Recent Results on the Helicity Structure of the Nucleon from Hermes

The HERMES experiment has measured double spin asymmetries of inclusive and semi–inclusive cross sections for the production of charged hadrons in deep– inelastic scattering of polarised positrons on polarised hydrogen and deuterium targets, in the kinematic range 0.023 < x < 0.6, and 1 GeV < Q < 15 GeV. For the data taken on the deuterium target, a RICH detector provides the complete identification of charged pions and kaons. From the inclusive measurements on deuterium, the polarised structure function g 1 has been extracted with high precision. Together with the semi–inclusive measurements, quark polarisations are extracted separately for the u, ū, d, d̄, and (s+ s̄) flavours in a LO QCD analysis. Furthermore, a possible breaking of flavour symmetry in the polarised light sea, ∆ū − ∆d̄, was measured for the first time and found to be consistent with zero within the experimental precision.