Proton--neutron mass difference, electroproduction, and photoabsorption in an analytic model of the virtual Compton amplitude

An analytic model of the forward (virtual) Compton amplitude is proposed which satisfies ν plane analyticity, s--u crossing symmetry, Regge behavior, and Bjorken scale invariance in appropriate limits. The model is constructed using the method of conformal mapping. Very good fits are obtained to the electroproduction data on νW2/sup p/, 2mW1/sup p/, νW2/sup p/ - νW2/sup n/, νW2/sup n//νW2/sup p/, νR/sub p/, and 1 + νW2/sup n//νW2/sup p/ and to the photoabsorption data on sigma/sub T/(γp), sigma/sub T/γn), and sigma/sub T/(γp) - sigma/sub T/(γn). The scale-invariance breaking at finite Q2 as exhibited by the data on νW/subp/2 plotted as a function of Q2 with ω fixed is also successfully reproduced. The p--n mass difference calculated using the Cottingham formula is finite. In particular, it is shown that a set of parameters exists for which the general condition for no logarithimic divergence even in the presence of Bjorken scaling giving by lim/sub Q2 → infinity/Q4Δt1/sup I/(-Q2,0) = (αm/π) integral01 dx [ΔF2(x) + 2xΔF1(x)] can be realized consistent with theoretical considerations and experimental data. The final result of our calculation of the mass difference is (-1.96 +- 0.52) MeV, which is to be compared with the experimental value of -1.293 MeV