  Electron spin relaxation in a spin-polarized quantum Hall state is studied.
Long spin relaxation times that are at least an order of magnitude longer than
those measured in previous experiments were observed and explained within the
spin-exciton relaxation formalism. Absence of any dependence of the spin
relaxation time on the electron temperature and on the spin-exciton density,
and specific dependence on the magnetic field indicate the definite relaxation
mechanism -- spin-exciton annihilation mediated by spin-orbit coupling and
smooth random potential.
