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Lambda baryon

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Baryon made of specific quark combinations

Lambda baryon
Quark structure of the lambda baryon.
Composition	Λ⁰ :uds Λ⁺ _c:udc Λ⁰ _b:udb
Statistics	Fermionic
Family	Baryons
Interactions	Strong,weak,electromagnetic, andgravity
Types	3
Mass	Λ⁰ :1115.683±0.006 MeV/c²^[1] Λ⁺ _c:2286.46±0.14 MeV/c² Λ⁰ _b:5619.60±0.17 MeV/c²
Spin	⁠1/2⁠ ħ
Isospin	0

Thelambda baryons (Λ) are a family ofsubatomic hadron particles containing oneup quark, onedown quark, and a third quark from a higherflavour generation, in a combination where thequantum wave function changes sign upon the flavour of any two quarks being swapped (thus slightly different from a neutralsigma baryon,Σ⁰
). They are thusbaryons, with totalisospin of 0, and have eitherneutral electric charge or theelementary charge +1.

Overview

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The lambda baryonΛ⁰
was first discovered in October 1950, by V. D. Hopper and S. Biswas of theUniversity of Melbourne, as a neutralV particle with aproton as a decay product, thus correctly distinguishing it as abaryon, rather than ameson,^[2] i.e. different in kind from theK meson discovered in 1947 by Rochester and Butler;^[3] they were produced by cosmic rays and detected in photographic emulsions flown in a balloon at 70,000 feet (21,000 m).^[4] Though the particle was expected to live for~10⁻²³ s,^[5] it actually survived for~10⁻¹⁰ s.^[6] The property that caused it to live so long was dubbedstrangeness and led to the discovery of the strange quark.^[5] Furthermore, these discoveries led to a principle known as theconservation 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).^[5] TheΛ⁰
with its uds quark decays via weak force to a nucleon and a pion − eitherΛ → p + π⁻ orΛ → n + π⁰.

In 1974 and 1975, an international team at theFermilab that included scientists from Fermilab and seven European laboratories under the leadership ofEric Burhop carried out a search for a new particle, the existence of which Burhop had predicted in 1963. He had suggested thatneutrino interactions could create short-lived (perhaps as low as 10⁻¹⁴ s) particles that could be detected with the use ofnuclear emulsion. Experiment E247 at Fermilab successfully detected particles with a lifetime of the order of 10⁻¹³ s. A follow-up experiment, WA17, which made use of the SPS accelerator at CERN confirmed the existence of theΛ⁺
_c (charmed lambda baryon), with a lifetime of(7.3±0.1)×10⁻¹³ s.^[7]^[8]

In 2011, the international team atJLab 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 mass1518.8 MeV/c² and width17.2 MeV/c², which seem to be smaller than their Breit–Wigner values.^[9] This was the first determination of the pole position for ahyperon.

The lambda baryon has also been observed in atomic nuclei calledhypernuclei. 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.^[10] 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 thePauli exclusion principle), and it binds the nucleus more tightly together due to its interaction via the strong force. In alithium isotope (⁷
_ΛLi), it made the nucleus 19% smaller.^[11]

Types of lambda baryons

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Lambda baryons are usually represented by the symbolsΛ⁰
,Λ⁺
_c,Λ⁰
_b, andΛ⁺
_t. In this notation, thesuperscript character indicates whether the particle is electrically neutral (⁰) or carries a positive charge (⁺). Thesubscript character, or its absence, indicates whether the third quark is astrange quark (Λ⁰
) (no subscript), acharm quark (Λ⁺
_c), abottom quark (Λ⁰
_b), or atop quark (Λ⁺
_t). Physicists expect to not observe a lambda baryon with a top quark, because theStandard Model of particle physics predicts that themean lifetime of top quarks is roughly5×10⁻²⁵ seconds;^[12] that is about⁠1/20⁠ of the mean timescale forstrong interactions, which indicates that the top quark would decay before a lambda baryon couldform 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, andQ,B,S,C,B′,T, would be of opposite signs.I,J, andP values in red have not been firmly established by experiments, but are predicted by thequark model and are consistent with the measurements.^[13]^[14] The top lambda(Λ⁺
_t) is listed for comparison, but is expected to never be observed, because top quarks decay before they have time toform hadrons.^[15]

Lambda baryons
Particle name	Symbol	Quark content	Rest mass [MeV/c²]	J^P	Q [e]	S	C	B′	T	Mean lifetime [s]	Commonly decays to
Lambda^[6]	Λ⁰	uds	1115.683±0.006	⁠1/2⁠⁺	0	−1	0	0	0	(2.631±0.020)×10⁻¹⁰	p⁺ +π⁻ or n⁰ +π⁰
charmed lambda^[16]	Λ⁺ _c	udc	2286.46±0.14	⁠1/2⁠⁺	+1	0	+1	0	0	(2.00±0.06)×10⁻¹³	decay modes^[17]
bottom lambda^[18]	Λ⁰ _b	udb	5620.2±1.6	⁠1/2⁠⁺	0	0	0	−1	0	1.409+0.055 −0.054×10⁻¹²	Decay modes^[19]
top lambda^{^‡}	Λ⁺ _t	udt	—	⁠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	J^P	Q [e]	S	C	B′	T	Mean lifetime [s]	Commonly decays to
Lambda^[6]	Λ⁰	uds	1115.683±0.006	0	⁠1/2⁠⁺	0	−1	0	0	0	(2.631±0.020)×10⁻¹⁰	p⁺ +π⁻ or n⁰ +π⁰
Sigma^[20]	Σ⁰	uds	1,192.642 ± 0.024	1	⁠1/2⁠⁺	0	−1	0	0	0	(7.4±0.7)×10⁻²⁰	Λ⁰ +γ (100%)

References

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^Zyla, P. A.; et al. (Particle Data Group) (2020)."Review of Particle Physics".Progress of Theoretical and Experimental Physics.2020 (8): 083C01.Bibcode:2020PTEP.2020h3C01P.doi:10.1093/ptep/ptaa104.hdl:11585/772320.
^Hopper, V.D.; Biswas, S. (1950). "Evidence Concerning the Existence of the New Unstable Elementary Neutral Particle".Phys. Rev.80 (6): 1099.Bibcode:1950PhRv...80.1099H.doi:10.1103/physrev.80.1099.
^Rochester, G. D.; Butler, C. C. (1947). "Evidence for the Existence of New Unstable Elementary Particles".Nature.160 (4077):855–7.Bibcode:1947Natur.160..855R.doi:10.1038/160855a0.PMID 18917296.S2CID 33881752.
^Pais, Abraham (1986).Inward Bound. Oxford University Press. pp. 21,511–517.ISBN 978-0-19-851971-3.
^^a ^b ^cThe Strange Quark
^^a ^b ^cAmsler, C.; et al. (Particle Data Group) (2008)."Λ"(PDF). Particle listings. Lawrence Berkeley Laboratory.
^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.JSTOR 769868.S2CID 123018692.
^Burhop, Eric (1933).The Band Spectra of Diatomic Molecules (MSc). University of Melbourne.
^Qiang, Y.; et al. (2010). "Properties of the Lambda(1520) resonance from high-precision electroproduction data".Physics Letters B.694 (2):123–128.arXiv:1003.5612.Bibcode:2010PhLB..694..123Q.doi:10.1016/j.physletb.2010.09.052.S2CID 119290870.
^"Media Advisory: The Heaviest Known Antimatter". bnl.gov. Archived fromthe original on 2017-02-11. Retrieved2013-03-10.
^Brumfiel, Geoff (1 March 2001)."The Incredible Shrinking Nucleus".Physical Review Focus. Vol. 7, no. 11.
^Quadt, A. (2006)."Top quark physics at hadron colliders"(PDF).European Physical Journal C.48 (3):835–1000.Bibcode:2006EPJC...48..835Q.doi:10.1140/epjc/s2006-02631-6.S2CID 121887478.
^Amsler, C.; et al. (Particle Data Group) (2008)."Baryons"(PDF). Particle summary tables.Lawrence Berkeley Laboratory.
^Körner, J.G.; Krämer, M.; Pirjol, D. (1994). "Heavy Baryons".Progress in Particle and Nuclear Physics.33:787–868.arXiv:hep-ph/9406359.Bibcode:1994PrPNP..33..787K.doi:10.1016/0146-6410(94)90053-1.S2CID 118931787.
^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 $t{\bar {t}}$ states and no top-flavored mesons or baryons ... .
^Amsler, C.; et al. (Particle Data Group) (2008)."Λ
_c"(PDF). Particle listings. Lawrence Berkeley Laboratory.
^Amsler, C.; et al. (Particle Data Group) (2008)."Λ⁺
_c"(PDF). Decay modes. Lawrence Berkeley Laboratory.
^Amsler, C.; et al. (Particle Data Group) (2008)."Λ
_b"(PDF). Particle listings. Lawrence Berkeley Laboratory.
^Amsler, C.; et al. (Particle Data Group) (2008)."Λ⁰
_b"(PDF). Decay modes. Lawrence Berkeley Laboratory.
^Zyla, P.A.; et al. (Particle Data Group) (2020-08-14)."Review of Particle Physics".Progress of Theoretical and Experimental Physics.2020 (8): 083C01.Bibcode:2020PTEP.2020h3C01P.doi:10.1093/ptep/ptaa104.hdl:10481/66389.

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Lambda baryon

Overview

Types of lambda baryons

See also

References

Further reading