Theorem on the Vanishing ofZ2: Evidence from Electron Scattering that the Proton Really Is a Composite Particle

Abstract

We prove that the asymptotic vanishing of the longitudinal part of the total virtual photoabsorption cross section on protons implies the vanishing of the wave-function renormalization constant $Z_2$. Present data indicate that $Z_2\lesssim 0.1$ (consistent with zero) suggesting that the proton is indeed a composite particle.