This entry contributed by Leonardo Motta
A conserved quantum number defined by assigning to every baryon and B = -1 to every antibaryon. mesons and leptons are then assigned B = 0. A process or reaction of fundamental particles cannot occur if it changes the total baryon number of a system.
For example, the process
( MeV) does not violate the conservation laws of charge, energy, linear momentum, or angular momentum. However, it does not occur because it violates the conservation of baryon number, i.e., B = 1 on the left and 0 on the right. It is fortunate that this process "never" happens, since otherwise all protons in the universe Eric Weisstein's World of Astronomy would gradually change into positrons! The apparent stability of the proton, and the lack of many other process that might otherwise occur, are thus correctly described by introducing the baryon number B together with a law of conservation of baryon number.
However, having stated that protons do not decay, it must also be noted that supersymmetric theories predict that protons actually do decay, although with a half-life of at least 1030 years, which is longer than the age of the universe. Eric Weisstein's World of Astronomy All attempts to detect the decay of protons have thus far been unsuccessful.
