In particle physics, the baryon number is an approximate conserved quantum number. The baryon number of a system is defined as the number of quarks divided by three minus the number of antiquarks in the system divided by three. CPT-symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity and time simultaneously. ... In the standard model of particle physics the Cabibbo Kobayashi Maskawa matrix (CKM matrix, sometimes earlier called KM matrix) is a unitary matrix which contains information on the strength of flavour changing weak decays. ... CP is the product of two symmetries: C for charge conjugation, which transforms a particle into its antiparticle, and P for parity, which creates the mirror image of a physical system. ... A phenomenon is said to be chiral if it is not identical to its mirror image (see Chirality (mathematics)). The spin of a particle may be used to define a handedness for that particle. ... Particles erupt from the collision point of two relativistic (100 GeV per nucleon) gold ions in the STAR detector of the Relativistic Heavy Ion Collider. ... In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves. ... A quantum number is any one of a set of numbers used to specify the full quantum state of any system in quantum mechanics. ...

Why one third? According to the laws of strong interaction there cannot be any bare color charge, i.e. the total color charge of a particle has to be zero ('white'), cf. confinement. This can only be achieved by either putting together a quark of one color with an antiquark of the corresponding anti-color, giving a meson with baryon number 0, by combining three quarks into a baryon with baryon number +1, or by combining three antiquarks into an anti-baryon with baryon number −1. Another possibility is the exotic pentaquark, consisting of 4 quarks and 1 anti-quark. The strong interaction or strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics. ... Confinement is the physics phenomenon that quarks cannot be isolated. ... In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. ... In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). ... A pentaquark is a subatomic particle consisting of a group of five quarks (compared to three quarks in normal baryons and two in mesons), or more specifically four quarks and one anti-quark. ...

Thus, quarks are always present in threes, if antiquarks are counted as "negative quarks". Historically, baryon number was defined long before the current model of quarks was established, so rather than changing the definition, particle physicists simply divided the previoiusly known quantum number by three. Nowadays it might be more accurate to speak of the conservation of quark number. For other uses of this term, see: Quark (disambiguation) 1974 discovery photograph of a possible charmed baryon, now identified as the Σc++ In particle physics, the quarks are subatomic particles thought to be elemental and indivisible. ... A quantum number is any one of a set of numbers used to specify the full quantum state of any system in quantum mechanics. ...

Particles without any quarks or antiquarks have baryon number 0. Such particles include leptons, the photon, and the W and Z bosons. In physics, a particle is a lepton if it has a spin of 1/2 and does not experience the strong nuclear force. ... In physics, the photon (from Greek φως, phōs, meaning light) is the quantum of the electromagnetic field; for instance, light. ... In physics, the W and Z bosons are the elementary particles that mediate the weak nuclear force. ...

The baryon number is nearly conserved in all interactions of the Standard Model. The loophole is the chiral anomaly. However, sphalerons are not all that common. Electroweak sphalerons can only change the baryon number by 3. A fundamental interaction is a mechanism by which particles interact with each other, and which cannot be explained by another more fundamental interaction. ... The Standard Model of Fundamental Particles and Interactions The Standard Model of particle physics is a theory which describes the strong, weak, and electromagnetic fundamental forces, as well as the fundamental particles that make up all matter. ... Chiral anomaly is the anomalous nonconservation of charge in a quantized theory of chiral fermions coupled to a background gauge field. ... A sphaleron is a rare process of the electroweak interaction of the Standard Model of particle physics. ...

'Conserved' means that the sum of the baryon number of all incoming particles is the same as the sum of the baryon numbers of all particles resulting from the reaction.

A violation of baryon number might lead to proton decay, but only if the baryon number changes by 1.

The still hypothetical idea of grand unified theory allows for the changing of a baryon into a bunch of leptons, thus violating the conservation of baryon and lepton number. Proton decay would be an example of such a process taking place. Grand unification, grand unified theory, or GUT is a theory in physics that unifies the strong interaction and electroweak interaction. ... In particle physics, the baryons are a family of subatomic particles including the proton and the neutron (collectively called nucleons), as well as a number of unstable, heavier particles (called hyperons). ... In physics, a particle is a lepton if it has a spin of 1/2 and does not experience the strong nuclear force. ... The standard model of particle physics states that protons are stable, i. ...

See also

• proton decay
• Lepton number
• B−L
• flavour (particle physics)
• Isospin
• Hypercharge