How is the electron depicted in simplest string theory and how the muon and tau? Are intermediate masses excluded by the model? I am curious to understand can string theory give the masses of electron, muon and tau. I suppose they are closed strings and vibration wavelength must be the length of the string divided by a digit $N=1,2,...$ But then the muon must have a mass of two electrons, the tau of three etc. I try to find something on Google but there is nothing. It will be good if one can show a picture.
 A: It's much more complicated than that. 
In the standard model (so, field theory rather than string theory), the mass of the electron comes from a "yukawa coupling" interaction between the left-handed part of the electron field, the right-handed part of the electron field, and the Higgs field. Something similar applies to muon and tau and also to the quarks; and there are also yukawa couplings between left- and right- parts of what we regard as different fermions, which give rise to decay probabilities (e.g. tau decays to muon). 
The actual mass is a product of the Higgs field "expectation value", and the yukawa coupling, which in the standard model is just an unexplained number. 
In string theory, mass also comes from the yukawa coupling. So there has to be a string state corresponding to the left-handed part, a string state corresponding to the right-handed part, and something (a physical degree of freedom) corresponding to the Higgs field. But in string theory the magnitude of the yukawa coupling will be determined by e.g. the geometry of the extra dimensions. 
