Lambda baryon

From Wikipedia, the free encyclopedia
Jump to: navigation, search

The Lambda baryons are a family of subatomic hadron particles that have the symbols Λ0, Λ+
, Λ0
, and Λ+
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 either a strange (Λ0), a charm (Λ+
), a bottom (Λ0
), or a top (Λ+
) 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 20 times shorter than 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 ft. [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 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.[7] 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.[8]


The symbols encountered in this list are: I (isospin), J (total angular momentum), P (parity), Q (charge), S (strangeness), C (charmness), B′ (bottomness), T (topness), B (baryon number), 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.[9][10] The top lambda (Λ+
) is listed for comparison, but is not expected to be observed, because top quarks decay before they have time to hadronize.[11]

Lambda baryons
Particle name Symbol Quark
Rest mass (MeV/c2) I JP Q (e) S C B' T Mean lifetime (s) Commonly decays to
Lambda[6] Λ0 uds 1115.683±0.006 0 12+ 0 −1 0 0 0 (2.631±0.020)×10−10 p+ + π or
n0 + π0
charmed Lambda[12] Λ+
udc 2286.46±0.14 0 12 + +1 0 +1 0 0 (2.00±0.06)×10−13 See Λ+
decay modes
bottom Lambda[13] Λ0
udb 5620.2±1.6 0 12 + 0 0 0 −1 0 1.409+0.055
See Λ0
decay modes
top Lambda Λ+
udt 0 12 + +1 0 0 0 +1

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

See also[edit]


  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. ^ V D Hopper, S Biswas (1950). "Evidence Concerning the Existence of the New Unstable Elementary Neutral Particle". Phys. Rev. 80: 1099. Bibcode:1950PhRv...80.1099H. doi:10.1103/physrev.80.1099. 
  3. ^ G.D.Rochester, C.C.Butler (1947). "Evidence for the Existence of New Unstable Elementary Particles". Nature 160: 855. Bibcode:1947Natur.160..855R. doi:10.1038/160855a0. 
  4. ^ Pais, Abraham (1986). Inward Bound. Oxford University Press, p 21, 511-517. 
  5. ^ a b c The Strange Quark
  6. ^ a b C. Amsler et al. (2008): Particle listings – Λ
  7. ^ "Media Advisory: The Heaviest Known Antimatter". 
  8. ^ Brumfiel, Geoff. "Focus: The Incredible Shrinking Nucleus". 
  9. ^ C. Amsler et al. (2008): Particle summary tables – Baryons
  10. ^ J. G. Körner et al. (1994)
  11. ^ 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 t anti-t states and no top-flavored mesons or baryons[...]." 
  12. ^ C. Amsler et al. (2008): Particle listings – Λ
  13. ^ C. Amsler et al. (2008): Particle listings – Λ