In the framework of the Standard Model of particle physics, protons are considered stable particles. However, certain theories beyond the Standard Model, such as Grand Unified Theories (GUTs) or some versions of string theory, propose that protons can decay.
Proton decay refers to the hypothetical process where a proton transforms into lighter particles. For example, one possible decay mode is the transformation of a proton into a positron (antielectron) and a neutral pion (a meson composed of an up quark and an anti-up quark). This decay process violates the conservation of baryon number, which is a fundamental principle in particle physics.
If proton decay were observed, it would have significant implications for both particle physics and cosmology. Here are a few key points:
Confirmation of GUTs: Grand Unified Theories predict that the fundamental forces of nature, namely the electromagnetic, weak, and strong forces, are unified at very high energies. Proton decay is a crucial prediction of some GUTs, and its observation would provide strong evidence in support of these theories.
Baryogenesis: The observed imbalance between matter and antimatter in the universe is an important puzzle in cosmology. If protons were found to decay, it would offer a possible mechanism for explaining this matter-antimatter asymmetry. The decay of protons could lead to an excess of matter over antimatter, contributing to the observed dominance of matter in the universe.
Unification of Forces: Proton decay is intimately linked to the idea of unifying the fundamental forces. It provides a connection between the strong nuclear force and the electroweak force, giving insights into the physics at energies beyond what we can currently access experimentally.
Experimental Challenges: Detecting proton decay is a significant experimental challenge due to its long predicted lifetime. Proton decay, if it occurs, has an extremely long half-life, estimated to be at least 10^33 years in many GUT scenarios. This means that dedicated and sensitive detectors with long observation times are necessary to search for proton decay.
To date, no experimental evidence of proton decay has been found. Experiments, such as those conducted by the Super-Kamiokande and the Deep Underground Neutrino Experiment (DUNE), continue to search for proton decay and explore its implications for our understanding of the fundamental laws of nature.