Liste der Baryonen
- This list is of all known and predicted baryons. See list of particles for a more detailed list of particles found in particle physics.
Baryons are made of quarks and as such are part of the subatomic particle family called the hadrons. Baryons are the sub-family of hadrons with a baryon number of 1, as opposed to the mesons which are the sub-family of hadrons with a baryon number of 0. Since baryons are composed of quarks they participate in the strong interaction, in contrast to leptons which are not composed of quarks and as such do not participate in the strong interaction. The protons and neutrons that make up most of the mass of the visible matter in the universe are both baryons, whereas electrons (the other major component of atoms) are leptons.
Traditionally, baryons were believed to be composed of only three quarks (triquarks) (quarks have a baryon number of Vorlage:Frac and antiquarks have a baryon number of −Vorlage:Frac). Recently, physicists have reported the existence of pentaquarks – "exotic" baryons made of four quarks and one antiquark – but their existence is not generally accepted within the particle physics community.[1][2] Each baryon has a corresponding antiparticle (antibaryon) where quarks are replaced by their corresponding antiquarks and vice versa. For example, a proton is made of two up quarks and one down quark; thus, the antiproton is made of two up antiquarks and one down antiquark.
Overview
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 increments of Vorlage:Frac ℏ (pronounced "h-bar"). The ℏ is often dropped because it is the "fundamental" unit of spin, and it is implied that "spin 1" means "spin 1 ℏ". In some systems of natural units, ℏ is chosen to be 1, therefore does not appear anywhere.
Quarks are fermionic particles of spin Vorlage:Frac (S = Vorlage:Frac). Because spin projections varies in increments of 1 (that is 1 ℏ), a single quark has a spin vector of length Vorlage:Frac, and has two spin projections (Sz = +Vorlage:Frac and Sz = −Vorlage:Frac). Two quarks can have their spins aligned, in which case the two spin vectors add to make a vector of length S = 1 and three spin projections (Sz = +1, Sz = 0, and Sz = −1). If two quarks have unaligned spins, the spin vectors add up to make a vector of length S = 0 and has only one spin projection (Sz = 0), etc. Since baryons are made of three quarks, their spin vectors can add to make a vector of length S = Vorlage:Frac which has four spin projections (Sz = +Vorlage:Frac, Sz = +Vorlage:Frac, Sz = −Vorlage:Frac, and Sz = −Vorlage:Frac), or a vector of length S = Vorlage:Frac with two spin projections (Sz = +Vorlage:Frac, and Sz = −Vorlage:Frac).[3]
There is another quantity of angular momentum, called the orbital angular momentum (quantum number L), that comes in increments of 1 ℏ, which represent the angular moment of due to particles orbiting around each other. The total angular momentum (quantum number J) of a particle is therefore the combination of intrinsic angular momentum (spin) and orbital angular momentum (J = S + L).[3]
Particles physicists are most interested in baryons with no orbital angular momentum (L = 0), therefore the two groups of baryons most studied are the S = Vorlage:Frac; L = 0 and S = Vorlage:Frac; L = 0, which corresponds to J = Vorlage:Frac and J = Vorlage:Frac, although they are not the only ones. It is also possible to obtain J = Vorlage:Frac particles from S = Vorlage:Frac and L = 1. How to distinguish between the S = Vorlage:Frac, L = 0 and S = Vorlage:Frac, L = 1 baryons is an active area of research in baryon spectroscopy.[4][5]
Parity
Parity refers to whether the wavefunction of a particle is even or odd. A positive parity (P = +) means that the wavefunction is even, while a negative (P = −) means the wavefunction is odd.[3]
- is an odd 1-dimensional wavefunction because
- is an even 1-dimensional wavefunction because
For baryons, the parity is related to the orbital angular momentum by the relation:[6]
Physicists are often particularly interested in baryons with no orbital angular momentum (L = 0), which are of even parity (P = +).
Isospin and charge
The concept of isospin was first proposed by Werner Heisenberg in 1932 to explain the similarities between protons and neutrons under the strong interaction.[7] Although they had different electric charges, their masses were so similar that physicists believed they were actually 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.[8]
This belief lasted until Murray Gell-Mann proposed the quark model in 1964 (containing originally only the u, d, and s quarks).Vorlage:Fact The success of the isospin model is now understood to be the result of the similar masses of the u and d quarks. Since the u and d quarks have similar masses, particles made of the same number them also have similar masses. The exact specific u and d quark composition determines the charge, as u quarks carry charge +Vorlage:Frac while d quarks carry charge −Vorlage:Frac. For example the four Deltas all have different charges (Vorlage:SubatomicParticle (uuu), Vorlage:SubatomicParticle (uud), Vorlage:SubatomicParticle (udd), Vorlage:SubatomicParticle (ddd)), but have similar masses (~1,232 MeV/c2) as they are each made of a total of three u and 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 "chared state". Since the "Delta particle" had four "charged states", it was said to be of isospin I = Vorlage:Frac. Its "charged states" Vorlage:SubatomicParticle, Vorlage:SubatomicParticle, Vorlage:SubatomicParticle, and Vorlage:SubatomicParticle, corresponded to the isospin projections Iz = +Vorlage:Frac, Iz = +Vorlage:Frac, Iz = −Vorlage:Frac, and Iz = −Vorlage:Frac respectively. Another example is the "nucleon particle". As there were two nucleon "charged states", it was said to be of isospin Vorlage:Frac. The positive nucleon Vorlage:SubatomicParticle (proton) was identified with Iz = +Vorlage:Frac and the neutral nucleon Vorlage:SubatomicParticle (neutron) with Iz = −Vorlage:Frac.[9] It was later noted that the isospin projections were related to the up and down quark content of particles by the relation:
![{\displaystyle I_{z}={\frac {1}{2}}[(n_{u}-n_{\bar {u}})-(n_{d}-n_{\bar {d}})],}](/media/api/rest_v1/media/math/render/svg/3bf92f196e22405396907ef7c55d70c7ebc91183)
where the n's are the number of up and down quarks and antiquarks.
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.
Flavour quantum numbers
The strangeness flavour quantum number S (not to be confused with spin) was noticed to go up and down along with particle mass. The higher the mass, the lower the strangeness (the more s quarks). Particles could be described with isospin projections (related to charge) and strangeness (mass) (see the uds octet and decuplet figures on the right). As other quarks where discovered, new quantum numbers were made to have similar description of udc and udb octets and decuplets. Since only the u and d mass are similar, this description of particle mass and charge in terms of isospin and flavour quantum numbers only works well for octet and decuplet made of one u, one d and one other quark and breaks down for the other octets and decuplets (for example ucb octet and decuplet). If the quarks all had the same mass, their behaviour would be called symmetric, as they would all behave in exactly the same way with respect to the strong interaction. Since quarks do not have the same mass, they do not interact in the same way (exactly like an electron placed in an electric field will accelerate more than a proton placed in the same field because of its lighter mass), and the symmetry is said to be broken.
It was noted that charge (Q) was related to the isospin projection (Iz), the baryon number (B) and flavour quantum numbers (S, C, B′, T) by the Gell-Mann–Nishijima formula:[9]
where S, C, B′, and T represent the strangeness, charmness, bottomness and topness flavour quantum numbers respectively. They are related to the number of strange, charm, bottom, and top quarks and antiquark according to the relations:
meaning that the Gell-Man–Nishijima formula is equivalent to the expression of charge in terms of quark content:
![{\displaystyle Q={\frac {2}{3}}[(n_{u}-n_{\bar {u}})+(n_{c}-n_{\bar {c}})+(n_{t}-n_{\bar {t}})]-{\frac {1}{3}}[(n_{d}-n_{\bar {d}})+(n_{s}-n_{\bar {s}})+(n_{b}-n_{\bar {b}})].}](/media/api/rest_v1/media/math/render/svg/2819b98f91f3a602eb120eb3e8d5575661f1150e)
Particle classification
Baryons are classified into groups according to their isospin (I) values and quark (q) content. There are six groups of triquarks – nucleon (Vorlage:SubatomicParticle), Delta (Vorlage:SubatomicParticle), Lambda (Vorlage:SubatomicParticle), Sigma (Vorlage:SubatomicParticle), Xi (Vorlage:SubatomicParticle), and Omega (Vorlage:SubatomicParticle). The rules for classification are defined by the Particle Data Group. These rules consider the up (Vorlage:SubatomicParticle), down (Vorlage:SubatomicParticle) and strange (Vorlage:SubatomicParticle) quarks to be light and the charm (Vorlage:SubatomicParticle), bottom (Vorlage:SubatomicParticle), and top (Vorlage:SubatomicParticle) to be heavy. The rules cover all the particles that can be made from three of each of the six quarks, even though baryons made of t quarks are not expected to exist because of the t quark's short lifetime. The rules do not cover pentaquarks.[10]
- Baryons with three Vorlage:SubatomicParticle and/or Vorlage:SubatomicParticle quarks are Vorlage:SubatomicParticle's (I = Vorlage:Frac) or Vorlage:SubatomicParticle's (I = Vorlage:Frac).
- Baryons with two Vorlage:SubatomicParticle and/or Vorlage:SubatomicParticle quarks are Vorlage:SubatomicParticle's (I = 0) or Vorlage:SubatomicParticle's (I = 1). If the third quark is heavy, its identity is given by a subscript.
- Baryons with one Vorlage:SubatomicParticle or Vorlage:SubatomicParticle quark are Vorlage:SubatomicParticle's (I = Vorlage:Frac). One or two subscripts are used if one or both of the remaining quarks are heavy.
- Baryons with no Vorlage:SubatomicParticle or Vorlage:SubatomicParticle quarks are Vorlage:SubatomicParticle's (I = 0), and subscripts indicate any heavy quark content.
- Baryons that decay strongly have their masses as part of their names. For example, Σ0 does not decay strongly, but Δ++(1232) does.
It is also a widespread (but not universal) practice to follow some additional rules when distinguishing between some states which would otherwise have the same symbol.[9]
- Baryons in total angular momentum J = Vorlage:Frac configuration which have the same symbols as their J = Vorlage:Frac counterparts are denoted by an asterisk ( * ).
- Two baryons can be made of three different quarks in J = Vorlage:Frac configuration. In this case, a prime ( ′ ) is used to distinguish between them.
- Exception: When two of the three quarks are one up and one down quark, one baryon is dubbed Λ while the other is dubbed Σ.
Quarks carry charge, so knowing the charge of a particle indirectly gives the quark content. For example, the rules above say that a Vorlage:SubatomicParticle contains a c quark and some combination of two u and/or d quarks. The c quark as a charge of (Q = +Vorlage:Frac), therefore the other two must be a u quark (Q = +Vorlage:Frac), and a d quark (Q = −Vorlage:Frac) to have the correct total charge (Q = 1).
Lists of baryons
These lists detail all known and predicted triquark baryons in total angular momentum J = Vorlage:Frac and J = Vorlage:Frac configurations with positive parity, as well as all the reported pentaquark baryons.
The symbols encountered in these lists are: I (isospin), J (total angular momentum), P (parity), u (up quark), d (down quark), s (strange quark), c (charm quark), b (bottom), Q (charge), B (baryon number), S (strangeness), C (charmness), B′ (bottomness), as well as a wide array of subatomic particles (hover for name).
Antiparticles are not listed in the tables; however, they simply would have all quarks changed to antiquarks (and antiquarks changed to quarks), and Q, B, S, C, B′, would be of opposite signs. Particles with † next to their names have been predicted by the standard model but not yet observed. I, J, and P values marked with *'s have not been firmly established by experiments, but are predicted by the quark model and are consistent with the measurements.[11][12]
JP = Vorlage:Frac+ baryons (triquarks)
[a] Vorlage:Note The masses of the proton and neutron are known with much better precision in atomic mass units (u) than in Electron volt/c2, due to the relatively poorly known value of the elementary charge. In atomic mass unit, the mass of the proton is 1.007 276 466 88(13) u while that of the neutron is 1.008 664 915 60(55) u.
[b] Vorlage:Note At least 1035 years. See proton decay.
[c] Vorlage:Note For free neutrons; in most common nuclei, neutrons are stable.
[d] Vorlage:Note The specific values of the name has not been decided yet, but will likely be close to Vorlage:SubatomicParticle(5810).
[e] Vorlage:Note Some controversy exists about this data.[26]
[f] Vorlage:Note This is actually a measurement of the average lifetime of b-baryons that decay to a jet containing a same sign Vorlage:SubatomicParticle∓Vorlage:SubatomicParticle∓ pair. Presumably the mix is mainly Vorlage:SubatomicParticle, with some Vorlage:SubatomicParticle.
JP = Vorlage:Frac+ baryons (triquarks)
Exotic baryons (pentaquarks)
This lists details pentaquarks reported to exist. However, other groups have looked for them and reported to have found nothing. Data is controversial to the point that the existence of pentaquarks is not generally accepted.
| Particle name | Symbol | Quark content |
Rest mass (MeV/c2) | I | JP | Q (e) | S | C | B' | Mean lifetime (s) | Commonly decays to |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Theta[39] | Vorlage:SubatomicParticle (1540) | Vorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticle | 1,533.6 ± 2.4 | 0 | Unknown | +1 | +1 | 0 | 0 | Unknown | Vorlage:SubatomicParticle + Vorlage:SubatomicParticle or Vorlage:SubatomicParticle + Vorlage:SubatomicParticle |
| charmed Theta[40] | Vorlage:SubatomicParticle (3100) | Vorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticle | 3,099 ± 8 | 0 | Unknown | 0 | 0 | −1 | 0 | Unknown | Unknown |
| Phi[41] | Vorlage:SubatomicParticle (1860) | Vorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticleVorlage:SubatomicParticle | 1,862 ± 2 | Vorlage:Frac | Unknown | 0 | −2 | 0 | 0 | Unknown | Unknown |
See also
References
References
- H. Garcilazo, J. Vijande, A. Valcarce: Faddeev study of heavy-baryon spectroscopy. In: Journal of Physics G: Nuclear and Particle Physics. 34. Jahrgang, Nr. 5, 2007, S. 961–976, doi:10.1088/0954-3899/34/5/014.
- V. M. Abazov et al.: Direct observation of the strange b baryon Xi(b)−. In: Physical Review Letters. 99. Jahrgang, 2007, arxiv:0706.1690, S. 052001, doi:10.1103/PhysRevLett.99.052001.
- T. Aaltonen et al.: First Observation of Heavy Baryons Vorlage:SubatomicParticle and Vorlage:SubatomicParticle. In: Physical Review Letters. 99. Jahrgang. CDF Collaboration, 2007, arxiv:0706.3868, S. 202001, doi:10.1103/PhysRevLett.99.202001.
- T. Aaltonen et al.: Observation and mass measurement of the baryon Vorlage:SubatomicParticle. In: Physical Review Letters. 99. Jahrgang, 2007, arxiv:0707.0589, S. 052002, doi:10.1103/PhysRevLett.99.052002.
- W.-M. Yao et al.: Review of Particle Physics. In: Journal of Physics G: Nuclear and Particle Physics. 33. Jahrgang. Particle Data Group, 2006, S. 1–1232, doi:10.1088/0954-3899/33/1/001.
- K. Carter: The rise and fall of the pentaquark. Fermi National Accelerator Laboratory and Stanford Linear Accelerator Center, 2006, abgerufen am 27. Mai 2008.
- S.Robbins: Physics Particle Overview – Baryons. Journey Through the Galaxy, 2006, abgerufen am 20. April 2008.
- D. M. Manley: Status of baryon spectroscopy. In: Journal of Physics: Conference Series. 5. Jahrgang, 2005, S. 230–237, doi:10.1088/1742-6596/9/1/043.
- H. Muir: Pentaquark discovery confounds sceptics. New Scientist, 2003, abgerufen am 27. Mai 2008.
- S. S. M. Wong: Introductory Nuclear Physics. 2nd edition Auflage. John Wiley & Sons, New York (NY) 1998, ISBN 0-471-23973-9, Chapter 3 – The Deuteron, S. 57–104.
- S. S. M. Wong: Introductory Nuclear Physics. 2nd edition Auflage. John Wiley & Sons, New York (NY) 1998, ISBN 0-471-23973-9, Chapter 2 – Nucleon Structure, S. 21–56.
- R. Shankar: Principles of Quantum Mechanics. 2nd edition Auflage. Plenum Press, New York (NY) 1994, ISBN 0-306-44790-8.
- J. G. Körner, M. Krämer, and D. Pirjol: Heavy Baryons. In: Progress in Particle and Nuclear Physics. 33. Jahrgang, 1994, arxiv:hep-ph/9406359, S. 787–868, doi:10.1016/0146-6410(94)90053-1.
- E. Wigner: On the Consequences of the Symmetry of the Nuclear Hamiltonian on the Spectroscopy of Nuclei. In: Physical Review. 51. Jahrgang, Nr. 2, 1937, S. 106–119.
- W. Heisenberg: Über den Bau der Atomkerne I. In: Zeitschrift für Physik. 77. Jahrgang, 1932, S. 1–11.
- W. Heisenberg: Über den Bau der Atomkerne II. In: Zeitschrift für Physik. 78. Jahrgang, 1932, S. 156–164.
- W. Heisenberg: Über den Bau der Atomkerne III. In: Zeitschrift für Physik. 80. Jahrgang, 1932, S. 587–596.
Further reading
- Particle Data Group – Review of Particle Physics (2006) and 2007 partial update for 2008.
- Particle Data Group – Complete baryon info
- Georgia State University – HyperPhysics
- ↑ H. Muir (2003)
- ↑ K. Carter (2003)
- ↑ a b c R. Shankar (1994)
- ↑ H. Garcilazo et. al. (2007)
- ↑ D. M. Manley (2009)
- ↑ S. S. M. Wong (1998a)
- ↑ "Über den Bau der Atomkerne" (Zeitschrift für Physik 77: 1-11)
- ↑ Physical Review 51: 106-119
- ↑ a b c S. S. M. Wong (1998b)
- ↑ W.-M. Yao et al. (2006): Naming scheme for hadrons
- ↑ W.-M. Yao et al. (2006): Particle summary tables - Baryon
- ↑ J. G. Körner et al. (1994)
- ↑ W.-M. Yao et al. (2006): Particle listings – Proton
- ↑ W.-M. Yao et al. (2006): Particle listings – Neutron
- ↑ W.-M. Yao et al. (2006): Particle listings – Lambda
- ↑ W.-M. Yao et al. (2006): Particle listings – Charmed Lambda
- ↑ W.-M. Yao et al. (2006): Particle listings – Bottom Lambda
- ↑ W.-M. Yao et al. (2006): Particle listings – Positive Sigma
- ↑ W.-M. Yao et al. (2006): Particle listings – Neutral Sigma
- ↑ W.-M. Yao et al. (2006): Particle listings – Negative Sigma
- ↑ a b c W.-M. Yao et al. (2006): Particle listings – Charmed Sigma(2455)
- ↑ a b T. Aaltonen et al. (2007a)
- ↑ W.-M. Yao et al. (2006): Particle listings – Neutral Xi
- ↑ W.-M. Yao et al. (2006): Particle listings – Negative Xi
- ↑ a b c d W.-M. Yao et al. (2006): Particle listings – Charmed baryons
- ↑ a b W.-M. Yao et al. (2006): Particle listings – Double charmed positive Xi
- ↑ a b W.-M. Yao et al. (2006): Particle listings – Bottom Xis
- ↑ Abazov et al. (2007)
- ↑ T. Aaltonen et al. (2007b)
- ↑ W.-M. Yao et al. (2006): Particle listings – Charmed Omega
- ↑ a b c d W.-M. Yao et al. (2006): Particle listings – Delta(1232)
- ↑ a b c d S. J. Robbins (2006)
- ↑ a b c W.-M. Yao et al. (2006): Particle listings – Sigma(1385)
- ↑ a b c W.-M. Yao et al. (2006): Particle listings – Charmed Sigma(2520)
- ↑ a b W.-M. Yao et al. (2006): Particle listings – Xi(1530)
- ↑ a b W.-M. Yao et al. (2006): Particle listings – Charmed Xi(2645)
- ↑ W.-M. Yao et al. (2006): Particle listings – Negative Omega
- ↑ W.-M. Yao et al. (2006): Particle listings – Neutral Charmed Omega(2770)
- ↑ W.-M. Yao et al. (2006): Particle listings – Positive Theta
- ↑ W.-M. Yao et al. (2006): Particle listings – Charmed Theta
- ↑ W.-M. Yao et al. (2006): Particle listings – Phi(1860)