Lambda baryon

The Lambda baryons 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 differing from a Sigma baryon). They are thus baryons, with total isospin of 0, and have either neutral electric charge or the elementary charge +1.

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 do not expect to 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; 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.

Overview

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, 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 70,000 feet (21,000 m). Though the particle was expected to live for ~1×10⁻²³ s, it actually survived for ~1×10⁻¹⁰ s. 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).

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)×10⁻¹³ s.

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 Lambda(1520) with mass = 1518.8 MeV and width = 17.2 MeV which seem to be smaller than their Breit–Wigner values. 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 (Λ⁷Li), it made the nucleus 19% smaller.

Types of lambda baryons

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.

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. 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.

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	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	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	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

• List of baryons

References

Bibliography

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• C. Caso; et al. (1998). "Review of Particle Physics". European Physical Journal C. 3 (1–4): 1–783. Bibcode:1998EPJC....3....1P. doi:10.1007/s10052-998-0104-x.
• J. G. Körner; M. Krämer; D. Pirjol (1994). "Heavy Baryons". Progress in Particle and Nuclear Physics. 33: 787–868. arXiv:hep-ph/9406359. Bibcode:1994PrPNP..33..787K. doi:10.1016/0146-6410(94)90053-1.
• R. Nave (12 April 2005). "The Lambda Baryon". HyperPhysics. Retrieved 2010-07-14.