Photon as a gauge boson for static fields Excuse me if my question is naive, but I have never taken a proper QFT.
I used to think of a photon as a quantum of EM field, quantum of light. But form QFT and particle physics prospective, photon is a mediator of electroweak force, gauge boson. It means that for static fields it has to be bound as well as for EM-waves. What is the mechanism? What are the good reference books/articles? Thanks in advance!
 A: It is not a gauge boson for a static field. Nothing is. It is the EM gauge boson, and the quanta of that field, representing an excitation. You can also say that it carries the EM force as it is exchanged between particles (though the word force already makes the statement more a classical analogy than a description of fact). 
More exactly any interaction where a photon line exists in a Feynman diagram is an electromagnetic interaction. 
The static force is described and calculated in Feynman diagrams as a virtual photon. Virtual because it does not have a physically possible energy-momentum, called off shell, and is not a real photon. There are alternative ways of describing it that are more physical, but this intuitive virtual photon allows you to calculate using the Feynman diagram rules. 
Any standard QFT book will cover this and make it both more clear and build up your intuition on it. If you have strong QM you might be able to, but a course or the easier one would be an easier first try (search in this site for some of those or a QFT for dummies book). Even Wikipedia should have plenty articles on it. Look for virtual photons or related terms. 
