Dispersion Analysis of the Strange Vector Form Factors of the Nucleon

Abstract

We analyze the nucleon matrix element of the strange quark vector current in a nucleon--model independent dispersive approach with input from the current world data set for the isoscalar electromagnetic form factors. The update of Jaffe's minimal 3-pole ansatz for the spectral functions yields a 40% larger (Sachs) strangeness radius, $(r_s^2)_{Sachs} = 0.20 {\rm fm}^2$, and a by 20% reduced magnitude of the strangeness magnetic moment, $\mu_s = -0.26$. In the pole approximation these values are shown to be upper bounds. After extending the ansatz in order to implement the asymptotic QCD momentum dependence (which the 3-pole form factors cannot reproduce), we find the magnitude of the 3-pole results reduced by up to a factor of 2.5. The signs of the leading moments originate primarily from the large $\phi$-meson couplings and are generic in the pole approximation.