Precision measurement of the spin-dependent asymmetry in the threshold region of quasielastic ³He [right arrow] (e [right arrow], e)́

Abstract

The first precision measurement of the spin-dependent asymmetry in the threshold region of 3He(e, e') was carried out in Hall A at the Jefferson Laboratory, using a longitudinally polarized continuous wave electron beam incident on a high-pressure polarized 3He gas target. The polarized electron beam was generated by illuminating a strained GaAs cathode with high intensity circularly polarized laser light, and an average beam polarization of about 70% was achieved. The 3He target was polarized based on the principle of spin-exchange optical pumping and the average target polarization was about 30%. The scattered electrons were detected in the two Hall A high resolution spectrometers, HRSe and HRSh. The data from HRSh were used for this analysis and covered both the elastic peak and the threshold region. Two kinematic points were measured in the threshold region, one with a central Q2-value of 0.1 (GeV/c)2 at an incident beam energy E0 = 0.778 GeV and the other with a central Q2-value of 0.2 (GeV/c)2 at Eo = 1.727 GeV. The average beam current was 10 tpA, which was mainly due to the limitation of the polarized 3He target. The measured asymmetry was compared with both plane wave impulse approximation (PWIA) calculations and non-relativistic full Faddeev calculations which include both final-state interactions (FSIs) and meson-exchange currents (MECs) effects. The poor description of the data by PWIA calculations at both Q2-values suggests the existence of strong FSI and MEC effects in the threshold region of 3He(e', e'). Indeed, the agreement between the data and full calculations is very good at Q2 = 0.1 (GeV/c)2.

(cont.) On the other hand, a small discrepancy at Q2 = 0.2 (GeV/c)2 is observed, which might be due to some Q2-dependent effects such as relativity and three-nucleon forces (3NFs), which are not included in the framework of non-relativistic Faddeev calculations. Some preliminary results show that 3NF effects are indeed non-negligible in the threshold region and more theoretical work is currently underway towards a better understanding of both effects. With the recent development and success of the chiral perturbation theory, it is hopeful that a future application of the resulting chiral NN forces to electrodisintegration of 3He might solve the present discrepancy.