Probing the mesonic structure of the nucleon by means of exclusive quasielastic pion knockout in (e, e′π) reactions at high energies

By including the Z diagram in an analysis performed in the laboratory frame (instantaneous form of dynamics), the notion of quasielastic pion knockout by protons and electrons [(p, 2p) and (e, e′p) reactions treated in terms of the relevant pole diagrams] is generalized to the relativistic case where a meson is quasielastically knocked out of a nucleon by an electron having an energy of a few GeV. The concept of the wave function is introduced for the pion (and for other mesons), and its relation to the vertex constant GπNN and the vertex function gπNN(k2) is indicated. The spectroscopic factor SNBℳ is defined as the normalization of the wave function for the meson ℳ. It is shown by two methods that, under the kinematical conditions of quasielastic knockout that include the condition Eπ≫mπ (Eπ is the energy of the knock-on pion) and the condition that the square Q2 of the virtual pion mass is large, the competing tree diagram is suppressed in relation to the pole diagram (this is not so in the case of pion photoproduction). From data of a p(e, e′π+)n experiment involving longitudinal virtual photons γL*, the momentum distribution |Ψpnπ(k)|2 of pions in the nucleon is extracted for the first time over the entire range of significant momenta k, and this result is used to determine the cutoff constant Λπ and the value of Spnπ≈0.2. The momentum distribution of positive rho mesons in the soft section of the spectrum is determined from experimental data on the process p(e, e′π+)n proceeding through the mechanism ρ++γT* → π+ involving transverse photons. A way to determine the momentum distribution of omega mesons through data on the process p(e, e′π0)p is indicated. Two forms of dynamics—instantaneous form and that of light-front dynamics (the latter does not involve the Z diagram)—are compared for the example where the calculations are performed for the spectroscopic factor SNBℳ.