Baryonnumber

Baryon number, denoted B, is a quantum number used to classify hadrons and their interactions in particle physics. Quarks carry B = +1/3, antiquarks carry B = −1/3, baryons (bound states of three quarks) have B = +1, and antibaryons have B = −1. Mesons, formed from a quark and an antiquark, have B = 0, since their constituents cancel in baryon number.

Under the Standard Model, baryon number is an accidental global symmetry of the perturbative Lagrangian. This means that in strong and electromagnetic interactions, and in most weak processes, total baryon number is conserved. Non-perturbative electroweak effects, such as sphaleron transitions, can violate B and L, but they conserve the combination B − L. Such violations are expected to be exceedingly rare at ordinary energies.

Conservation of baryon number explains the stability of ordinary matter and provides constraints on particle decays. If baryon number were not conserved, protons could decay into lighter non-baryonic particles. Experimental searches have not observed proton decay and have established very long lifetime bounds for various decay channels.

In cosmology and particle theory, baryon number violation is a key ingredient in explanations for the matter–antimatter asymmetry of the Universe. Any viable theory of baryogenesis must incorporate processes that violate B, along with CP violation and departure from thermal equilibrium.

Related topics include baryons, baryogenesis, proton decay, and grand unified theories.