In the scalar field Lagrangian the mass term is given by

$$m^2 \phi^2$$

But the equivalent term in Maxwell's Lagrangian for electromagnetism is

$$m^2A_{\mu}A^{\mu}$$

But I don't know why the latter is correct. Could anyone help me getting to it?

In the scalar field Lagrangian the mass term is given by

$$m^2 \phi^2.$$

But the equivalent term in Maxwell's Lagrangian for electromagnetism is

$$m^2A_{\mu}A^{\mu}.$$

But I don't know why the latter is correct. Could anyone help me getting to it?