Explicit symmetry breaking
In theoretical physics, explicit symmetry breaking is the breaking of a symmetry of a theory by terms in its defining equations of motion (most typically, to the Lagrangian or the Hamiltonian) that do not respect the symmetry. Usually this term is used in situations where these symmetry-breaking terms are small, so that the symmetry is approximately respected by the theory. An example is the spectral line splitting in the Zeeman effect, due to a magnetic interaction perturbation in the Hamiltonian of the atoms involved.
Explicit symmetry breaking is also associated with electromagnetic radiation. A system of accelerated charges results in electromagnetic radiation when the geometric symmetry of the electric field in free space is explicitly broken by the associated electrodynamic structure under time varying excitation of the given system. This is quite evident in an antenna where the electric lines of field curl around or have rotational geometry around the radiating terminals in contrast to linear geometric orientation within a pair of transmission lines which does not radiate even under time varying excitation. 
- Castellani, E. (2003) "On the meaning of Symmetry Breaking" in Brading, K. and Castellani, E. (eds) Symmetries in Physics: New Reﬂections, Cambridge: Cambridge University Press
- Sinha & Amaratunga (2016) "Explicit Symmetry Breaking in Electrodynamic Systems and Electromagnetic Radiation" Morgan Claypool, Institute of Physics, UK