The distribution of hadrons produced in deeply inelastic electron–proton
collisions depends on the azimuthal angle between lepton scattering plane and
hadron production plane in the photon-proton centre-of-mass frame. In
addition to the well known up-down asymmetry induced by the azimuthal
dependence of the Born level subprocess, there is also a non-vanishing
left-right asymmetry, provided the incoming electron is polarized. This
asymmetry is time-reversal-odd and induced by absorptive corrections
to the Born level process. We investigate the numerical magnitude of azimuthal
asymmetries in semi-inclusive hadron production at HERA with particular
emphasis on a possible determination of the time-reversal-odd asymmetry.
