# Can neutrinos interact by the EM interaction and gravity?

A definition of a lepton is:

A particle that does not interact by the strong force but does by the 3 other forces.$^1$

Neutrinos are leptons, so from the above definition one would expect it to interact through the weak interactions, EM and gravity. However I have seen arguments like the following:

This interaction must be weak since it contains neutrinos.$^2$

Clearly this implies that neutrinos only interact through the weak interaction and thus contradicts the definition given above.

So can neutrinos actually interact through EM and gravity? If so why do you often get arguments like the one in Martin and Shaw?

$^1$Adapted from: Matolyak, J. and Haija, A. 2014. Essential Physics. Boca Raton: CRC Press, page 442

$^2$e.g. Martin, B.R. and Shaw, G., 2013. Particle physics. John Wiley & Sons. page 337

• One has to keep in mind that all the statements isolating the effect of forces for elementary particles are really talking of first order Feynman diagram interactions. Higher order diagrams can involve all the forces for all the particles, but because so many vertices are involved in higher orders in a multiplicative way, the higher order contribution to the interaction crossection is very small. Thus the quoted statement holds. Commented Jul 24, 2016 at 18:38
• @annav 'Thus the quoted statement holds.' Which one? The first or the second or both? Commented Jul 24, 2016 at 18:40
• Just define a lepton as a fermion that is color-neutral, and the apparent problem dissolves. It's not clear what such a "definition" of a lepton is supposed to improve over just listing electron, muon, tauon and their neutrinos, anyway. Commented Jul 24, 2016 at 19:23
• the first quote. Both are sloppy generalizations in my opinion. for the neutrno see my answer here physics.stackexchange.com/questions/269472/… Commented Jul 25, 2016 at 3:57

Finally, interestingly enough, I looked at the Wikipedia page for leptons (here). It simply defines a lepton as a spin $\frac{1}{2}$ particle that does not undergo strong interactions. It further states that leptons are divided into two classes: charged leptons (also known as electron-like leptons) and neutral leptons (aka neutrinos).