Matrix Elements for Low-Energyp−dRadiative Capture

Abstract

The radiative capture reaction p 1 d ! 3He 1 g has been the focus of many experimental and theoretical studies over the past 20 years, and during this time experiments with polarized protons and deuterons have come to be fairly common. Initially, the interest in polarization experiments arose in part from the observation that the “tensor analyzing powers” for p-d capture are sensitive to D-state components in the 3He wave function [1–4]. Additionally, it was known that measurements of the proton analyzing power and the deuteron vector analyzing power are sensitive to M1 transitions [5,6] which can be influenced by meson exchange processes and non-nucleonic degrees of freedom. Recently, the focus of the work in this field has expanded somewhat as theorists continue to make progress in developing techniques for performing exact quantum calculations in few-body systems. Within just the last few years we have seen publication of the first calculations [7] of p-d radiative capture which incorporate both realistic NN interactions and a correct treatment of Coulomb forces. In view of this new capability, one may now view p-d capture experiments as a means for testing, in a more general way, our understanding of the spin structure of the A ­ 3 system and of the fundamental NN interaction. The purpose of this Letter is to present a new set of measurements for p-d radiative capture at an energy just below the deuteron breakup threshold, Ec.m. ­ 2 MeV. We also describe a new method for the analysis of subthreshold capture data, which is based on Watson’s theorem [8]. This new technique has made it possible to carry out a partial-wave analysis of the data which is significantly more extensive in scope than any previous analysis of this kind for p-d capture. The value of a partial-wave analysis should be readily apparent. In this kind of analysis one determines (by fitting data) a set of parameters which specify the contributions to the reaction amplitude from the individual angular momentum states. In general, a partial-wave analysis provides insight about the details of the reaction process (see, for example, Ref. [9]), and, in addition, makes it possible to compare theory and experiment at a more fundamental level.