Proton decay

The status of proton decay is described, including general motivations for baryon number violation, and the present and future experimental situation. Grand unification with and without supersymmetry is considered, including possible evidence from coupling constant unification and implications for proton decay and neutrino mass. 1 Motivations Baryon number is almost certainly not absolutely conserved. • There is no compelling reason to think that baryon number is conserved. The only convincing mechanism we have for ensuring an absolute conservation law is gauge invariance. For example, electromagnetic gauge invariance guarantees that electric charge is conserved and implies the existence of a massless photon. However, there is no analogous baryton – i.e., there is no long range force coupling to B. We know from Eötvos-type experiments [1] that if there were such a gauge boson its coupling would have to be incredibly small, g 2 B /4π < 6 × 10 −48. Hence, baryonic gauge invariance cannot be invoked and there is no good reason to suspect absolute conservation. • Black holes do not remember baryon number. If a proton were to drop into a black hole its quantum numbers would disappear from the universe, violating baryon number. • It has been known for some time [2] that baryon number is violated in the weak interactions via the weak anomaly, as shown in Figure 1. The idea is that the vacuum state is not unique and there are degenerate vacua characterized by different values for B. There are nonperturbative tunnelling amplitudes to make transitions from one vacuum to another. However, these are incredibly slow, characterized by rates proportional to exp(−4π sin 2 θ W /α) ∼ 10 −172 , and are irrelevant for proton decay. However, it has been realized and emphasized recently [3] that thermal fluctuations at the time of the electroweak phase transition could lead to transitions, and this has significant implications for the baryon asymmetry of the universe.