The lambda baryons (Λ) are a family of
subatomic hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...
particles containing one
up quark
The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (two up qua ...
, one
down quark
The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two u ...
, and a third quark from a higher
flavour
Flavor or flavour is either the sensory perception of taste or smell, or a flavoring in food that produces such perception.
Flavor or flavour may also refer to:
Science
*Flavors (programming language), an early object-oriented extension to Lisp ...
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
baryon
In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classi ...
s, with total
isospin
In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions ...
of 0, and have either
neutral electric charge or the
elementary charge
The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a funda ...
+1.
Overview

The lambda baryon was first discovered in October 1950, by V. D. Hopper and S. Biswas of the
University of Melbourne
The University of Melbourne is a public research university located in Melbourne, Australia. Founded in 1853, it is Australia's second oldest university and the oldest in Victoria. Its main campus is located in Parkville, an inner suburb n ...
, as a neutral
V particle with a
proton as a decay product, thus correctly distinguishing it as a
baryon
In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classi ...
, rather than a
meson
In particle physics, a meson ( or ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticle ...
, 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 . Though the particle was expected to live for ,
[The Strange Quark](_blank)
/ref> it actually survived for . 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 with its uds quark decays via weak force to a nucleon and a pion − either or .
In 1974 and 1975, an international team at the Fermilab
Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy United States Department of Energy National Labs, national laboratory specializing in high-energy parti ...
that included scientists from Fermilab and seven European laboratories under the leadership of Eric Burhop
Eric Henry Stoneley Burhop, (31 January 191122 January 1980) was an Australian physicist and humanitarian.
A graduate of the University of Melbourne, Burhop was awarded an 1851 Exhibition Scholarship to study at the Cavendish Laboratory under ...
carried out a search for a new particle, the existence of which Burhop had predicted in 1963. He had suggested that neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
interactions could create short-lived (perhaps as low as 10−14 s) particles that could be detected with the use of nuclear emulsion A nuclear emulsion plate is a type of particle detector first used in nuclear and particle physics experiments in the early decades of the 20th century. https://cds.cern.ch/record/1728791/files/vol6-issue5-p083-e.pdf''The Study of Elementary Partic ...
. 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 .
In 2011, the international team at JLab
jLab is a numerical computational environment implemented in Java. The main scripting engine of jLab is GroovySci, an extension of Groovy. Additionally, the interpreted J-Scripts (similar to MATLAB) and dynamic linking to Java class code ar ...
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
In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formulated ...
), and it binds the nucleus more tightly together due to its interaction via the strong force. In a lithium
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...
isotope (), it made the nucleus 19% smaller.
Types of lambda baryons
Lambda baryons are usually represented by the symbols and In this notation, the superscript
A subscript or superscript is a character (such as a number or letter) that is set slightly below or above the normal line of type, respectively. It is usually smaller than the rest of the text. Subscripts appear at or below the baseline, whil ...
character indicates whether the particle is electrically neutral (0) or carries a positive charge (+). The subscript
A subscript or superscript is a character (such as a number or letter) that is set slightly below or above the normal line of type, respectively. It is usually smaller than the rest of the text. Subscripts appear at or below the baseline, whil ...
character, or its absence, indicates whether the third quark is a strange quark (no subscript), a charm quark
The charm quark, charmed quark or c quark (from its symbol, c) is the third-most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks. Examples of hadrons containi ...
a bottom quark
The bottom quark or b quark, also known as the beauty quark, is a third-generation heavy quark with a charge of − ''e''.
All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exce ...
or a top quark
The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling y_ is very close to unity; in the Standard ...
Physicists expect to not observe a lambda baryon with a top quark, because the Standard Model of particle physics predicts that the mean lifetime
A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and (lambda) is a positive rate c ...
of top quarks is roughly seconds;[
] that is about of the mean timescale for strong interaction
The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called th ...
s, which indicates that the top quark would decay before a lambda baryon could form a hadron.
The symbols encountered in this list are: (''isospin
In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions ...
''), (''total angular momentum quantum number
In quantum mechanics, the total angular momentum quantum number parametrises the total angular momentum of a given particle, by combining its orbital angular momentum and its intrinsic angular momentum (i.e., its spin).
If s is the particle's s ...
''), (''parity
Parity may refer to:
* Parity (computing)
** Parity bit in computing, sets the parity of data for the purpose of error detection
** Parity flag in computing, indicates if the number of set bits is odd or even in the binary representation of the r ...
''), ('' charge''), ('' strangeness''), ('' charmness''), ('' bottomness''), ('' topness''), u (''up quark
The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (two up qua ...
''), d (''down quark
The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two u ...
''), s ('' strange quark''), c (''charm quark
The charm quark, charmed quark or c quark (from its symbol, c) is the third-most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks. Examples of hadrons containi ...
''), b (''bottom quark
The bottom quark or b quark, also known as the beauty quark, is a third-generation heavy quark with a charge of − ''e''.
All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exce ...
''), t (''top quark
The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling y_ is very close to unity; in the Standard ...
''), as well as other subatomic particles.
Antiparticles are not listed in the table; however, they simply would have all quarks changed to antiquarks, and would be of opposite signs. and 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 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:
See also
* List of baryons
Baryons are composite particles made of three quarks, as opposed to mesons, which are composite particles made of one quark and one antiquark. Baryons and mesons are both hadrons, which are particles composed solely of quarks or both quarks and a ...
References
Further reading
*
*
*
{{DEFAULTSORT:Lambda baryon
Baryons
Strange quark