Measurement of Transverse Target-spin Asymmetries at Hermes

Azimuthal asymmetries are observed in the cross section of semi-inclusive charged pion electroproduction from a transversely polarized hydrogen target at Hermes. Their si- multaneous dependence on the azimuthal angles of both the pion and the target spin axis allows the extraction of two Fourier components that are interpreted in terms of Collins and Sivers effects. For π+-production both components are found to be positive, while for π−-production the Sivers asymmetry vanishes and the Collins asymmetry is found to be negative and surprisingly large in magnitude.