Decays
In a decay, the original particle disappears and two or more less massive particles are produced.
For example:
Particle decays are like radioactive decays of atomic nuclei. When a nucleus decays radioactively, some of the decay products are constituents that were present before the decay, but others, such as photons or electrons, are entirely new objects produced by the decay process.
When a fundamental particle decays, all the produced particles are new objects that were not present before the decay. A single type of fundamental particle can have many possible sets of decay products.
Basic Rules of Decays
Any decay that can happen, will happen.
Decay rates depend on the type of interaction and on the amount of energy "released," that is, energy converted from mass energy to kinetic energy.
Decays can happen only if all conservation laws can be obeyed. For example:
- Conservation of energy states that the sum of the energies of the final product particles must be equal to the mass energy (mc2) of the initial particle. This implies that the sum of the masses of the final particles must be less than the mass of the initial one.
- Conservation of electric charge states that the sum of the charges of the final product particles must be the same as the charge of the initial particle.
Some atomic nuclei are stable, others decay. Nuclear stability and radioactive decays can all be explained from the same set of conservation laws and underlying interaction types as those for isolated particles.
As far as we know today, electrons, protons, photons, and neutrinos are the only fundamental particles that never decay. All other isolated particles are unstable and decay with a definite half-life decay distribution. They are produced in accelerator experiments, and in cosmic ray interactions, but are not part of ordinary matter.
For example, isolated neutrons have a half-life of 14.8 minutes. They decay by weak interactions to produce a proton, an electron, and an anti-electron neutrino.
Decays that Do Not Occur
In contrast, the experimental lower limit on the half-life for the proton decay is 100,000,000,000,000,000,000,000,000,000,000 or 1032 years! If the half-life were any shorter, proton decays would have been observed in experiments searching for them, yet they have not been seen. (You might wonder how such a limit can be set. A cube of water 10 meters on each side contains 1033 protons and neutrons. With appropriate instructions, one can watch this water very carefully!)
Conservation of energy and electric charge would allow the decay of a proton into a positron and a photon. If this were an allowed electromagnetic process, it would occur with a half-life of a tiny fraction of a second instead of 1032 years. So, there must be another conservation law that forbids this particular decay. In fact, we find two laws (rules 6 and 7) that forbid it:
- Conservation of Baryon Number : A proton has baryon number +1, but a photon and a positron each have baryon number zero (+1 does not equal zero).
- Conservation of Electron Number: A positron (anti-electron) has electron number -1, but a proton and a photon each have electron number zero (-1 does not equal zero).
If proton decay is discovered in some future experiment, we will know that there are some extremely rare types of interactions that do not respect these laws.
