Can particle with fractional charge exist in isolation? Since quarks are ruled out I wonder if it is possible for free fractional charge to exist not counting virtual particles?
 A: All the data gathered over the last 200 years or so show only integer multiples of charges, and the models that classify the behavior of particles are successful in predicting new data. Fractional charges arouse in the models to describe the symmetries of hadrons which were experimentally found to be composite.
The models have not been invalidated, within the mainstream  there are no free fractional charges. They have not been observed and that (non observation) is what the models describe.
A: Good question. Not very desirable from the point of view of supporters of the quark model which is a part of the standard model, but still.
I'm afraid, that no fractional charges is possible in Nature. Not in isolation, not with confinment.
The root of mistake with fractional charges is belief (not proven) of physical community that proton and neutron are similar particles which are distinguished by quantum number isospin.
If you thust that proton and neutron is similar particles you have to trust in fractional charges.
But, if you would analize what is neutron, you would quickly go to conclusion, that neutron have to be a composite of proton and negative pion. As well as atomic nuclei.
In than case you would quickly go to conclustion,  than fractictional charges are not needed.
I wonder how many centuries will it take from physical community to understand what SU(3) symmetry really shows, apart from mistake with wrong identification of neutron as elementary particle like proton, made in wild 1934 year :)
