My book says that "Later on, to keep the number of particles either odd or even on both sides another particle called anti neutrino, was also assumed to be emitted along with the beta particle. This particle is uncharged and its rest mass is zero. Then the complete transformation equation becomes $$ n \to p^+ + e^− + \bar{\nu}. $$ Is it correct that to keep number even or odd, please explain. An answer not using very much complicated words will be appreciated.
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Electrons, protons, and neutrons are all fermions, with spins of $\pm 1/2$.
Spin has to be conserved, and you can't add up an odd number of $1/2$'s and get an integer. If you start with an odd number of fermions, the products have to contain an odd number of fermions. So this reaction has to produce at least one unseen fermion.
I guess that they invoked Occam's razor when they assumed that there was exactly one unseen fermion (rather than, say, three).
Peter Shor
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