  Spin relaxation from a triplet excited state to a singlet ground state in a
semiconductor quantum dot is studied by employing an electrical pump-and-probe
method. Spin relaxation occurs via cotunneling when the tunneling rate is
relatively large, confirmed by a characteristic square dependence of the
relaxation rate on the tunneling rate. When cotunneling is suppressed by
reducing the tunneling rate, the intrinsic spin relaxation is dominated by
spin-orbit interaction. We discuss a selection rule of the spin-orbit
interaction based on the observed double-exponential decay of the triplet
state.
