Lambda baryon

The Lambda baryons are a family of subatomic hadron particles that have the symbols
Λ0
,
Λ+
c,
Λ0
b, and
Λ+
t and have +1 elementary charge or are neutral. They are baryons containing three different quarks: one up, one down, and one third quark, which can be a strange (
Λ0
), a charm (
Λ+
c), a bottom (
Λ0
b), or a top (
Λ+
t) quark. The top Lambda is not expected to be observed as the Standard Model predicts the mean lifetime of top quarks to be roughly 5×10−25 s.[1] This is about one-twentieth the timescale for strong interactions, and, therefore it does not form hadrons.

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 ~1×10−23 s,[5] it actually survived for ~1×10−10 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]

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 x 10−13 s.[7][8]

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 (Λ7Li), it made the nucleus 19% smaller.[10]

List

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 (hover for name).

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.[11][12] 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.[13]

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[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
charmed Lambda[14]
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[15]
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. ↑ A. Quadt (2006). "Top quark physics at hadron colliders". European Physical Journal C. 48 (3): 835–1000. Bibcode:2006EPJC...48..835Q. doi:10.1140/epjc/s2006-02631-6.
  2. ↑ 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.
  3. ↑ Rochester, G.D.; Butler, C.C. (1947). "Evidence for the Existence of New Unstable Elementary Particles". Nature. 160 (4077): 855. Bibcode:1947Natur.160..855R. doi:10.1038/160855a0.
  4. ↑ Pais, Abraham (1986). Inward Bound. Oxford University Press, p 21, 511-517.
  5. 1 2 3 The Strange Quark
  6. 1 2 C. Amsler et al. (2008): Particle listings –
    Λ
  7. ↑ Massey, Harrie; Davis, D. H. (November 1981). "Eric Henry Stoneley Burhop 31 January 1911 – 22 January 1980" (PDF). 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. ↑ "Media Advisory: The Heaviest Known Antimatter". bnl.gov.
  10. ↑ Brumfiel, Geoff. "Focus: The Incredible Shrinking Nucleus".
  11. ↑ C. Amsler et al. (2008): Particle summary tables – Baryons
  12. ↑ J. G. Körner et al. (1994)
  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 3-540-63667-6. OCLC 38965994. Because the top quark decays before it can be hadronized, there are no bound states and no top-flavored mesons or baryons[...].
  14. ↑ C. Amsler et al. (2008): Particle listings –
    Λ
    c
  15. ↑ C. Amsler et al. (2008): Particle listings –
    Λ
    b

Bibliography

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