Transversity Distribution and Polarized Fragmentation Function from Semi-inclusive Pion Electroproduction

A method is discussed to determine the hitherto unknown u-quark transversity distribution δu(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 δu(x) and of the ratio H 1 (z)/D u 1 (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. The systematic uncertainty of the method is dominated by the assumption of u-quark dominance.