Composition |
|
---|---|
Statistics | Fermionic |
Family | Baryons |
Interactions | Strong, weak, electromagnetic, and gravity |
Types | 3 |
Mass | |
Spin | 1⁄2 |
Isospin | 0 |
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.
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,
[2] i.e. different in kind from the
K meson discovered in 1947 by Rochester and Butler;
[3] they were produced by cosmic rays and detected in photographic emulsions flown in a balloon at 70,000 feet (21,000 m).
[4] Though the particle was expected to live for ~10−23 s,
[5] it actually survived for ~10−10 s.
[6] The property that caused it to live so long was dubbed strangeness and led to the discovery of the strange quark.
[5] 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).
[5] The
Λ0
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
Λ+
c (charmed lambda baryon), with a lifetime of (7.3±0.1)×10−13 s.
[7]
[8]
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. [9] 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.
[10] 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 (7
ΛLi
), it made the nucleus 19% smaller.
[11]
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 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 5×10−25 seconds;
[12] 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.
[13]
[14] The top lambda (
Λ+
t) is listed for comparison, but is expected to never be observed, because top quarks decay before they have time to
form hadrons.
[15]
Particle name | Symbol | Quark content |
Rest mass ( MeV/ c²) | I | J P | Q ( e) | S | C | B′ | T | Mean lifetime ( s) | Commonly decays to |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Lambda [6] | 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 [16] | Λ+ c |
u d c |
2286.46±0.14 | 0 | 1/2+ | +1 | 0 | +1 | 0 | 0 | (2.00±0.06)×10−13 | decay modes [17] |
bottom lambda [18] | 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 [19] |
top lambda ‡ | Λ+ t |
u d t |
— | 0 | 1/2+ | +1 | 0 | 0 | 0 | +1 | — | ‡ |
‡ ^ 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/ c²) | I | J P | Q ( e) | S | C | B′ | T | Mean lifetime ( s) | Commonly decays to |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Lambda [6] | 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 |
Sigma [20] | Σ0 |
u d s |
1,192.642 ± 0.024 | 1 | 1/2+ | 0 | −1 | 0 | 0 | 0 | 7.4 ± 0.7 × 10−20 |
Λ0 + γ (100%) |
Because the top quark decays before it can be hadronized, there are no bound states and no top-flavored mesons or baryons ... .
Composition |
|
---|---|
Statistics | Fermionic |
Family | Baryons |
Interactions | Strong, weak, electromagnetic, and gravity |
Types | 3 |
Mass | |
Spin | 1⁄2 |
Isospin | 0 |
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.
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,
[2] i.e. different in kind from the
K meson discovered in 1947 by Rochester and Butler;
[3] they were produced by cosmic rays and detected in photographic emulsions flown in a balloon at 70,000 feet (21,000 m).
[4] Though the particle was expected to live for ~10−23 s,
[5] it actually survived for ~10−10 s.
[6] The property that caused it to live so long was dubbed strangeness and led to the discovery of the strange quark.
[5] 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).
[5] The
Λ0
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
Λ+
c (charmed lambda baryon), with a lifetime of (7.3±0.1)×10−13 s.
[7]
[8]
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. [9] 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.
[10] 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 (7
ΛLi
), it made the nucleus 19% smaller.
[11]
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 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 5×10−25 seconds;
[12] 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.
[13]
[14] The top lambda (
Λ+
t) is listed for comparison, but is expected to never be observed, because top quarks decay before they have time to
form hadrons.
[15]
Particle name | Symbol | Quark content |
Rest mass ( MeV/ c²) | I | J P | Q ( e) | S | C | B′ | T | Mean lifetime ( s) | Commonly decays to |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Lambda [6] | 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 [16] | Λ+ c |
u d c |
2286.46±0.14 | 0 | 1/2+ | +1 | 0 | +1 | 0 | 0 | (2.00±0.06)×10−13 | decay modes [17] |
bottom lambda [18] | 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 [19] |
top lambda ‡ | Λ+ t |
u d t |
— | 0 | 1/2+ | +1 | 0 | 0 | 0 | +1 | — | ‡ |
‡ ^ 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/ c²) | I | J P | Q ( e) | S | C | B′ | T | Mean lifetime ( s) | Commonly decays to |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Lambda [6] | 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 |
Sigma [20] | Σ0 |
u d s |
1,192.642 ± 0.024 | 1 | 1/2+ | 0 | −1 | 0 | 0 | 0 | 7.4 ± 0.7 × 10−20 |
Λ0 + γ (100%) |
Because the top quark decays before it can be hadronized, there are no bound states and no top-flavored mesons or baryons ... .