Lambda baryon
Template:Short description Template:Infobox Particle
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, Template:SubatomicParticle). They are thus baryons, with total isospin of 0, and have either neutral electric charge or the elementary charge +1.
Overview
The lambda baryon Template:Subatomic particle 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 Template:Convert.[3] Though the particle was expected to live for Template:Val,[4] it actually survived for Template:Val.[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, 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).[4] The Template:Subatomic particle with its uds quark decays via weak force to a nucleon and a pion − either Template:Nowrap or Template:Nowrap.
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 Template:SubatomicParticle (charmed lambda baryon), with a lifetime of Template:Val.[6][7]
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.[8] 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.[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 (Template:PhysicsParticle), it made the nucleus 19% smaller.[10]
Types of lambda baryons
Lambda baryons are usually represented by the symbols Template:Math Template:Math Template:Math and Template:Math 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 Template:Nobr (no subscript), a charm quark Template:Nobr a bottom quark Template:Nobr or a top quark Template:Nobr 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 Template:Val seconds;[11] that is about Template:Sfrac 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: Template:Mvar (isospin), Template:Mvar (total angular momentum quantum number), Template:Mvar (parity), Template:Mvar (charge), Template:Mvar (strangeness), Template:Mvar (charmness), Template:Mvar (bottomness), Template:Mvar (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 Template:Mvar would be of opposite signs. Template:Mvar and Template:Mvar values in red have not been firmly established by experiments, but are predicted by the quark model and are consistent with the measurements.[12][13] The top lambda Template:Math is listed for comparison, but is expected to never be observed, because top quarks decay before they have time to form hadrons.[14]
‡ Template:Note 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:
| Particle name | Symbol | Quark content |
Rest mass (MeV/[[speed of light|Template:Mvar]]²) | [[Isospin|Template:Mvar]] | [[Total angular momentum quantum number|Template:Mvar]][[Parity (physics)|Template:Mvar]] | [[charge (physics)|Template:Mvar]] ([[elementary charge|Template:Mvar]]) | [[strangeness|Template:Mvar]] | [[charm (quantum number)|Template:Mvar]] | [[bottomness|Template:Mvar]] | [[topness|Template:Mvar]] | Mean lifetime ([[second|Template:Mvar]]) | Commonly decays to |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Lambda[5] | Template:Math | Template:Subatomic particleTemplate:Subatomic particleTemplate:Subatomic particle | Template:Val | 0 | Template:Nowrap | 0 | −1 | 0 | 0 | 0 | Template:Val | Template:Nobr Template:Nobr |
| Sigma[19] | Template:Math | Template:SubatomicParticleTemplate:SubatomicParticleTemplate:SubatomicParticle | 1,192.642 ± 0.024 | 1 | Template:Nowrap | 0 | −1 | 0 | 0 | 0 | Template:Nowrap | Template:Nobr (100%) |
See also
References
Further reading
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