CLASSICAL PHOTOABSORPTION SUM-RULES

We use a quantum loop expansion to derive sum rule constraints on polarized photoabsorption cross sections in the Standard Model, generalizing earlier results obtained by Altarelli, Cabibbo, and Maiani. We show that the logarithmic integral of the spin-dependent photoabsorption cross section integral(nu th)(infinity) d nu/nu Delta sigma(Born)(nu) vanishes for any 2 --> 2 process gamma a bc in the classical, tree-graph approximation in the Standard Model, supersymmetric gauge theories, and other quantum field theories where the Drell-Hearn-Gerasimov sum rule is valid. Here nu = p . q/M and Delta sigma(nu) = sigma p(nu) - sigma(A)(nu) is the difference between the photoabsorption cross section for parallel and antiparallel photon and target helicities. Tests of the sum rule for the reactions gamma e --> W nu and gamma gamma --> W+W- can provide new tests of the canonical magnetic and quadrupole couplings of the Standard Model. We also extend the sum rule to certain virtual photon processes.