Dynamical mean-field theory for pairing and spin gap in the attractive Hubbard model

We solve the attractive Hubbard model for arbitrary interaction strengths w ithin dynamical mean-field theory. We compute the transition temperature fo r superconductivity and analyze electron pairing in the normal phase. The n ormal state is a Fermi liquid at weak coupling and a non-Fermi-liquid state with a spin gap at strong coupling. Away from half filling, the quasiparti cle weight vanishes discontinuously at the transition between the two norma l states.