# Can photon couple to scalar field?

• arroy_0205
In QFT, photons are the carriers of the electromagnetic force. This means that they can interact with other charged particles. However, they don't feel any force at all if they pass between charges.

#### arroy_0205

I have seen in one paper that photon is coupled to dilaton field which is scalar and motivated by string theory. I do not understand this. Photon is carrier of electromagnetic field and so I thought it can only couple to electrically charged fields. Can anyone explain?

Photons can couple to scalar fields. There just happens to be none in the Standard Model (in the spontaneously broken state). Don't want to search for my Rosiek paper now but I am pretty sure the photon couples to the Selectrons of SUSY, for example. In standard particle physics photons only couple to charged fields but in exotic physics they might couple to gravity which the dilaton (according to the WP article; I never encountered those guys myself) seems to be related to.

Scalar fields carry charge as long as they are complex.

Yes that is true but the one I talked about was real scalar field. That is the source of confusion. This I did not write explicitly in my question.

arroy_0205 said:
I have seen in one paper that photon is coupled to dilaton field which is scalar and motivated by string theory. I do not understand this. Photon is carrier of electromagnetic field and so I thought it can only couple to electrically charged fields. Can anyone explain?

Well, I am not up-to-snuff on string theory per se, but I would say that any scalar object with internal charges, whether they cancel or not, will couple to a photon through a trilinear coupling involving another appropriate vector particle. For neutral scalar mesons, for example, the coupling to two photons is an available channel...

photons can couple to fields that are electrically neutral through higher-dimension operators (and therefore are suppressed). All you need to do is maintain gauge and Lorentz invariance, so for example (with a real scalar field $\phi$):

$$\phi F_{\mu\nu}F^{\mu\nu}$$

is allowed. However, it's physical effects are typically quite small. But this is how you can describe photons scattering off (neutral) atoms in the atmosphere, for example.