Analyzing power puzzle in low energy elasticNdscattering

Abstract

First results of rigorous 3N Faddeev calculations including realistic NN and 3N forces (3NF) are presented. We compare the theoretical analyzing power A" in elastic Nd scattering at E&,b = 3.0 MeV to recently measured nd data. As 3NF we take the Tucson-Melbourne 2m-exchange model. We study its dependence on the~N form-factor cutoff parameter and also the interplay of three realistic NN forces with that 3NF. The discrepancy between theory and experiment, present for NN forces only, turns out to be aggravated when including that 3NF. Further efforts to be taken are indicated.PACS number(s): 25.10.+s, 21.30.+ y, 24.70.+s, 25.40.Dn A great amount of proton-proton (pp) and neutronproton (np) data has been measured over the years and there exist several so-called realistic nucleon-nucleon (NN) potentials, among thetn meson-theoretical ones, which describe that data set very well. Are those forces also relevant for systems of three and more nucleons? The advent of supercomputers in recent years allowed to solve the 3N scattering problem rigorously for any type of NN forces. Our extensive analysis of available threenucleon (3N), proton-deuteron (pd) and neutron-deuteron (nd) elastic scattering data in fact revealed, that the simple dynamical mechanism of three nucleons interacting pairwise with realistic NN interactions clearly dominates this process. The predictions of these modern NN potentials give a very good description of the total cross section for nd scattering, of angular distributions and nearly all polarization data [1] in elastic Nd scattering. Also quite a few Nd breakup data are equally well described by that simple dynamical picture, leaving very little room for additional dynamics. There is, however, an interesting exception in the analyzing power in elastic Nd scattering. While for incident nucleon lab energies of 30 MeV and higher good agreement is found between the theoretical predictions and the A data [1], a sizeable discrepancy of about 30% exists in the low energy region, below E&,b =30 MeV, between the theoretical predictions based on modern NN potentials and both pd and nd A" data. That discrepancy lies in and around the A"maximum at 6, =125' [1]. It exists for energies below as well as above the deuteron breakup threshold. In Fig. 1 we exemplify that discrepancy at a neutron lab energy E&", b=3.0 MeV. The predictions of the currently most prominent four diS'erent potentials, AV14 [2], Bonn B [3], Nijmegen [4], and Paris [5], clearly underestimate the recently measured A data [6] in the region of its maximum. The deviation of the AV14 prediction from the other ones is due to slight on-shell differences between the AV14 Po & 2 NN phase-shift parameters and those phases of the other three potentials. This points to a pos-0.070 0.035 0.000 Bc.m. (deg) 180 FIG. 1. The neutron-deuteron analyzing power data at El", b =3.0 MeV [6] (full dots) compared to different potential predictions: Bonn Bsolid line, AV14short-dashed line, Nijmegendotted line, and Parislong-dashed line. sible reason for that size...

Three-Nucleon Force in the 4He Scattering System

“…In the large model space AV18 plus UIX yields -27.81 MeV close to the experimental -28.296 MeV, to which the parameters of the UIX are fitted to. Although the parameters of the V * 3 TNI were chosen as to give only minute changes in the three-nucleon system, for 4 He it resulted in 650 keV additional binding.…”

“…The 4 He atomic nucleus is one of the best studied few-body systems, both experimentally and theoretically, as summarized in the recent A = 4 compilation [1]. Besides the many textbook examples of gross structure, there are subtle points yielding large effects that are only qualitatively understood.…”

“…Furthermore very many operators can contribute to a TNI and the lack of stringent conditions in the three-nucleon system on the structure of the TNI makes its application to other systems desirable. In [2] it was shown that although a realistic N N force can generally reproduce the 4 He system, there remain differences, most notably in the analyzing powers. Since the intensely studied 4 He system [1] is unfortunately very difficult to describe due to the many resonances and the 4 He bound state, the much simpler systems p− 3 He and n− 3 H where data exist in the energy range of interest were investigated in [8].…”

“…Three-nucleon interactions (TNIs) are added to give the necessary small corrections but they still fail to reproduce certain properties of the three nucleon system, most notably the A y analyzing power in N d scattering [4]. Yet the 30% deviation of A y can be resolved by tiny changes in the N d scattering phase shifts (on the order of 0.1 degrees [5,6,7]).…”

“…Besides the many textbook examples of gross structure, there are subtle points yielding large effects that are only qualitatively understood. Except for [2] none of the existing calculations aims at a complete understanding of the many features of 4 He, which is not surprising in view of the number of different phenomena studied so far [1]. With the recent compilation [1] and the comprehensive R-matrix analysis [3] of a large amount of scattering data below E x = 30 MeV, a new, microscopic calculation for the 4 He system in this energy range using modern realistic two-and three-nucleon forces is most desirable.…”