We extend a class of parton models to a fully gauge invariant theory for the full Compton amplitude.The existence of local electromagnetic interactions is shown to always give rise to a constant real part in the high energy behavior of the amplitude TI(v ,q'). In the language of Reggeisation this is interpreted as a fixed pole at J = 0 in Tl and v T2, with residue polynomial in the photon mass squared.

A general nonperturbative model for the entire Compton amplitude which incorporates Bjorken scaling, gauge invariance, and Regge behavior is presented. We show that a covariantly regularized model based on the infinite-momentum-frame techniques of Drell, Levy, and Yan is equivalent to the manifestly covariant nonperturbative parton model of Landshoff, Polkinghorne, and Short. We also demonstrate that a general consequence of composite theories of hadrons with field-theoretic constituents which incorporates the above properties is the existence of a constant energy-independent and q2-independent term in T , ( v , q 2 ) (a "Kronecker delta" 6,, term) and a J = 0 fixed pole in T, (v, q2). Sum rules for general Compton amplitudes are derived and a discussion of mass renormalization for electromagnetic self-energy corrections of hadrons is presented. We demonstrate that such sum rules are always finite, even in the presence of Regge behavior, when subtraction terms in the underlying parton-proton u-channel dispersion relation are taken into account. Analytic continuation in a is thus justified.