Bhabha vs. Moller scattering as a contact-interaction analyzer at a polarized Linear Collider

We discuss electron-electron contact-interaction searches in the Bhabha and Moller processes at planned Linear Colliders run in the electron-positron and electron-electron modes, with longitudinally polarized beams. Our analysis is based on the measurement, for the two processes, of polarized differential cross sections that allow to disentangle the different, independent, four-fermion couplings. We evaluate by a model-independent analysis the sensitivities to the contact coupling constants, emphasizing the role of the available beam polarization and the complementarity of the two processes in giving the best limits. We also make a comparison with the potential of $e^+e^-\to\mu^+\mu^-$ at the same energy and beams polarization.Comment: 18 latex pages, 15 eps figs., some misprints corrected and added ref