Lambda baryon

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, ). They are thus baryons, with total isospin of 0, and have either neutral electric charge or the elementary charge +1.

Overview
The lambda baryon Lambda0 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, i.e. different in kind from the K meson discovered in 1947 by Rochester and Butler; they were produced by cosmic rays and detected in photographic emulsions flown in a balloon at 70000 ft. Though the particle was expected to live for $1,115.683 MeV/c2$, it actually survived for $2,286.46 MeV/c2$. The property that caused it to live so long was dubbed strangeness and led to the discovery of the strange quark. 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). The Lambda0 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 (charmed lambda baryon), with a flight time of $5,619.6 MeV/c2$.

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. 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. 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, it made the nucleus 19% smaller.

Types of lambda baryons
Lambda baryons are usually represented by the symbols $Lambda0,$ $Charmed Lambda+,$ $Bottom Lambda0,$ and $Top Lambda+.$ 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 $(Lambda0)$ (no subscript), a charm quark $(Charmed Lambda+)$, a bottom quark $(Bottom Lambda0)$, or a top quark $(Top Lambda+)$. 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 $1/2$ seconds; that is about $~ seconds$ 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: $~ seconds$ (isospin), $7.3 s$ (total angular momentum quantum number), $5$ (parity), $1⁄20$ (charge), $I$ (strangeness), $J$ (charmness), $P$ (bottomness), $Q$ (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 $S$ would be of opposite signs. $C$ and $B′$ values in red have not been firmly established by experiments, but are predicted by the quark model and are consistent with the measurements. The top lambda $(Top Lambda+)$ is listed for comparison, but is expected to never be observed, because top quarks decay before they have time to form hadrons.

‡ 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: