9 jan. 2017 Learning physics/ https://en.wikipedia.org/wiki/Baryon_number.

9 jan. 2017 Learning physics/ https://en.wikipedia.org/wiki/Baryon_number

In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as

${\displaystyle B={\frac {1}{3}}\left(n_{\text{q}}-n_{\bar {\text{q}}}\right),}$

where n_q is the number of quarks, and n_q is the number of antiquarks. Baryons (three quarks) have a baryon number of +1, mesons (one quark, one antiquark) have a baryon number of 0, and antibaryons (three antiquarks) have a baryon number of −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.

Flavour in
particle physics

Flavour quantum numbers
Isospin: I or I3 Charm: C Strangeness: S Topness: T Bottomness: B′
Related quantum numbers
Baryon number: B Lepton number: L Weak isospin: T or T3 Electric charge: Q X-charge: X
Combinations
Hypercharge: Y Y = (B + S + C + B′ + T) Y = 2 (Q − I3) Weak hypercharge: YW YW = 2 (Q − T3) X + 2YW = 5 (B − L)
Flavour mixing
CKM matrix PMNS matrix Flavour complementarity
v t e

In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as

Particles not formed of quarks

Particles without any quarks have a baryon number of zero.
Such particles include leptons (electron, muon, tau and their neutrinos) and gauge bosons (photon, W and Z bosons, gluons, and the Higgs boson); or the hypothetical graviton.

Conservation

See also: Conservation law (physics)

The baryon number is conserved in nearly all the interactions of the Standard Model. '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.
An exception is the chiral anomaly proposed by some extensions of the standard model.
However, sphalerons are not all that common. Electroweak sphalerons can only change the baryon number by 3.
No experimental evidence of sphalerons has yet been observed.
The still hypothetical idea of a grand unified theory allows for the changing of a baryon into several leptons (see B − L), thus violating the conservation of both baryon and lepton numbers.^[2] Proton decay would be an example of such a process taking place, but has never been observed.

See also

• Lepton number
• Flavour (particle physics)
• Isospin
• Hypercharge
• Proton decay
• B − L
•