Selectron and squark production inep collisions at HERA

For the SUSY Standard Model we give a detailed discussion how HERA can be used for disentangling the mixing structure in the neutralino sector. We present 3-dimensional plots for the total cross section and for the polarization asymmetry for selectron-squark production, varying both the sfermion masses and a mixing parameter. The main results are: (i) production rates at HERA are reasonably large for a wide range of mixing parameters; (ii) the polarization asymmetry depends only weakly upon the sfermion masses and thus is a very useful tool in determining mixing parameters or mass splittings between left and right handed selectron masses.