baryonnummer

The baryonnummer, commonly referred to as the baryon number, is a quantum number used in particle and nuclear physics. It is an additive property assigned to particles that helps distinguish baryons from other particles. In the standard assignments, baryons such as protons and neutrons have B = +1, antibaryons have B = -1, and mesons and most leptons have B = 0. In the quark model, each quark carries B = 1/3, so baryons (three-quark states) sum to B = +1 and antibaryons (three antiquarks) sum to B = -1.

Conservation of baryon number is an important feature of the Standard Model, where it is an accidental global symmetry. It is preserved in perturbative processes, making baryon-number-conserving reactions common in ordinary nuclear and particle interactions. However, non-perturbative effects in the electroweak sector, such as sphaleron processes, can violate B and L while conserving B − L, particularly at high temperatures in the early universe or in extreme conditions.

The concept is central to discussions of baryogenesis, the set of processes that produced the matter–antimatter asymmetry observed today. Explaining why there is more matter than antimatter requires, among other ingredients, baryon-number violation, CP violation, and departures from thermal equilibrium (the Sakharov conditions). Experimental searches for B-violating phenomena, notably proton decay, test theories beyond the Standard Model; current limits place very long possible lifetimes for protons, constraining many proposed B-violating models.