In addition to the proton and electron, the decay also produces a particle called a
neutrino.Though neutrinos are fascinating in their own
right, our discussion of radioactivity can get away with ignoring them: they have no electric
charge and practically no mass, and they almost never interact with anything.
Wolfgang Pauli first suggested, around 1930, that
neutrinos might exist, because without them, momentum didn't seem to be
conserved in nuclear reactions. (Notice how paying attention to
conservation laws can lead to important discoveries!) Neutrinos were
actually detected in the 1950's.
At one time, neutrinos were thought to have no mass at all. However, they've been in the news
recently because there is now compelling evidence that they do have some mass--maybe about one
ten-millionth the mass of an electron. If you want to read more about this discovery, here are
some useful links: