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 Quote by arivero But suppose that the up quark were massless. Could the neutral pion decay into a pair of up, anti-up quarks?
The pion is already composed of u-ubar quarks, so I don't know what a decay to u-ubar would mean. Since quarks are confined, you'll end up with a pi0 anyway, so that's not much of a decay.

There is an additional subtlety - a pi0 composed of massless quarks would itself be massless (it becomes a Goldstone) and massless particles do not decay.