ar X iv : n uc l - th / 9 70 60 77 v 1 2 7 Ju n 19 97 The Three - Nucleon System Near the N - d Threshold

The three-nucleon system is studied at energies a few hundred keV above the N-d threshold. Measurements of the tensor analyzing powers T 20 and T 21 for p-d elastic scattering at E c.m. = 432 keV are presented together with the corresponding theoretical predictions. The calculations are extended to very low energies since they are useful for extracting the p-d scattering lengths from the experimental data. The interaction considered here is the Argonne V18 potential plus the Urbana three-nucleon potential. The calculation of the asymptotic D-to S-state ratio for 3 H and 3 He, for which recent experimental results are available, is also presented. 1 Study of the three-nucleon system at low energies provides a stringent test of our understanding of the nuclear dynamics and the nucleon-nucleon (NN) interaction. In Ref. [1], detailed comparisons have been performed between experimental data and the corresponding theoretical predictions for N-d scattering for 0.6 ≤ E c.m. ≤ 2.0 MeV. Good agreement was observed for the cross section and the tensor analyzing powers T 20 , T 21 , and T 22 , but significant differences were found in the vector analyzing power A y for N-d scattering and the analyzing power iT 11 for p-d scattering. It is also important to experimentally test the theoretical calculations at lower energies. One motivation is to extract the p-d scattering lengths, for which past experimental results [2] have persistently disagreed with theoretical estimates [3]. A further advantage is that at low energies the contributions from higher partial waves are reduced, so that the remaining important partial waves can be better investigated. Another motivation for these comparisons is to test the theoretical p-d scattering wavefunctions which have recently been used to calculate the astrophysical S-factor and analyzing powers for the 2 H(p, γ) 3 He reaction at very low energies [4]. Below E c.m. = 0.6 MeV, the cross section is mainly governed by S-wave scattering. As a result the polarization observables, which are influenced by other partial waves, are small and difficult to determine experimentally. The present paper reports measurements of T 20 and T 21 for p-d elastic scattering at E c.m. = 432 keV. These data are compared to calculations utilizing the Pair-Correlated Hyperspherical Harmonic (PHH) basis [5] to construct the scattering wave function. The corresponding reactance matrix (R matrix) is obtained by means of the Kohn variational principle, as described in Ref. [6]. …