n}=30$ MeV. In their own right, $^{12}\mathrm{C}$$(n,n)$$^{12}\mathrm{C}$ ${A}_{y}(\ensuremath{\theta})$ data are of crucial importance for improving both the parametrization of $n\text{\ensuremath{-}}$$^{12}\mathrm{C}$ scattering and our knowledge of the level scheme of $^{13}\mathrm{C}$. The present ${A}_{y}(\ensuremath{\theta})$ data are compared with published data and previous phase-shift-analysis results.

The analyzing power A(y)(theta) for neutron elastic scattering from C-12 has been measured for 33 neutron energies between E-n=2.2 and 8.5 MeV in the angular range from 25(degrees) to 145(degrees) in the laboratory system. The primary motivation for these measurements is the need for an accurate knowledge of A(y)(theta) for C-12(n,n)C-12 elastic scattering to enable corrections to high-precision neutron-proton and neutron-deuteron A(y)(theta) data in the neutron-energy range below E-n=30 MeV. In their own right, C-12(n,n)C-12 A(y)(theta) data are of crucial importance for improving both the parametrization of n-C-12 scattering and our knowledge of the level scheme of C-13. The present A(y)(theta) data are compared with published data and previous phase-shift-analysis results.

New results for the neutron-deuteron analyzing power A(y)(theta) at E-n = 1.2 and 1.9 MeV and their comparison to proton-deuteron data reveal a sizeable and unexpected difference which increases with decreasing center-of-mass energy. This finding calls for the theoretical treatment of a subtle electromagnetic effect presently not incorporated in rigorous three-nucleon scattering calculations, before it is justified to invoke charge-dependent three-nucleon forces and/or other new physics. (C) 2002 Elsevier Science B.V. All rights reserved.

New results for the neutron-deuteron analyzing power Ay(θ) at En = 1.2 and 1.9 MeV and their comparison to proton-deuteron data reveal a sizeable and unexpected difference which increases with decreasing center-of-mass energy. This finding calls for the theoretical treatment of a subtle electromagnetic effect presently not incorporated in rigorous three-nucleon scattering calculations, before it is justified to invoke charge-dependent three-nucleon forces and/or other new physics.

This paper reports two recent high-accuracy determinations of the 1 S 0 neutron-neutron scattering length, a nn . One was done at the Los Alamos National Laboratory using the π− d capture reaction to produce two neutrons with low relative momentum. The neutron-deuteron (nd) breakup reaction was used in other measurement, which was conducted at the Triangle Universities Nuclear Laboratory. The results from the two determinations were consistent with each other and with previous values obtained using the π − d capture reaction. The value obtained from the nd breakup measurements is a nn = −18.7 ± 0.1 (statistical) ± 0.6(systematic) fm, and the value from the π− d capture experiment is a nn = −18.50 ± 0.05 ± 0.53 fm. The recommended value is a nn = − 18.5 ± 0.3 fm.

The S-1(0) neutron-neutron (nn) scattering length's currently accepted value (a(nn)=-18.6+/-0.3 fm) is derived exclusively from two pi (-)-d capture-reaction experiments, in disagreement with the average -16.7+/-0.5 fm extracted from kinematically-complete nd breakup experiments. This discrepancy may be due to deficiencies in the analyses of n-d breakup data and/or three-nucleon force (3NF) effects. A kinematically-complete n+d-->n(1)+n(2)+p breakup experiment at an incident neutron energy of 13.0 MeV was performed recently at TUNL. The value of a(nn) was extracted from the direct comparison of experimental and rigorously-calculated theoretical nd breakup differential cross sections at four production angles of the nn pair. Using modem nucleon-nucleon potential models in the three-nucleon cross-section calculations we obtained a(nn)=-18.7+/-0.6 fm, in agreement with the pi (-)-d result. We found no significant effect due to 3NFs on our a(nn) value.

The present paper reports high-accuracy cross-section data for the 2H(n,nnp) reaction in the neutron-proton (np) and neutron-neutron (nn) final-state-interaction (FSI) regions at an incident mean neutron energy of 13.0 MeV. These data were analyzed with rigorous three-nucleon calculations to determine the 1S0 np and nn scattering lengths, a_np and a_nn. Our results are a_nn = -18.7 +/- 0.6 fm and a_np = -23.5 +/- 0.8 fm. Since our value for a_np obtained from neutron-deuteron (nd) breakup agrees with that from free np scattering, we conclude that our investigation of the nn FSI done simultaneously and under identical conditions gives the correct value for a_nn. Our value for a_nn is in agreement with that obtained in pion-deuteron capture measurements but disagrees with values obtained from earlier nd breakup studies.

The common theme of this talk is “probing the nuclear dynamics at low energies”. This subtitle implies the study of nuclear systems at rather large internucleon distances. We will focus our attention on the three-nucleon (3N) system and our goal here is to explore the role of 3N forces in the 3N continuum. The result anticipated from such a study may turn out to be trivial, considering our restriction to low energies, i.e., sizeable internucleon distances. Traditionally, one expects 3N forces in the continuum to be important only at fairly high energies (EN > 100 MeV) where the three nucleons can come closer together. However, experimental evidence, some of which may be controversial, makes 3N-force studies at low energies not only interesting, but also necessary.