Baryon number (B) conservation means that the number of quarks minus the number of anti-quarks in any interaction remains constant. Lepton number (L) conservation means that the number of leptons (electrons, muons, tau leptons, and neutrinos) minus the number of anti-leptons in any interaction remains constant.
The Standard Model separately conserves B and L in all interactions except sphaleron interactions, which have been never observed and are theoretically confined to extremely high energy scales and mass-energy densities.
The conservation of baryon number and lepton number is established remarkably robustly in experiments.
Some of the main experimental searches that have not detected B and L non-conservation are the searches for neutrinoless double beta decay, the search for tree-level flavor changing neutral currents, and the search for proton decay. These non-detections have ruled out or tightly constrained many theories in physics including Majorana neutrino mass and most of the simpler grand unified theories (GUTs), such as SU(5).
Baryon number (B) conservation underlies the apparent stability of ordinary matter by forbidding the decay of nucleons, while lepton number (L) conservation plays a central role in the structure of lepton interactions and the possible origin of neutrino mass.
In the Standard Model, B and L are accidental global symmetries rather than imposed fundamental principles. However, they are expected to be violated in many extensions of the theory, including frameworks of unification and processes in the early Universe.
This review summarizes the status of experimental tests of B and L conservation and discusses them within a unified framework for interpreting current and future searches across different processes and experimental approaches, outlining historical and theoretical motivation, key physical processes, as well as their broader connections and complementarity to other searches.
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