X iv : n uc l - th / 9 80 70 37 v 1 1 0 Ju l 1 99 8 Microscopic calculation of 6 Li elastic and transition form factors

Variational Monte Carlo wave functions, obtained from a realistic Hamilto-nian consisting of the Argonne v 18 two-nucleon and Urbana-IX three-nucleon interactions, are used to calculate the 6 Li ground-state longitudinal and transverse form factors as well as transition form factors to the first four excited states. The charge and current operators include one-and two-body components , leading terms of which are constructed consistently with the two-nucleon interaction. The calculated form factors and radiative widths are in good agreement with available experimental data. 1 Calculations of the 6 Li elastic and inelastic form factors have relied in the past on relatively simple shell-model [1–3] or α-d [4] cluster wave functions. These calculations have typically failed to provide a satisfactory, quantitative description of all measured form factors. More phenomenologically successful models have been based on αNN [5–8] clusterization, or on extensions of the basic spherical-cluster α-d model in which the deuteron is allowed to deform [9]. However, while these models do provide useful insights into the structure of the A=6 nuclei, their connection with the underlying two-(and three-) nucleon dynamics is rather tenuous. The 6 Li form factor calculations we report on here are within the context of a realistic approach to nuclear dynamics based on two-and three-nucleon interactions—the Argonne v 18 [10] and Urbana IX [11] interactions, respectively, or AV18/UIX model—and consistent two-body charge and current operators [12–14]. Up until very recently, calculations of this type were limited to the A=2–4 systems, as reviewed in Ref. [15]. Indeed, the deuteron structure functions and threshold electrodisintegration, the trinucleon charge and magnetic form factors, and α charge form factor have been the observables of choice for testing the quality of interactions and associated two-body currents. However, the availability of realistic six-body wave functions for the ground and low-lying excited states of 6 Li [16] makes it now possible to extend and test our understanding of the electromagnetic structure of nuclei in a new regime—that of p-shell nuclei—and to verify to what extent the inability of reproducing simultaneously the observed elastic and transition form factors is due to the inadequacy of cluster or shell-model wave functions. The AV18/UIX model reproduces the experimental binding energies and charge radii of 3 H, 3 He, and 4 He in numerically exact calculations, based on the Faddeev [17], pair-correlated hyperspherical harmonics [18], and Green's function Monte Carlo (GFMC) [16] methods. For A=6 systems the GFMC results are somewhat …