The Spin and Flavour Dependence of High-Energy Photoabsorption

Abstract

We review the present data on high-energy, spin-dependent photoabsorption. We find a strong isotriplet term in $({\sigma }_A-{\sigma }_P)$ which persists from $Q^2\sim 0.25$GeV${}^2$ to high $Q^2$ polarised deep inelastic scattering. For $Q^2\sim 4$GeV${}^2$ and $x$ between 0.01 and 0.12 the isotriplet part of $g_1$ behaves as $g_1^{(p-n)}\sim x^{-\frac{1}{2}}$, in contrast to soft Regge theory which predicts that $g_1^{(p-n)}$ should converge as $x\to 0$. The isotriplet, polarised structure function $2x{g}_1^{(p-n)}$ is significantly greater than the isotriplet, unpolarised structure function $F_2^{(p-n)}$ in this kinematic region. We analyse the low $Q^2$ photoabsorption data from E-143 and SMC and use this data to estimate the high-energy Regge contribution to the Drell-Hearn-Gerasimov sum-rule.

Comments: 19 pages, LaTeX, 3 figures generated as g1,g1F2 and A1.tex