Inclusive probability to record an electron in elastic electromagnetic scattering by a spin one-half hadron wave packet

Abstract

The inclusive probability to record an electron in elastic electromagnetic scattering of an electron by a spin one-half hadron is obtained, the initial quantum states of the electron and the hadron being described by the density matrices of a general form. Contrary to the Rosenbluth formula for the differential cross-section for this process, the first nontrivial contribution to the inclusive probability turns out to be of order $\alpha$ and not $\alpha^2$. This contribution describes the interference between the trivial contribution to the $S$-matrix and the leading contribution to its connected part. The explicit expression for this interference terms is derived. For example, for Gaussian wave packets the interference contribution to the inclusive probability is of relative order $2\times 10^{-2}$ in comparison with the contribution of the free passed particle for the nonrelativistic proton and the $10$ MeV electron with the normal deviation of momenta in the electron and hadron wave packets of order $10$ keV. It is shown that the same interference term arises when the electron is scattered by the classical electromagnetic field produced by the hadron electromagnetic current averaged with respect to the free evolving density matrix of the hadron, even in the case of a single hadron. The interference term describes coherent scattering of the electron by the hadron wave packet and is immune to the quantum recoil experienced by a hadron due to scattering. The effective electron mass operator is found on the mass-shell.