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In particle physics, a baryon is a type of composite subatomic particle that contains an odd number of , conventionally three. proton and neutron are examples of baryons; because baryons are composed of , they belong to the hadron family of particles. Baryons are also classified as because they have half-integer spin.
The name "baryon", introduced by Abraham Pais, comes from the Ancient Greek word for "heavy" (βαρύς, barýs), because, at the time of their naming, most known elementary particles had lower masses than the baryons. Each baryon has a corresponding antiparticle (antibaryon) where their corresponding antiquarks replace quarks. For example, a proton is made of two and one down quark; and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.
Baryons participate in the residual strong force, which is force carrier by particles known as . The most familiar baryons are and , both of which contain three quarks, and for this reason they are sometimes called triquarks. These particles make up most of the mass of the visible matter in the universe and compose the atomic nucleus of every atom (, the other major component of the atom, are members of a different family of particles called ; leptons do not interact via the strong force). containing five quarks, called , have also been discovered and studied.
A census of the Universe's baryons indicates that 10% of them could be found inside galaxies, 50 to 60% in the , and the remaining 30 to 40% could be located in the warm–hot intergalactic medium (WHIM).
Baryons, alongside , are , composite particles composed of . Quarks have of B = and antiquarks have baryon numbers of B = −. The term "baryon" usually refers to triquarks—baryons made of three quarks ( B = + + = 1).
Other have been proposed, such as —baryons made of four quarks and one antiquark ( B = + + + − = 1),H. Muir (2003)K. Carter (2003) but their existence is not generally accepted. The particle physics community as a whole did not view their existence as likely in 2006,W.-M. Yao et al. (2006): Particle listings – Θ+ and in 2008, considered evidence to be overwhelmingly against the existence of the reported pentaquarks.C. Amsler et al. (2008): [http://pdg.lbl.gov/2008/reviews/pentaquarks_b801.pdf Pentaquarks However, in July 2015, the LHCb experiment observed two resonances consistent with pentaquark states in the Λ → J/ψKp decay, with a combined statistical significance of 15σ.
In theory, heptaquarks (5 quarks, 2 antiquarks), nonaquarks (6 quarks, 3 antiquarks), etc. could also exist.
The very existence of baryons is also a significant issue in cosmology because it is assumed that the Big Bang produced a state with equal amounts of baryons and antibaryons. The process by which baryons came to outnumber their is called baryogenesis.
The concept of isospin was first proposed by Werner Heisenberg in 1932 to explain the similarities between protons and neutrons under the strong interaction.W. Heisenberg (1932) Although they had different electric charges, their masses were so similar that physicists believed they were the same particle. The different electric charges were explained as being the result of some unknown excitation similar to spin. This unknown excitation was later dubbed isospin by Eugene Wigner in 1937.E. Wigner (1937)
This belief lasted until Murray Gell-Mann proposed the quark model in 1964 (containing originally only the u, d, and s quarks).M. Gell-Mann (1964) The success of the isospin model is now understood to be the result of the similar masses of u and d quarks. Since u and d quarks have similar masses, particles made of the same number then also have similar masses. The exact specific u and d quark composition determines the charge, as u quarks carry charge + while d quarks carry charge −. For example, the four Delta baryon all have different charges ( (uuu), (uud), (udd), (ddd)), but have similar masses (~1,232 MeV/c2) as they are each made of a combination of three u or d quarks. Under the isospin model, they were considered to be a single particle in different charged states.
The mathematics of isospin was modeled after that of spin. Isospin projections varied in increments of 1 just like those of spin, and to each projection was associated a "Quantum state". Since the "Delta baryon" had four "charged states", it was said to be of isospin I = . Its "charged states" , , , and , corresponded to the isospin projections I3 = +, I3 = +, I3 = −, and I3 = −, respectively. Another example is the "nucleon particle". As there were two nucleon "charged states", it was said to be of isospin . The positive nucleon (proton) was identified with I3 = + and the neutral nucleon (neutron) with I3 = −.S.S.M. Wong (1998a) It was later noted that the isospin projections were related to the up and down quark content of particles by the relation:
In the "isospin picture", the four Deltas and the two nucleons were thought to be the different states of two particles. However, in the quark model, Deltas are different states of nucleons (the N++ or N− are forbidden by Pauli's exclusion principle). Isospin, although conveying an inaccurate picture of things, is still used to classify baryons, leading to unnatural and often confusing nomenclature.
It was noted that charge ( Q) was related to the isospin projection ( I3), the baryon number ( B) and flavour quantum numbers ( S, C, B′, T) by the Gell-Mann–Nishijima formula:
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{align}
meaning that the Gell-Mann–Nishijima formula is equivalent to the expression of charge in terms of quark content:
are particles of spin Because spin projections vary in increments of (that is a single quark has a spin vector of and has two spin projections and Two quarks can have their spins aligned, in which case the two spin vectors add to make a vector of length and three spin projections and If two quarks have unaligned spins, the spin vectors add up to make a vector of length and has only one spin projection etc. Since baryons are made of three quarks, their spin vectors can add to make a vector of length which has four spin projections and or a vector of length with two spin projections and
There is another quantity of angular momentum, called the orbital angular momentum (azimuthal quantum number ), that comes in increments of which represent the angular moment due to quarks orbiting around each other. The total angular momentum (total angular momentum quantum number of a particle is therefore the combination of intrinsic angular momentum (spin) and orbital angular momentum. It can take any value from in increments
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| + | |
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| +, + | |
| −, − | |
| + | |
| −, −, − | |
| +, +, +, + | |
| −, −, −, − |
Particle physicists are most interested in baryons with no orbital angular momentum ( L = 0), as they correspond to —states of minimal energy. Therefore, the two groups of baryons most studied are the S = ; L = 0 and S = ; L = 0, which corresponds to J = + and J = +, respectively, although they are not the only ones. It is also possible to obtain J = + particles from S = and L = 2, as well as S = and L = 2. This phenomenon of having multiple particles in the same total angular momentum configuration is called degeneracy. How to distinguish between these degenerate baryons is an active area of research in baryon spectroscopy.H. Garcilazo et al. (2007)D.M. Manley (2005)
Based on this, if the wavefunction for each particle (in more precise terms, the quantum field for each particle type) were simultaneously mirror-reversed, then the new set of wavefunctions would perfectly satisfy the laws of physics (apart from the weak interaction). It turns out that this is not quite true: for the equations to be satisfied, the wavefunctions of certain types of particles have to be multiplied by −1, in addition to being mirror-reversed. Such particle types are said to have negative or odd parity ( P = −1, or alternatively P = –), while the other particles are said to have positive or even parity ( P = +1, or alternatively P = +).
For baryons, the parity is related to the orbital angular momentum by the relation:S.S.M. Wong (1998b)
As a consequence, baryons with no orbital angular momentum ( L = 0) all have even parity ( P = +).
It is also a widespread (but not universal) practice to follow some additional rules when distinguishing between some states that would otherwise have the same symbol.
Quarks carry a charge, so knowing the charge of a particle indirectly gives the quark content. For example, the rules above say that a contains a c quark and some combination of two u and/or d quarks. The c quark has a charge of ( Q = +), therefore the other two must be a u quark ( Q = +), and a d quark ( Q = −) to have the correct total charge ( Q = +1).
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