  We realize an open version of the Dicke model by coupling two hyperfine
ground states using two cavity-assisted Raman transitions. The interaction due
to only one of the couplings is described by the Tavis-Cummings model and we
observe a normal mode splitting in the transmission around the dispersively
shifted cavity. With both couplings present the dynamics are described by the
Dicke model and we measure the onset of superradiant scattering into the cavity
above a critical coupling strength.
