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 lifetime 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.
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]
^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. ISBN978-3-540-63667-0. OCLC38965994. Because the top quark decays before it can be hadronized, there are no bound states and no top-flavored mesons or baryons ... .
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Amsler, C.; et al. (Particle Data Group) (2008). " Λ c"(PDF). Particle listings. Lawrence Berkeley Laboratory.
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Amsler, C.; et al. (Particle Data Group) (2008). " Λ+ c"(PDF). Decay modes. Lawrence Berkeley Laboratory.
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Amsler, C.; et al. (Particle Data Group) (2008). " Λ b"(PDF). Particle listings. Lawrence Berkeley Laboratory.
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Amsler, C.; et al. (Particle Data Group) (2008). " Λ0 b"(PDF). Decay modes. Lawrence Berkeley Laboratory.
