The first term above is the Stueckelberg field strength, M 1 and M 2 are topological mass parameters and σ σ is the axion. After symmetry breaking in the electroweak sector the photon remains massless. The model predicts a new type of gauge boson dubbed Z S t ′ which inherits a very distinct narrow decay width in this model. The St sector of the StSM decouples from the SM in limit M 2 / M 1 → → 0 .