Spin–isospin selectivity in three-nucleon forces

Abstract

Precision data are presented for the break-up reaction, 2 H( p, pp)n, within the framework of nuclear-force studies. The experiment was carried out at KVI using a polarized-proton beam of 190 MeV impinging on a liquid-deuterium target and by exploiting the detector, BINA. Some of the vector-analyzing powers are presented and compared with state-of-the-art Faddeev calculations including three-nucleon forces effect. Significant discrepancies between the data and theoretical predictions were observed for kinematical configurations which correspond to the 2 H( p, 2 He)n channel. These results are compared to the 2 H( p, d)p reaction to test the isospin sensitivity of the present three-nucleon force models. The current modeling of two and three-nucleon forces is not sufficient to describe consistently polarization data for both isospin states. Understanding the exact nature of the nuclear force is one of the long-standing questions in nuclear physics. In 1935, Yukawa successfully described the pair-wise nucleon-nucleon (NN) interaction as an exchange of a boson [1]. Current NN models are mainly based on Yukawa's idea and provide an excellent description of the high-quality database of proton-proton and neutron-proton scattering [2] and of the properties of the deuteron. However, for the simplest three-nucleon system, triton, three-body calculations employing NN forces clearly underestimate the experimental binding energies [3], demonstrating that NN forces are not sufficient to describe the three-nucleon system accurately. Some of the discrepancies between experimental data and calculations solely based on the NN interaction can be resolved by introducing an additional three-nucleon force (3NF). Most of the current models for the 3NF are based on a refined version of Fujita-Miyazawa's 3NF model [4], in which a 2π-exchange mechanism is incorporated by an intermediate ∆ excitation of one of the nucleons [5,6].The structure of the 3NF can be studied via a measurement of observables in three-nucleon scattering processes. More detailed information on the spin dependence of the 3NF can be obtained by measuring polarization observables such as the analyzing powers. For this, a series of * Electronic address: messchendorp@kvi.nl extensive studies of 3NF effects in elastic-scattering reactions have been performed at KVI and other laboratories. Precision measurements of the vector analyzing power of the proton in elastic proton-deuteron scattering have been performed at various beam energies ranging from 90 to 250 MeV [7,8,9,10,11]. Also, vector and tensor analyzing powers in elastic deuteron-proton scattering have been obtained at various beam energies ranging from 75 to 270 MeV [12,13,14,15,16,17]. In these measurements, systematic discrepancies between data and theoretical predictions which rigorously solve the Faddeev equations and using only NN potentials were observed. A large part of the discrepancies were removed by adding a 3NF to the NN potentials. Nevertheless, there are still unresolved problems specially at higher...

Vector and tensor analyzing powers in deuteron-proton breakup at 130 MeV

“…In contrast to the data presented here, the measurements of the breakup reaction at higher energies also hint at problems with a proper description of the vector analyzing powers [28][29][30][31]. The observed discrepancies seem to be stronger in proton analyzing powers than in the deuteron ones.…”

“…This study shows a good description of the data by the NN interaction alone but reveals also certain regions where discrepancies cannot be cured by the inclusion of any of the existing 3NF models. It is important to mention here that very recently the problems with describing polarization observables was also demonstrated in the data measured for the p-d breakup at 135 MeV [28] and 190 MeV [29], as well as in the case of the d-p breakup data: at 270 MeV [30] and in several configurations at 135 MeV/A [31].…”

Vector analyzing powers of the deuteron-proton elastic scattering and breakup at 100 MeV

“…Certain problems with describing (proton) vector analyzing powers were demonstrated by the data measured for the p-d breakup at 135 MeV [45] and 190 MeV [46]. Vector (deuteron) analyzing power and spin correlation coefficients for the d-p breakup at 270 MeV (135 MeV/nucleon) are relatively well described by pure NN interactions, while the remaining discrepancies are not removed by adding 3NF [47].…”

Dynamics of Three-Nucleon System Studied in Deuteron–Proton Breakup Experiments

“…On the other hand, certain discrepancies are observed for the tensor analyzing powers of the breakup reaction at the same beam energies, even if (or when) the 3NF is included in the calculations [16]. The experimental studies of the 2 H(p,pp)n reaction at 135 and 190 MeV [17,18] show a large (and growing with beam energy) discrepancy between the measured data and the theoretical predictions for the vector analyzing power A p y . It is located at small relative azimuthal angles of the two breakup protons, and even increases with the three-nucleon force included.…”