Stability of Photons

                                                                                                                                                                 

Light Photons

While a non-zero photon mass has been under theoretical and experimental study for years, the possible implication of a finite life time of a photon remains a point to discuss. The tight experimental upper bound of the photon mass restricts the kinematically allowed final states of photon decay to the lightest neutrinos or particles beyond standard model if this channel is the concern of experimental studies.

A non-zero photon mass is often dismissed on theoretical grounds as insertion of mass term into QED calculations breaks gauge invariance and might therefore disturbs renormalizability. i.e. consistency of the theory at quantum level.

The question of a photon mass in QED is then purely experimental as there is no dispute over a small mass assigned to the photon (m>0) [This would take Electroweak gauge group SU(2)_LX U(1)_γ   ---- the hypercharge U(1)_γ,  that can be used for calculations. ] The resulting mass for the hypercharge gauge boson eventually generates again a massive photon.

Particles beyond the standard model could not only increase the rate of γ -->  v₁ v₂ , but also serve as final states  themselves, as some extensions to the standard model feature additional massless states, for example, sterile neutrinos, hidden photons, Goldstone bosons and axions. These weakly sub- eV particles are less constrained than neutrinos and photon decay could be the indirect or direct effect of these states. Although, mainly of academic interest we mention that a massive photon provides the possibility of faster-than-light particles- and a decaying photon always predicts them.

The question of photon decay is therefore relevant even if the lightest neutrinos have an inaccessible heavy final state.

                                           (Ref. Julian Heeck, How stable is the photon? Phy. Rev. Letters,2013)