According to our current understanding of particle physics, protons are considered stable particles and do not decay into anything lighter. In fact, protons are believed to be the lightest stable particles in the universe.
On the other hand, free neutrons, outside the context of a stable atomic nucleus, are not stable and undergo radioactive beta decay. In beta decay, a neutron can transform into a proton, an electron, and an electron antineutrino. The process is described by the weak force, one of the fundamental forces in nature. This decay occurs through the conversion of a down quark in the neutron to an up quark, changing the neutron (udd) into a proton (uud). The electron and the electron antineutrino are also emitted to conserve charge and other properties.
However, within the confines of an atomic nucleus, the situation is different. Neutrons and protons can be bound together in stable nuclei, and the stability of the nucleus depends on the balance between the nuclear force and the electrostatic repulsion between protons. In some rare cases, certain isotopes can undergo processes such as beta decay or other nuclear reactions, resulting in a change in the number of protons and neutrons. These processes are typically associated with radioactive isotopes and nuclear reactions rather than spontaneous decay of individual protons or neutrons.
To summarize, protons are considered stable, while free neutrons can decay into a proton, an electron, and an electron antineutrino. However, within an atomic nucleus, the stability and decay properties are determined by the specific isotope and nuclear reactions involved.