How stable are protons?

Protons are considered to be very stable based on discoveries. Protons have never been seen to decay, either it exists in atoms, or it's drifting freely in the atmosphere, it is still remarkably stable. Nevertheless, in physics, no theory is laid down against the decaying of the proton.

Even a stable proton might be considered separately in physics, as several theories demand that protons should decay. A good reason that proves proton to be stable is that protons are the lightest baryon. Every other baryon, such as neutron decays into a proton.

Protons live for such a long time that we do not know maybe they exist forever. The reason for this is that no Standard Model's interactions violate the baryon number. It is up to physics to improve beyond the Standard Model to get proton decayed. The experiment actually shows that proton would have a lifetime for more than 1034 years. It is considered to have a lifetime beyond the whole galaxies.

Protons are stable because the proton is the lightest baryon. The neutron is nearly the same mass as the proton (about 0.1% difference in mass), and when you put conglomerations of protons and neutrons together, neutrons become stable (these are called nuclei).

All other baryons must decay into the proton. The beneficial fact is that we don't know how long the proton lives, but that it lives a long time should not be considered anomalous. The reason that it lives so long and that we don't know how long its life expectancy is because none of the interactions of the Standard Model violate baryon number. There is a non-perturbative process that violates baryon number by 3 units and can cause tritium or Helium-3 to decay with an enormous lifetime (10^120 years).

Hence, only physics beyond the Standard Model (which as yet remains undiscovered) can cause the proton to decay. The most natural scale for that to be at is at the GUT scale or above. These effects lead to lifetimes that are between 10^35 and 10^40 years. It is possible to enhance the rate as well. But we really don't know.