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It is intriguing (to me) that while fractionally charged particles exist in the standard model, they are always bound into composite particles of integer charge. The standard model explains this by QCD: fractionally charged particles all have nonzero color charge, and so can't be free. But I'm wondering whether it would be possible to introduce a fractionally charged free particle, or whether the theory becomes inconsistent in that case. Is there some deep reason that colorless particles must have integral charge, other than the fact that the standard model carefully assigns color and electric charges to make this happen?