# Baryon number

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 This box:

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

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

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

## Baryon number vs. quark number

Quarks do not only carry electric charge, but also additional charges such as color charge and weak isospin. Because of a phenomenon known as color confinement, a hadron cannot have a net color charge; that is, the total color charge of a particle has to be zero ("white"). A quark can have one of three "colors", dubbed "red", "green", and "blue".

For normal hadrons, a white color can thus be achieved in one of three ways:

• A quark of one color with an antiquark of the corresponding anticolor, giving a meson with baryon number 0,
• Three quarks of different colors, giving a baryon with baryon number +1,
• Three antiquarks into an antibaryon with baryon number −1.

The baryon number was defined long before the quark model was established, so rather than changing the definitions, particle physicists simply gave quarks one third the baryon number. Nowadays it might be more accurate to speak of the conservation of quark number.

In theory, exotic hadrons can be formed by adding pairs of quark and antiquark, provided that each pair has a matching color/anticolor. For example, a pentaquark (four quarks, one antiquark) could have the individual quark colors: red, green, blue, blue, and antiblue.

## 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 and gluons); or the hypothetical graviton.

## Conservation

The baryon number is nearly conserved in 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. However, sphalerons are not all that common. Electroweak sphalerons can only change the baryon number by 3.

The still hypothetical idea of grand unified theory allows for the changing of a baryon into several leptons (see BL), thus violating the conservation of both baryon and lepton numbers. Proton decay would be an example of such a process taking place, but has never been observed.

## References

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• baryon number — noun a number equal to the difference between the number of baryons and the number of antibaryons in any subatomic structure; it is conserved in all types of particle interactions • Hypernyms: ↑number * * * noun : a number equal to the number of… …   Useful english dictionary

• baryon number — Physics. a quantum number assigned to elementary particles, baryons having baryon number 1, antibaryons 1, and all other observable particles 0; quarks have baryon number 1/3 and antiquarks 1/3. Cf. conservation of baryon number. * * * …   Universalium

• baryon number conservation — barioninio skaičiaus tvermė statusas T sritis fizika atitikmenys: angl. baryon number conservation vok. Baryonenzahlerhaltung, f rus. сохранение барионного числа, n pranc. conservation du nombre baryonique, f …   Fizikos terminų žodynas

• baryon number — noun A quantum number related to the difference between the numbers of quarks and antiquarks in a system of subatomic particles …   Wiktionary

• conservation of baryon number — Physics. the principle that the total baryon number remains constant in all processes involving the interaction of elementary particles. * * * …   Universalium

• law of conservation of baryon number — barioninio krūvio tvermės dėsnis statusas T sritis fizika atitikmenys: angl. law of conservation of baryon number vok. Erhaltungssatz der Baryonenzahl, m rus. закон сохранения барионного заряда, m pranc. loi de conservation de la charge… …   Fizikos terminų žodynas

• conservation of baryon number — Physics. the principle that the total baryon number remains constant in all processes involving the interaction of elementary particles …   Useful english dictionary

• baryon — aryon n. 1. any of the elementary particles having a mass equal to or greater than that of a proton and that participate in strong interactions; a hadron with a baryon number of +1. Syn: heavy particle. [WordNet 1.5] …   The Collaborative International Dictionary of English

• Baryon — Baryons are the family of subatomic particles with a baryon number of 1. The term baryon is usually used to refer to triquarks baryons made of three quarks. Exotic baryons made of four quarks and one antiquark are known as the pentaquarks, but… …   Wikipedia

• baryon — baryonic /bar ee on ik/, adj. /bar ee on /, n. Physics. a proton, neutron, or any elementary particle that decays into a set of particles that includes a proton. Cf. quark model. [1950 55; < Gk barý(s) heavy + (FERMI)ON] * * * Any member of one… …   Universalium