# In what sense are photons emergent?

Recently I read in an essay by Wilczek:

"Photons are mixtures of weak B3 and hypercharge C mesons. It is those objects, not the emergent photon, whose properties are ideally simple."

Until now I thought that photons are elementary massless spin-1 bosons that arise as gauge bosons for the $U(1)$ symmetry in (quantized) electrodynamics. They can be described by a four-vector $A_\mu$ including two unphysical degrees of freedom that can be eliminated by introducing the gauge-invariant four-rotation $F_{\mu\nu} := \partial_\mu A_\nu - \partial_\nu A_\mu$ so that only the two polarisations (helicity $h = \pm 1$) remain.

Where does the emergence of the photon come from? Is this related to electroweak symmetry breaking and the Higgs field? Why are mesons (hadrons) mentioned??

• See the Wikipedia article on EW symmetry breaking. Above the EW transition neither the photon nor the Z exist. They appear at low energies due to mixing of two of bosons that exist before the EW symmetry is broken. I guess Wilczek's point is that the higher symmetry before the EW transition makes the physics simpler. – John Rennie Feb 8 '15 at 16:27
• Photons are not a mixture of mesons (which would be quark-antiquark pairs). They simply are a mixture of two other electroweak gauge bosons such that their combination remains massless after the Higgs SSB. (Mass eigenstates are the "naive" physical states) I've never heard the word emergence being used to describe that, and I do not understand the question. – ACuriousMind Feb 8 '15 at 16:30
• Exactly, this sentence with "mixtures of mesons" is what I dont understand in the essay by Wilczek. I understand the spontaneous breaking of EW symmetry, but I only knew mesons as hadrons until now! – quantumorsch Feb 8 '15 at 16:37
• It seems obvious from the context that Wilczek means gauge bosons not mesons. – John Rennie Feb 8 '15 at 16:44
• I vaguely remember reading of an 'archaic' use of "meson" to mean intermediate vector boson. For an example from 1967, see this which refers to a "... the W meson or intermediate boson..." – Alfred Centauri Feb 8 '15 at 22:31

Yes. There is a particular mixture of the $W^0$ and $B$ bosons that propagates freely in the Higgs field condensate; this freely propagating state is the photon.