Analytic parametrization of 4He charge form factor

An N/D method of analytic representation for form factors, recently found successful for the deuteron, is used to analyze all the available data on the 4He charge form factor and obtain useful information. The nearest anomalous cut positions in the t plane relevant for this analysis are calculated using possible exchanges at the photon-helium vertex. In contrast to the deuteron case, all familiar and simple intermediate states yield anomalous cut positions above the three-pion cut. Although including the contributions of the three-pion cut in the D function and the five-pion cut in the N function yields a reasonable fit except around the second maximum, the best fit is obtained using the anomalous cut at t/sub a/ = 0.247 GeV2 instead of the five-pion cut. An exponential weight function, necessary for optimized polynomial expansion in Laguerre polynomials in the N function, seems to be essential in reducing the chi2 value as in the case of the deuteron. The dip in the form-factor data is parametrized in terms of a zero of the N function and the formula predicts a second zero and a third maximum which can be verified in future experiments. The existing data favor an asymptotic behavior of the type exp[-const(lnt)2](lnt)4/t 2 and the formula extrapolates smoothly into the timelike region. A plot of charge density against nuclear radius shows a central depression and the root-mean-square and half-density radii of the distribution are computed