In particle physics , a baryon is a type of composite subatomic particle consisting of an odd number of valence quarks (at least 3). [1] Baryons belong to the hadron family of particles ; Hadrons are made of quarks . Baryons are also classified as fermions because they have quasi-integer spin .

The name “baryon”, introduced by Abraham Pais , [2] comes from the Greek word for “heavy” (βαρύς, barys, ) , because at the time of their naming, most known elementary particles had smaller masses than baryons. Each baryon has a corresponding antiparticle (antibaryon), where their corresponding antiquarks take the place of the quarks. For example, a proton is made up of two up quarks and one down quark ; And its corresponding antiparticle, the antiproton , is made up of two up antiquarks and one down antiquark.

Because they are made of quarks, baryons participate in the strong interaction , which is mediated by particles known as gluons . The most familiar baryons are the proton and the neutron , both of which contain three quarks, and for this reason they are sometimes called triquarks . These particles make up most of the mass of visible matter in the universe and make up the nucleus of each atom . ( Electrons , the other major component of the atom, are members of a different family of particles calledleptons ; Leptons do not interact through strong force.) Exotic baryons containing five quarks, called pentaquarks , have also been found and studied.

A census of the universe’s bays indicates that 10% of them can be found inside galaxies, 50 to 60% in the perihelion medium, [3] and the remaining 30 to 40% located in the warm-hot intergalactic medium (WHIM). can be. ,


The baryons are strongly interacting ; That is, they act by the strong nuclear force and are described by the Fermi–Dirac statistics , which apply to all particles obeying the Pauli exclusion principle. This is in contrast to bosons, which do not obey the exclusion principle.

Along with mesons there are baryons, hadrons, particles made of quarks. In quarks B  = . is the baryon number of1/3And in antiquarks B  = − . have baryon numbers1/3, The term ” baryon ” usually refers to triquarks – baryons made of three quarks ( b  = .)1/3 +  1/3 +  1/3 = 1)।

Other exotic baryons have been proposed, such as the pentaquark—a baryon composed of four quarks and one antiquark ( b  =  1/3 +  1/3 +  1/3 +  1/3 –  1/3 = 1), [5] [6] but their existence is not generally accepted. The particle physics community as a whole did not view its existence as likely in 2006, [7] and, in 2008, overwhelmingly considered the evidence against the existence of the reported pentaquark. [8] However, in July 2015, the LHCb experiment observed two resonances corresponding to pentaquark states.0b _→ J / K
P decay, with a combined statistical significance of 15σ.

In theory, heptaquarks (5 quarks, 2 antiquarks), nonquarks (6 quarks, 3 antiquarks), etc. could also exist.

Baryonic matter

Almost all matter encountered or experienced in everyday life is baryonic matter, which includes atoms of any kind, and gives them the property of mass. Non-baryonic matter, as the name implies, is any type of matter that is not composed primarily of baryons. This may include neutrinos and free electrons, dark matter, supersymmetric particles, axons, and black holes.

The existence of baryons is also an important issue in cosmology because it is believed that the Big Bang produced a state with equal amounts of baryons and antibaryons. The process by which baryons came to outnumber them is called anti-particle baryogenesis.


The experiments are consistent with the number of quarks in the universe being constant and, to be more specific, the number of baryons being constant (if antimatter is counted as negative); citation needed ] In technical parlance, the total baryon number appears to be conserved .Within the prevailing Standard Model of particle physics, the number of baryons can change into multiples of three due to the action of sphalerons, although this is rare and not observed under experiment. Some grand unified theories of particle physics also predict that a single proton may decay, changing the baryon number by one; However, this has not yet been observed under experiment. The excess of baryons compared to baryons in the present universe is believed to be due to non-conservation of baryon numbers in the early universe, although this is not well understood.


Isospin and charge

The concept of isospin was first proposed by Werner Heisenberg in 1932 to explain the similarity between protons and neutrons under strong interactions. [11] Although they had different electrical charges, their masses were so similar that physicists believed they were the same particle. The different electric charges were explained as the result of some unknown excitation, similar to spin. This unknown stimulus was later dubbed isospin by Eugene Wigner in 1937. [12]

This belief lasted until Murray Gell-Mann proposed the quark model in 1964 (which originally included only u, d and s quarks). [13] The success of the isospin model is now understood to be the result of the similar mass of U and D quarks. Since u and d quarks have the same mass, the same number of particles also have the same mass. The exact specific U and D quark structure determines the charge, as the U quark charges +2/3While D quarks charge -1/3, For example, all four deltas have different charges (Δ++(uh),Δ+(UUD),Δ0(UDD),Δ
(ddd)), but have the same mass (~1,232 MeV/c 2 ) because they are each made up of a combination of three u or d quarks. Under the isospin model, they were considered to be single particles in different charged states.

The mathematics of isospin was modeled after spin. The isospin projections vary in increments of 1, just like spin, and there is a “charged state” associated with each projection. Since the “delta particle” had four “charged states”, it was called isospin I  = . was told of3/2, its “charged state”Δ+++0, And Δ, corresponding to isospin estimates 3  = +3/2, 3  = +1/2, 3  = −1/2, and 3  = -3/2, respectively. Another example is the “nucleon particle”. Since the two nucleons were “charged states”, it was called isospin.1/2, positive nucleon
Identity of (proton) 3  = + . made from1/2and neutral nucleon
(neutron) 3  = − . with1/2, [14] It was later noted that the isospin projections are related to the quark content above and below the particles:

I_{\mathrm {3} }={\frac {1}{2}}[(n_{\mathrm {u} }-n_{\mathrm {\bar {u}} })-(n_{\mathrm {d} }-n_{\mathrm {\bar {d}} })],

where n is the number of up and down quarks and antiquarks.

In the “isospin picture”, the four deltas and two nuclei were considered as separate states of the two particles. However, in the quark model, deltas are different states of the nucleon (n ++ or n- are prohibited by Pauli’s exclusion principle). Isospin, although conveys a wrong picture of things, is still used to classify baryons, leading to unnatural and often confusing nomenclature.

Flavor quantum number

oddity taste quantum number s(not to be confused with spin) and was seen to go up and down with particle mass. The higher the mass, the lower the strangeness (more quarks). Particles can be described with isospin projections (related to charge) and strangeness (mass) (see uds octet and decuplet figures on the right). As other quarks were discovered, new quantum numbers were created for the same description of the UDC and UDB octet and decouple. Since only u and d masses are the same, this description of particle mass and charge in terms of isospin and flavor quantum numbers works well for octet and decouplet composed of only one u, one d, and one other quark, and for Breaks down into other octaves and decuplets (for example, UCB octaves and decuplets). If all quarks had the same mass, their behavior would be symmetric ., because they all behaved similarly to the strong interaction. Since quarks do not have the same mass, they do not interact equally (just as an electron placed in an electric field will move faster than a proton placed in the same field due to its lighter mass), and is called symmetry. Has to be broken.

It was noted that the charge ( Q ) was related to the isospin projection ( I ) , the baryon number ( B ) and the flavor quantum number ( S , C , B ′, T ) by the Gell–Mann–Nishijima formula :

{\displaystyle Q=I_{3}+{\frac {1}{2}}\left(B+S+C+B^{\prime }+T\right),}

where s , c , b , and t represent strangeness, attractiveness, bottom and top taste quantum numbers, respectively. They are related to the number of strange, attractive, bottom and top quarks and antiquarks according to the relationship:

{\displaystyle {\begin{aligned}S&=-\left(n_{\mathrm {s} }-n_{\mathrm {\bar {s}} }\right),\\C&=+\left(n_{\mathrm {c} }-n_{\mathrm {\bar {c}} }\right),\\B^{\prime }&=-\left(n_{\mathrm {b} }-n_{\mathrm {\bar {b}} }\right),\\T&=+\left(n_{\mathrm {t} }-n_{\mathrm {\bar {t}} }\right),\end{aligned}}}

Which means that the Gell–Mann–Nishijima formula is equivalent to the expression of charge in terms of quark content:

{\displaystyle Q={\frac {2}{3}}\left[(n_{\mathrm {u} }-n_{\mathrm {\bar {u}} })+(n_{\mathrm {c} }-n_{\mathrm {\bar {c}} })+(n_{\mathrm {t} }-n_{\mathrm {\bar {t}} })\right]-{\frac {1}{3}}\left[(n_{\mathrm {d} }-n_{\mathrm {\bar {d}} })+(n_{\mathrm {s} }-n_{\mathrm {\bar {s}} })+(n_{\mathrm {b} }-n_{\mathrm {\bar {b}} })\right].}

Spin, Orbital Angular Momentum, and Total Angular Momentum

Spin (quantum number s ) is a vector quantity that represents the “intrinsic” angular momentum of a particle. it comes in the growth of1/2 H (pronounced “j-bar”). Often dropped because it is the “fundamental” unit of spin, and it is implied that “spin 1” means “spin 1 “. In some systems of natural units, , is chosen to be 1, and therefore does not appear anywhere.

Quarks are particles of fermionic spin1/2( s  =  1/2) because the spin estimates vary in increments of 1 (that is, 1), a single quark has a spin vector of length1/2, and there are two spin projections ( z  = +1/2and Z  = -1/2) two quarks can align their spins, in which case the two spin vectors  combine to form a vector of length S  = 1 and three spin projections ( z  = +1, z  = 0, and z = -1). Huh . If two quarks have unaligned spins, the spin vectors  combine to form a vector of length S = 0 and there is only one spin projection ( z  = 0) etc. Since baryons are made up of three quarks, their spin vectors can add up to the length S  = . draw a vector of3/2, which has four spin projections ( z  = +3/2, Z  = +1/2, Z  = -1/2, and Z  = -3/2), or length S  = . a vector of1/2with two spin projections ( z  = +1/2, and Z  = -1/2) [15]

There is another quantity of angular momentum, called orbital angular momentum (azimuthal quantum number L ), which comes in increments of 1, which represent the angular moment due to quarks orbiting around each other. The total angular momentum (the total angular momentum quantum number j of a particle) is therefore a combination of internal angular momentum (spin) and orbital angular momentum. This J = | ,One can take any value from L – S | to j = | L + S | , in increments of 1.

orbital angular
momentum, L
Total angular
momentum, J ,
Notation , JP
13/2,1/23/2 , 1/2
25/2,3/2+5/2+ , 3/2+
37/2,5/27/2 , 5/2
15/2,3/2,1/25/2 , 3/2 , 1/2
27/2,5/2,3/2,1/2+7/2+ , 5/2+ , 3/2+ , 1/2+
39/2,7/2,5/2,3/29/2 , 7/2 , 5/2 , 3/2

Particle physicists  are most interested in baryons with no orbital angular momentum ( L = 0), because they correspond to ground states—states of minimum energy. Therefore, the two groups of baryons most studied are S  =  1/2, L  = 0 and S  =  3/2, L  = 0, which is J  = . corresponds to1/2+ and j  =  3/2+ , respectively, although they are not alone. It is also possible to obtain J  =  obtain3/2S to Particle =  1/2and l  = 2, as well as s  =  3/2and L  = 2. This phenomenon of having many particles in the same total angular momentum configuration is called degeneration . How to distinguish between these degenerate baryons is an active area of ​​research in baryon spectroscopy. [16] [17]


If the universe were reflected in a mirror, most of the laws of physics would be the same – things would behave the same way, regardless of what we call “left” and what we call “right”. This concept of mirror reflection is called “internal parity” or simply “parity” ( p ). Gravity, the electromagnetic force, and the strong interaction all behave the same way, regardless of whether the universe is reflected in a mirror or not, and thus are said to preserve equivalence (p-symmetry). However, the weak interaction separates the “left” from the “right”, a phenomenon known as parity violation (p-violation).

Based on this, if the wavefunctions for each particle (in more precise terms, the quantum field for each particle type) are simultaneously mirror-reversed, then the new set of wavefunctions would completely satisfy the laws of physics (with respect to the weak interaction). in addition). It turns out that this is not quite true: in order to satisfy the equations, the wavefunctions of certain types of particles have to be multiplied by -1 in addition to being mirror-reversed. Such particle types are called negative or odd parity ( P  = -1, or alternatively P  = -), while other particles are called positive or even parity ( P  = +1, or alternatively P  = +). Is.

For baryons, parity is related to orbital angular momentum:


As a result, baryons with no orbital angular momentum ( L  = 0) all have parity ( P  = +).


Baryons are classified into groups according to their isospin ( I ) values ​​and quark ( Q ) content. There are six groups of baryons: nucleons (No), delta (Δ), lambda (Λ), sigma (Σ), She (X), and omega (Ω) The rules of classification are defined by the particle data set. These rules consider the above (You), below (d) and weird (s) quarks to be light and attraction (C), below (b), and top (So) quarks being heavy . The rules cover all particles that can be created from three of each of the six quarks, even though baryons made from top quarks are not expected to exist due to the short lifetimes of the top quarks. The rules don’t cover pentaquarks. [19]

  • With three (any combination) baryon tum and/or d quarks there are no s ( I =1/2) or Eberians ( I =3/2)
  • Two . The baryon containing tum and/or d quarks are barion ( I = 0) or baryon ( I = 1). If the third quark is heavier, it is identified by a subscript.
  • a . The baryons containing tum or d quarks are baryons ( I =1/2) If one or both quarks are heavy then one or two subscripts are used.
  • The numbered baryons are tum or d quarks barians ( I = 0), and the subscripts indicate any heavy quark content.
  • Baryons that decay have their mass as part of their name. For example, 0 does not decay strongly, but ++ (1232) does.

It is a widespread (but not universal) practice to follow some additional rules when distinguishing between states that would otherwise have the same symbol. [14]

  • Baryon J in total angular momentum  =  3/2Configuration in which their J  = . have similar symbols1/2Equivalents are denoted by asterisks ( * ).
  • J  = . Two baryons can be made from three different quarks in1/2layout. In this case, a prime() is used to differentiate between them.
    • Exception : When two of the three quarks are an up and a down quark, one is called a baryon and the other is called a baryon.

Quarks have a charge, so knowing the charge of a particle indirectly gives the amount of the quark. For example, the above rule says that aΛ+c It consists of an ac quark and some combination of two u and/or d quarks. c is the charge of the quark ( Q  = +2/3), so there must be two more quarks ( Q  = +2/3), and Ed Quark ( Q  = −1/3) to be the true total charge ( Q  = +1).

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