Baryon number

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In particle physics, the baryon number (B) is an additive quantum number of a system. It is defined as $${\displaystyle B={\frac {1}{3}}(n_{\text{q}}-n_{\rm {\overline {q}}}),}$$ where ⁠${\displaystyle n_{\rm {q}}}$⁠ is the number of quarks, and ⁠${\displaystyle n_{\rm {\overline {q}}}}$⁠ is the number of antiquarks. Baryons (three quarks) have B = +1, mesons (one quark, one antiquark) have B = 0, and antibaryons (three antiquarks) have B = −1. Exotic hadrons like pentaquarks (four quarks, one antiquark) and tetraquarks (two quarks, two antiquarks) are also classified as baryons and mesons depending on their baryon number. In the standard model B conservation is an accidental symmetry^[1] which means that it appears in the standard model but is often violated when going beyond it. Physics beyond the Standard Model theories that contain baryon number violation are, for example, Standard Model with extra dimensions,^[2] Supersymmetry, Grand Unified Theory and String theory.