Composition | Elementary particle |
---|---|
Statistics | Fermionic |
Family | Quark |
Generation | Second |
Interactions | strong, weak, electromagnetic force, gravity |
Symbol | s |
Antiparticle | Strange antiquark ( s ) |
Theorized |
Murray Gell-Mann (1964) George Zweig (1964) |
Discovered | 1968, SLAC |
Mass | 95+9 −3 MeV/c2 [1] |
Decays into | Up quark |
Electric charge | −1/3 e |
Color charge | Yes |
Spin | 1/2 ħ |
Weak isospin | LH: −1/2, RH: 0 |
Weak hypercharge | LH: 1/3, RH: −2/3 |
The strange quark or s quark (from its symbol, s) is the third lightest of all
quarks, a type of
elementary particle. Strange quarks are found in
subatomic particles called
hadrons. Examples of hadrons containing strange quarks include
kaons (
K
),
strange D mesons (
D
s),
Sigma baryons (
Σ
), and other
strange particles.
According to the IUPAP, the symbol s is the official name, while "strange" is to be considered only as a mnemonic. [2] The name sideways has also been used because the s quark (but also the other three remaining quarks) has an I3 value of 0 while the u ("up") and d ("down") quarks have values of +1/2 and −1/2 respectively. [3]
Along with the
charm quark, it is part of the
second generation of matter. It has an
electric charge of −+1/3
e and a
bare mass of 95+9
−3
MeV/c2.
[1] Like all
quarks, the strange quark is an
elementary
fermion with
spin
1/2, and experiences all four
fundamental interactions:
gravitation,
electromagnetism,
weak interactions, and
strong interactions. The
antiparticle of the strange quark is the strange antiquark (sometimes called antistrange quark or simply antistrange), which differs from it only in that some of its properties have
equal magnitude but opposite sign.
The first strange particle (a particle containing a strange quark) was discovered in 1947 ( kaons), but the existence of the strange quark itself (and that of the up and down quarks) was only postulated in 1964 by Murray Gell-Mann and George Zweig to explain the eightfold way classification scheme of hadrons. The first evidence for the existence of quarks came in 1968, in deep inelastic scattering experiments at the Stanford Linear Accelerator Center. These experiments confirmed the existence of up and down quarks, and by extension, strange quarks, as they were required to explain the eightfold way.
In the beginnings of particle physics (first half of the 20th century),
hadrons such as
protons,
neutrons and
pions were thought to be
elementary particles. However, new hadrons were discovered and the "
particle zoo" grew from a few particles in the early 1930s and 1940s to several dozens of them in the 1950s. Some particles were much longer lived than others; most particles decayed through the
strong interaction and had
lifetimes of around 10−23 seconds. When they decayed through the
weak interactions, they had lifetimes of around 10−10 seconds. While studying these decays,
Murray Gell-Mann (in 1953)
[4]
[5] and
Kazuhiko Nishijima (in 1955)
[6] developed the concept of
strangeness (which Nishijima called eta-charge, after the
eta meson (
η
)) to explain the "strangeness" of the longer-lived particles. The
Gell-Mann–Nishijima formula is the result of these efforts to understand strange decays.
Despite their work, the relationships between each particle and the physical basis behind the strangeness property remained unclear. In 1961, Gell-Mann [7] and Yuval Ne'eman [8] independently proposed a hadron classification scheme called the eightfold way, also known as SU(3) flavor symmetry. This ordered hadrons into isospin multiplets. The physical basis behind both isospin and strangeness was only explained in 1964, when Gell-Mann [9] and George Zweig [10] [11] independently proposed the quark model, which at that time consisted only of the up, down, and strange quarks. [12] Up and down quarks were the carriers of isospin, while the strange quark carried strangeness. While the quark model explained the eightfold way, no direct evidence of the existence of quarks was found until 1968 at the Stanford Linear Accelerator Center. [13] [14] Deep inelastic scattering experiments indicated that protons had substructure, and that protons made of three more-fundamental particles explained the data (thus confirming the quark model). [15]
At first people were reluctant to identify the three-bodies as quarks, instead preferring Richard Feynman's parton description, [16] [17] [18] but over time the quark theory became accepted (see November Revolution). [19]
By the end of the summer ... [Gell-Mann] completed his first paper, 'Isotopic Spin and Curious Particles' and send it of to Physical Review. The editors hated the title, so he amended it to 'Strange Particles'. They wouldn't go for that either—never mind that almost everybody used the term—suggesting insteand [ sic] 'Isotopic Spin and New Unstable Particles'.