Implication of the running mass of the ρ0 meson for the dilepton mass spectrum and the μ+μ-/e+e- ratio in K+→π+l+l- decays

We make an attempt to resolve the discrepancy of the observed e+e- mass spectrum in the K+→π+e+e- decay with that predicted by meson dominance. To this end we investigate the properties of the ρ0 propagator. We use dispersion relations to evaluate the running mass mρ2(t) of the ρ0 resonance without adjustable parameters. To improve the convergence of the dispersion integral, the momentum dependence of strong vertices is taken from the flux-tube-breaking model of Kokoski and Isgur. The obtained behavior of mρ2(t) at small momentum squared t makes the K+→π+e+e- form factor rise faster with increasing t than in the original meson dominance calculation and more in agreement with the published data. As a consequence, the meson dominance prediction of the μ+μ-/e+e- ratio changes slightly, from 0.224 to 0.236. We do not see any possibility to accommodate into the meson dominance approach an even steeper e+e- spectrum, indicated by the preliminary data of the E865 Collaboration at BNL AGS. copyright 1999 The American Physical Society