Lambda baryon

Baryon made of specific quark combinations

Λ⁰
:
u

d

s
Λ⁺
_c:
u

d

c
Λ⁰
_b:
u

d

b

StatisticsFermionicFamilyBaryonsInteractionsStrong, weak, electromagnetic, and gravityTypes3Mass

Λ⁰
: 1115.683±0.006 MeV/c²^[1]
Λ⁺
_c: 2286.46±0.14 MeV/c²
Λ⁰
_b: 5619.60±0.17 MeV/c²

Spin1⁄2Isospin0

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

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,^[2] i.e. different in kind from the K 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 dubbed strangeness and led to the discovery of the strange quark.^[5] 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).^[5] 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.^[7]^[8]

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.^[9] 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.^[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 the Pauli exclusion principle), and it binds the nucleus more tightly together due to its interaction via the strong force. In a lithium isotope (⁷
_ΛLi
), it made the nucleus 19% smaller.^[11]

Types of lambda baryons

Lambda baryons are usually represented by the symbols
Λ⁰
,
Λ⁺
_c,
Λ⁰
_b, 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 (
Λ⁰
_b), 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;^[12] 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.^[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 to form 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	u d s	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	u d c	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	u d b	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	u d t	—	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	u d s	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	u d s	1,192.642 ± 0.024	1	1/2⁺	0	−1	0	0	0	7.4 ± 0.7 × 10⁻²⁰	Λ⁰ + γ (100%)

References

^ 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 ^c The Strange Quark
^ ^a ^b ^c Amsler, 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 from the original on 2017-02-11. Retrieved 2013-03-10.
^ 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.