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,
Σ⁰
). They are thus baryons, with total isospin of 0, and have either neutral electric charge or the elementary charge +1.

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 ~10⁻²³ s, it actually survived for ~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, where in 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
Λ⁰
with its uds quark structure can decay by the weak force like this: its s (ie: strange) quark decays to (1) a u quark, which then joins with the remaining u & d quarks to form a proton, and (2) the weak force's
W⁻
boson which then decays to a
π⁻
particle.

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. 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 (Λ⁷Li), it made the nucleus 19% smaller.

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 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.

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	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

Further reading

• Amsler, C.; et al. (2008). "Review of Particle Physics" (PDF). Physics Letters B. 667 (1–5): 1–6. Bibcode:2008PhLB..667....1A. doi:10.1016/j.physletb.2008.07.018.
• Caso, C.; 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.
• Nave, R. (12 April 2005). "The Lambda baryon". HyperPhysics. Retrieved 14 July 2010.