# Physics:Lambda baryon

Short description: Baryon made of specific quark combinations
Statistics Fermionic Strong, weak, electromagnetic, and gravity 3 ​1⁄2 0

The lambda baryons (Λ) are a family of subatomic hadron particles containing one up quark, one down quark, and a third quark from a higher flavour generation, in a combination where the quantum wave function changes sign upon the flavour of any two quarks being swapped (thus slightly different from a neutral sigma baryon, Σ0). They are thus baryons, with total isospin of 0, and have either neutral electric charge or the elementary charge +1.

## Overview

Quark structure of the lambda baryon.

The lambda baryon Λ0 was first discovered in October 1950, by V. D. Hopper and S. Biswas of the University of Melbourne, as a neutral V particle with a proton as a decay product, thus correctly distinguishing it as a baryon, rather than a meson,[1] i.e. different in kind from the K meson discovered in 1947 by Rochester and Butler;[2] they were produced by cosmic rays and detected in photographic emulsions flown in a balloon at 70,000 feet (21,000 m).[3] Though the particle was expected to live for ~10−23 s,[4] it actually survived for ~10−10 s.[5] The property that caused it to live so long was dubbed strangeness and led to the discovery of the strange quark.[4] Furthermore, these discoveries led to a principle known as the conservation of strangeness, wherein lightweight particles do not decay as quickly if they exhibit strangeness (because non-weak methods of particle decay must preserve the strangeness of the decaying baryon).[4] The Λ0 with its uds quark decays via weak force to a nucleon and a pion − either Λ → p + π or Λ → n + π0.

In 1974 and 1975, an international team at the Fermilab that included scientists from Fermilab and seven European laboratories under the leadership of Eric Burhop carried out a search for a new particle, the existence of which Burhop had predicted in 1963. He had suggested that neutrino interactions could create short-lived (perhaps as low as 10−14 s) particles that could be detected with the use of nuclear emulsion. Experiment E247 at Fermilab successfully detected particles with a lifetime of the order of 10−13 s. A follow-up experiment WA17 with the SPS confirmed the existence of the Λ+c (charmed lambda baryon), with a flight time of (7.3±0.1)×10−13 s.[6][7]

In 2011, the international team at JLab used high-resolution spectrometer measurements of the reaction H(e, e′K+)X at small Q2 (E-05-009) to extract the pole position in the complex-energy plane (primary signature of a resonance) for the Λ(1520) with mass = 1518.8 MeV and width = 17.2 MeV which seem to be smaller than their Breit–Wigner values.[8] This was the first determination of the pole position for a hyperon.

The lambda baryon has also been observed in atomic nuclei called hypernuclei. These nuclei contain the same number of protons and neutrons as a known nucleus, but also contains one or in rare cases two lambda particles.[9] In such a scenario, the lambda slides into the center of the nucleus (it is not a proton or a neutron, and thus is not affected by the Pauli exclusion principle), and it binds the nucleus more tightly together due to its interaction via the strong force. In a lithium isotope (7ΛLi), it made the nucleus 19% smaller.[10]

## Types of lambda baryons

Lambda baryons are usually represented by the symbols Λ0, Λ+c, Λ0b, and Λ+t. In this notation, the superscript character indicates whether the particle is electrically neutral (0) or carries a positive charge (+). The subscript character, or its absence, indicates whether the third quark is a strange quark (Λ0) (no subscript), a charm quark (Λ+c), a bottom quark (Λ0b), or a top quark (Λ+t). Physicists expect to not observe a lambda baryon with a top quark, because the Standard Model of particle physics predicts that the mean lifetime of top quarks is roughly 5×10−25 seconds;[11] that is about 1/20 of the mean timescale for strong interactions, which indicates that the top quark would decay before a lambda baryon could form a hadron.

The symbols encountered in this list are: I (isospin), J (total angular momentum quantum number), P (parity), Q (charge), S (strangeness), C (charmness), B′ (bottomness), T (topness), u (up quark), d (down quark), s (strange quark), c (charm quark), b (bottom quark), t (top quark), as well as other subatomic particles.

Antiparticles are not listed in the table; however, they simply would have all quarks changed to antiquarks, and Q, B, S, C, B′, T, would be of opposite signs. I, J, and P values in red have not been firmly established by experiments, but are predicted by the quark model and are consistent with the measurements.[12][13] The top lambda (Λ+t) is listed for comparison, but is expected to never be observed, because top quarks decay before they have time to form hadrons.[14]

Lambda baryons
Particle name Symbol Quark
content
Rest mass (MeV/c²) I JP Q (e) S C B′ T Mean lifetime (s) Commonly decays to
Lambda[5] Λ0 Up quarkDown quarkStrange quark 1115.683±0.006 0 1/2+ 0 −1 0 0 0 (2.631±0.020)×10−10 Proton+ + Pion- or
Neutron0 + Pion0
charmed lambda[15] Λ+c Up quarkDown quarkCharm quark 2286.46±0.14 0 1/2+ +1 0 +1 0 0 (2.00±0.06)×10−13 decay modes[16]
bottom lambda[17] Λ0b Up quarkDown quarkb 5620.2±1.6 0 1/2+ 0 0 0 −1 0 1.409+0.055
−0.054
×10−12
Decay modes[18]
top lambda Λ+t Up quarkDown quarkTop quark 0 1/2+ +1 0 0 0 +1

^ Particle unobserved, because the top-quark decays before it has sufficient time to bind into a hadron ("hadronizes").

The following table compares the nearly-identical Lambda and neutral Sigma baryons:

Neutral strange baryons
Particle name Symbol Quark
content
Rest mass (MeV/c²) I JP Q (e) S C B′ T Mean lifetime (s) Commonly decays to
Lambda[5] Λ0 Up quarkDown quarkStrange quark 1115.683±0.006 0 1/2+ 0 −1 0 0 0 (2.631±0.020)×10−10 Proton+ + Pion- or
Neutron0 + Pion0
Sigma[19] Σ0 Up quarkDown quarkStrange quark 1,192.642 ± 0.024 1 1/2+ 0 −1 0 0 0 7.4 ± 0.7 × 10−20 Lambda0 + Photon (100%)

## References

1. Hopper, V.D.; Biswas, S. (1950). "Evidence Concerning the Existence of the New Unstable Elementary Neutral Particle". Phys. Rev. 80 (6): 1099. doi:10.1103/physrev.80.1099. Bibcode1950PhRv...80.1099H.
2. Rochester, G. D.; Butler, C. C. (1947). "Evidence for the Existence of New Unstable Elementary Particles". Nature 160 (4077): 855–7. doi:10.1038/160855a0. PMID 18917296. Bibcode1947Natur.160..855R.
3. Pais, Abraham (1986). Inward Bound. Oxford University Press. pp. 21, 511–517. ISBN 978-0-19-851971-3.
4. The Strange Quark
5. Amsler, C. (2008). "Λ". Lawrence Berkeley Laboratory.
6. Massey, Harrie; Davis, D. H. (November 1981). "Eric Henry Stoneley Burhop 31 January 1911 – 22 January 1980". Biographical Memoirs of Fellows of the Royal Society 27: 131–152. doi:10.1098/rsbm.1981.0006.
7. Burhop, Eric (1933). The Band Spectra of Diatomic Molecules (MSc). University of Melbourne.
8. Qiang, Y. (2010). "Properties of the Lambda(1520) resonance from high-precision electroproduction data". Physics Letters B 694 (2): 123–128. doi:10.1016/j.physletb.2010.09.052. Bibcode2010PhLB..694..123Q.
9. Brumfiel, Geoff (1 March 2001). "The Incredible Shrinking Nucleus". Physical Review Focus 7 (11).
10.
11. Amsler, C. (2008). "Baryons". Lawrence Berkeley Laboratory.
12. Körner, J.G.; Krämer, M.; Pirjol, D. (1994). "Heavy Baryons". Progress in Particle and Nuclear Physics 33: 787–868. doi:10.1016/0146-6410(94)90053-1. Bibcode1994PrPNP..33..787K.
13. Ho-Kim, Quang; Pham, Xuan Yem (1998). "Quarks and SU(3) Symmetry". Elementary Particles and their Interactions: Concepts and phenomena. Berlin: Springer-Verlag. p. 262. ISBN 978-3-540-63667-0. OCLC 38965994. "Because the top quark decays before it can be hadronized, there are no bound $\displaystyle{ t \bar{t} }$ states and no top-flavored mesons or baryons ... ."
14. Amsler, C. (2008). "Λc". Lawrence Berkeley Laboratory.
15. Amsler, C. (2008). "Λ+c". Lawrence Berkeley Laboratory.
16. Amsler, C. (2008). "Λb". Lawrence Berkeley Laboratory.
17. Amsler, C. (2008). "Λ0b". Lawrence Berkeley Laboratory.
18. Zyla, P.A. et al. (2020-08-14). "Review of Particle Physics". Progress of Theoretical and Experimental Physics 2020 (8): 083C01. doi:10.1093/ptep/ptaa104. Bibcode2020PTEP.2020h3C01P.