One signature could be similar to that of a parton model. Suppose leptons are composed of internal particles, preons or rishons or what-ever-ons. At low energy the lepton will appear to be composed of the valence partons (lepto-partons?), which might just be the lepton itself. As one transforms to a high energy frame, then in the limit this momentum goes to infinity the Lorentz contraction of the lepton makes other modes, or higher energy partons in excited states, apparent in scattering experiments. There would then be a Bjorken scaling to scattering amplitudes which act as signatures of the internal constituents of a lepton.
Another signature could be some deviation in the magnetic moment of the electron. The magnetic moment is
μs = −gsμbohrS/ℏ.
For a Dirac electron with the EM field "turned off" the g-factor is
gs = 2. In QED this is
gs = 2.00231930436. If the electron is a constituent particle then there might at some scale be a deviation from the QED expected result.
What might these constituents be? Most likely any such deviation would to my mind be some stringy physics which due to extra large dimension and related matters is exhibiting an influence on a scale we can detect. I don’t like the idea of quarks and leptons as composite objects. This is largely because the energy in binding this system together would be much larger than the masses of the partons. This would present us with horrendous problems far surpassing those seen with quarks and QCD.
This post imported from StackExchange Physics at 2014-03-24 04:41 (UCT), posted by SE-user Lawrence B. Crowell