“Lambda baryon”的意思、由来-开放百科全书

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 {{Subatomic particle|Lambda0}}, {{Subatomic particle|Charmed Lambda+}}, {{Subatomic particle|Bottom Lambda0}}, and {{Subatomic particle|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 {{Nowrap|({{Subatomic particle|Lambda0}})}} (no subscript), a charm quark {{Nowrap|({{Subatomic particle|Charmed Lambda+}})}}, a bottom quark {{Nowrap|({{Subatomic particle|Bottom Lambda0}})}}, or a top quark {{Nowrap|({{Subatomic particle|Top Lambda+}})}}. 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 {{val|5|e=-25}} seconds;^[1] that is about {{sfrac|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 {{Subatomic particle|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,^[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 {{convert|70000|ft}}.^[4] Though the particle was expected to live for {{val|1|p=~|e=-23|u=seconds}},^[5] it actually survived for {{val|1|p=~|e=-10|u=seconds}}.^[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]

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 {{SubatomicParticle|Charmed Lambda+}} (charmed lambda baryon), with a flight time of {{val|7.3|0.1|e=-13|u=s}}.^[6]^[7]

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.^[8] 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.^[9] 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.^[10]

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.^[11]^[12] The top lambda ({{Subatomic particle|Top Lambda+}}) is listed for comparison, but is not expected to be observed, because top quarks decay before they have time to hadronize.^[13]

^† {{note|Undiscovered}} Particle unobserved, because the top-quark decays before it hadronizes.

See also

References

1. ^{{cite journal| first=A. | last=Quadt| year=2006| title=Top quark physics at hadron colliders| journal=European Physical Journal C| volume=48| issue=3 |pages=835–1000| doi=10.1140/epjc/s2006-02631-6| bibcode = 2006EPJC...48..835Q }}
2. ^{{Cite journal | last=Hopper | first=V.D. | last2=Biswas | first2=S. | title=Evidence Concerning the Existence of the New Unstable Elementary Neutral Particle | journal=Phys. Rev. | volume=80 | issue=6 | page=1099 | year=1950 | doi=10.1103/physrev.80.1099 | bibcode=1950PhRv...80.1099H}}
3. ^{{Cite journal | last=Rochester | first=G. D. | last2=Butler| first2=C. C. | title=Evidence for the Existence of New Unstable Elementary Particles | journal=Nature | volume=160 | issue=4077 | page=855 | year=1947 | doi=10.1038/160855a0 | bibcode=1947Natur.160..855R }}
4. ^{{Cite book | last=Pais | first=Abraham | title=Inward Bound | publisher=Oxford University Press| pages= 21, 511–517 | year=1986}}
5. ^¹²The Strange Quark
6. ^{{cite journal |title=Eric Henry Stoneley Burhop 31 January 1911 – 22 January 1980 |first1=Harrie |last1=Massey |authorlink=Harrie Massey |first2=D. H. |last2=Davis |journal=Biographical Memoirs of Fellows of the Royal Society |volume=27 |date=November 1981 |pages=131–152 |jstor=769868|doi=10.1098/rsbm.1981.0006}}
7. ^{{cite thesis |type=MSc |first=Eric |last=Burhop |url=http://trove.nla.gov.au/work/21419586?versionId=256398321933 |title=The Band Spectra of Diatomic Molecules |publisher=University of Melbourne |year=1933 }}
8. ^{{cite journal |first=Y. |last=Qiang |display-authors=etal |title=Properties of the Lambda(1520) resonance from high-precision electroproduction data |journal=Physics Letters B |date=2010 |volume=694 |issue=2 |pages=123–128 |doi=10.1016/j.physletb.2010.09.052 |arxiv=1003.5612 }}
9. ^{{cite web|title=Media Advisory: The Heaviest Known Antimatter|url=http://www.bnl.gov/rhic/news2/news.asp?a=1236&t=pr|publisher=bnl.gov}}
10. ^{{cite web|last=Brumfiel|first=Geoff|title=Focus: The Incredible Shrinking Nucleus|url=http://physics.aps.org/story/v7/st11}}
11. ^C. Amsler et al. (2008): Particle summary tables – Baryons
12. ^J. G. Körner et al. (1994)
13. ^{{Cite book | last=Ho-Kim | first=Quang | first2 = Xuan Yem | last2=Pham | title=Elementary Particles and Their Interactions: Concepts and Phenomena | year= 1998 | publisher=Springer-Verlag | location=Berlin | isbn=978-3-540-63667-0 | oclc=38965994 | page=262 | chapter=Quarks and SU(3) Symmetry | quote=Because the top quark decays before it can be hadronized, there are no bound

states and no top-flavored mesons or baryons[...].}}
14. ^¹C. Amsler et al. (2008): Particle listings – {{Subatomic particle|Lambda}}
15. ^C. Amsler et al. (2008): Particle listings – {{Subatomic particle|Charmed Lambda}}
16. ^C. Amsler et al. (2008): Particle listings – {{Subatomic particle|Bottom Lambda}}

Overview

Types of lambda baryons

See also

References

Bibliography