The particle that has the least mass is a beta particle.
If we look at the particles listed individually, we can see that the proton and the neutron are both massive particles found in the nucleus of an atom. They are responsible for the mass of the atom. An alpha particle is just of the same mass as a helium nucleus.
A beta particle is actually an electron. Recall that an electron has negligible mass.The mass of an electron is about 1/840 times that of the proton.
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