Effects of heavy bosonic excitations on QED vacuum

We discuss the contribution of axion-like excitations (ALE) to the vacuum birrefringence in the limit $m_A\gtrsim \omega$, where $m_A$ is the mass of the excitation and $\omega$ the energy of test photons interacting with an external (intense) magnetic field. The relevance of this term with respect to the QED contribution depends on the ratio $g_A\mathrm{/}{m}_A$ and, from present bounds on the mass and the coupling constant $g_A$, we find that in the present low energy regime, it ranges from ${10}^{-14}$ to ${10}^2$ suggesting an interesting alternative to explore.