As for mathematics, it might be worth mentionning that there's an AMS prize rewarding undergraduates who published "outstanding" results (by themselves or as a joint work), the Frank and Brennie Morgan prize.
That said, there is no indication on the number of candidates considered each year for the prize. Perhaps the fact that they have honorable mentions means that really only one or two undergraduates each year manage to publish at research-level, the rest of those publications not being worth mentionning.
