Polarization Phenomena in Small-Angle Photoproduction of e+e- Pairs and the GDH Sum Rule.pdf

a r X i v : n u c l - t h / 9 6 0 7 0 0 4 v 1 3 J u l 1 9 9 6 MKPH-T-96-15 POLARIZATION PHENOMENA IN SMALL-ANGLE PHOTOPRODUCTION OF e+e? PAIRS AND THE GDH SUM RULE A.I. L’VOV a Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 117924, Russia S. SCOPETTA Department of Physics, University of Perugia, via A. Pascoli, I-06100 Perugia, Italy and Institut fu?r Kernphysik, Universita?t Mainz, D-55099, Germany D. DRECHSEL, S. SCHERER Institut fu?r Kernphysik, Universita?t Mainz, D-55099, Germany We discuss a possibility to measure the spin-dependent part of the forward Compton scattering amplitude through interference effects of the Bethe-Heitler and virtual Compton scattering mechanisms in photoproduction of e+e? pairs at small angles. 1 Introduction In studies of spin-dependent structure functions of nucleons and nuclei with real and virtual photons, the verification of the Gerasimov–Drell–Hearn sum rule is of special interest 1. The GDH sum rule is based in essence only on the assumption of spin-independence of high-energy forward Compton scattering and thus provides a very clean test of the spin dynamics. The forward Compton scattering amplitude on a spin-1/2 target is de- scribed by two even functions f1,2 of the photon energy ω, f = (e′ · e) f1(ω) + iωσ · (e′ × e) f2(ω), (1) which, at ω = 0, are constrained by the low-energy theorem: f1(0) = ?αZ 2 M , f2(0) = ? ακ 2 2M2 . (2) Here M, eZ, κ are the mass, electric charge, and anomalous magnetic moment of the target, and α = e2/4π ? 1/137. Within the framework of the Regge pole model, these functions behave like f1(ω) ∝ ωαR(0), f2(ω) ∝ ωαR(0)?1 for ω → ∞, (3) aTalk given at the Workshop on Virtual Compton Scattering, Clermont-Ferrand, June 1996 1 where αR(0) is the intercept of the leading t-channel Regge exchange con- tributing to the amplitudes. With the usual assumption of αR(0)～ 1, both f1,2 satisfy once-subtracted dispersion relations. The optical theorem allows to find the imaginary parts of f1,