When a free neutron decays to a proton, an electron and an antineutrino are created i.e. . The rest mass of neutron is larger than that of proton and thus the Q-value is positive. If we consider a free proton decays to a neutron, a positron and a neutrino are created i.e. . The Q-value is thus negative which is impossible. Also, a lower mass particle cannot be decayed into a large mass particle. Hence, a free proton does not decay to a neutron.