Transversity distribution and polarized fragmentation function from semi-inclusive pion electroproduction

Abstract. A method is discussed to determine the hitherto unknown u-quark transversity distribution $\delta u(x)$ from a planned HERMES measurement of a single target-spin asymmetry in semi-inclusive pion electroproduction off a transversely polarized target. Assuming u-quark dominance, the measurement yields the shapes of the transversity distribution $\delta u(x)$ and of the ratio $H_1^{\perp (1)u}(z)/D_1^u(z)$, of polarized and unpolarized u-quark fragmentation functions. The unknown relative normalization can be obtained by identifying the transversity distribution with the well-known helicity distribution at large x and small $Q^2$. The systematic uncertainty of the method is dominated by the assumption of u-quark dominance.