  We investigate the finite-size Dicke model with photon leakage. It is shown
that the symmetry breaking states, which are characterized by non-vanishing
$\langle \hat{a} \rangle \neq 0$ and correspond to the ground states in the
superradiant phase in the thermodynamic limit, are stable, while the
eigenstates of the isolated finite-size Dicke Hamiltonian conserve parity
symmetry. We introduce and analyze an effective master equation that describes
the dynamics of a pair of the symmetry breaking states that are the degenerate
lowest energy eigenstates in the superradiant region with photon leakage. It
becomes clear that photon leakage is essential to stabilize the symmetry
breaking states and to realize the superradiant phase without the thermodynamic
limit. Our theoretical analysis provides an alternative interpretation using
the finite-size model to explain results from cold atomic experiments showing
superradiance with the symmetry breaking in an optical cavity.
