Lambda baryon

The Lambda baryons 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 differing from a Sigma baryon). 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,[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, where in 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 structure can decay by the weak force like this: its s (ie: strange) quark decays to (1) a u quark, which then joins with the remaining u & d quarks to form a proton, and (2) the weak force's
W−
 boson which then decays to a
π−
 particle.[6]

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 Lambda(1520) with mass = 1518.8 MeV and width = 17.2 MeV which seem to be smaller than their Breit–Wigner values.[9] 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 (Λ7Li), 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 do not expect to 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 not expected to be observed, because top quarks decay before they have time to hadronize.[15]

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
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
charmed Lambda[16]
Λ+
c
u

d

c
 2286.46±0.14 0 1/2+ +1 0 +1 0 0 (2.00±0.06)×10−13 See
Λ+
c decay modes
bottom Lambda[17]
Λ0
b
u

d

b
 5620.2±1.6 0 1/2+ 0 0 0 −1 0 1.409+0.055
−0.054×10−12		 See
Λ0
b decay modes
top Lambda†
Λ+
t
u

d

t
 — 0 1/2+ +1 0 0 0 +1 — —

† ^ Particle unobserved, because the top-quark decays before it hadronizes.

See also

References
  1. 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.
  2. 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.
  3. Pais, Abraham (1986). Inward Bound. Oxford University Press. pp. 21, 511–517.
  4. The Strange Quark
  5. Amsler, C.; et al. (2008). "
    Λ
    "     (PDF). Particle Data Group. Particle listings. Lawrence Berkeley Laboratory.
  6. lambda zero decays to a proton and a pi-minus via the weak interaction
  7. 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.
  8. Burhop, Eric (1933). The Band Spectra of Diatomic Molecules (MSc). University of Melbourne.
  9. 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. doi:10.1016/j.physletb.2010.09.052.
  10. "Media Advisory: The Heaviest Known Antimatter". bnl.gov.
  11. Brumfiel, Geoff (1 March 2001). "The Incredible Shrinking Nucleus". Physical Review Focus. 7 (11).
  12. 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.
  13. Amsler, C.; et al. (2008). "Baryons" (PDF). Particle Data Group. Particle summary tables. Lawrence Berkeley Laboratory.
  14. 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.
  15. 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 states and no top-flavored mesons or baryons ... .
  16. Amsler, C.; et al. (2008). "
    Λ
    c    "     (PDF). Particle Data Group. Particle listings. Lawrence Berkeley Laboratory.
  17. Amsler, C.; et al. (2008). "
    Λ
    b    "     (PDF). Particle Data Group. Particle listings. Lawrence Berkeley Laboratory.

Further reading
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