Free protons, which are hydrogen nuclei, are considered stable particles. They do not decay over time under normal circumstances. This stability arises due to the conservation of certain fundamental properties, such as electric charge and baryon number. A proton has a positive electric charge, and there is no lighter particle with a positive charge to decay into while conserving charge.
On the other hand, free neutrons are not stable and undergo decay through the weak nuclear force. Neutrons consist of three quarks: two down quarks and one up quark. The decay of a free neutron occurs via the weak interaction, where one of the down quarks within the neutron is transformed into an up quark, converting the neutron into a proton. In this decay process, a W^- boson is emitted, which subsequently decays into an electron and an electron antineutrino:
n → p + e^- + ν_e
The weak interaction violates the conservation of certain properties, such as the total number of baryons (baryon number) and the charge parity (C-parity). In this decay process, the baryon number is conserved because the total number of baryons (protons and neutrons) on both sides of the reaction is the same. However, the neutron has a baryon number of 1, while the proton has a baryon number of 1/3 (since it is made up of three quarks). Therefore, the decay of a neutron leads to a change in the number of protons and neutrons.
It's worth noting that while free neutrons decay with a half-life of about 14 minutes, neutrons within atomic nuclei can be stable due to the additional binding energy provided by the strong nuclear force, which helps to hold the nucleus together.