We investigate selectron pair production and decay in e-e- scattering and e+e- annihilation with polarized beams taking into account neutralino mixing as well as ISR and beamstrahlung corrections. One of the main advantages of having both modes at disposal is their complementarity concerning the threshold behaviour of selectron pair production. In e-e- the cross sections at threshold for seleectron_R selectron_R and selectron_L selectron_L rise proportional to the momentum of the selectron and in e+ e- that for selectron_R selectron_L. Measurements at threshold with polarized beams can be used to determine the selectron masses precisely. Moreover we discuss how polarized electron and positron beams can be used to establish directly the weak quantum numbers of the selectrons. We also use selectron pair production to determine the gaugino mass parameter M_1. This is of particular interest for scenarios with non-universal gaugino masses at a high scale resulting in |M_1| << |M_2| at the electroweak scale. Moreover, we consider also the case of a non-vanishing selectron mixing and demonstrate that it leads to a significant change in the phenomenology of selectrons.
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