9
1.2. The fermions of the Standard Model
hadrons, not fractionally charged quarks. The explanation for this novel behaviour of quarks is now believed to lie in the nature of the interquark force
(QCD). We shall briefly discuss this force in section 1.3.6, and treat it in detail
in volume 2. The consensus at present is that QCD does imply the ‘confinement’ of quarks – that is, they do not exist as isolated single particles
1 , only
as groups confined to hadronic volumes.
When Gell-Mann and Zweig made their proposal, three types of quark
were enough to account for the observed hadrons: in addition to the u and
d quarks, the ‘strange’ quark s was needed to describe the known strange
particles such as the hyperon Λ
0 (uds), and the strange mesons like K
0 (d¯ s).
In 1964, Bjorken and Glashow (1964) discussed the possible existence of a
fourth quark on the basis of quark–lepton symmetry, but a strong theoretical
argument for the existence of the c (‘charm’) quark, within the framework of
gauge theories of electroweak interactions, was given by Glashow, Iliopoulos
and Maiani (1970), as we shall discuss in volume 2. They estimated that
the c quark mass should lie in the range 3–4 GeV. Subsequently, Gaillard
and Lee (1974) performed a full (one-loop) calculation in the then newlydeveloped renormalizable electroweak theory, and predicted m c ≈ 1.5 GeV.
The prediction was spectacularly confirmed in November of the same year with
the discovery (Aubert et al. 1974, Augustin et al. 1974) of the J/ψ system,
which was soon identified as a c¯ c composite (and dubbed ‘charmonium’), with
a mass in the vicinity of 3 GeV. Subsequently, mesons such as D
0 (c¯ u) and
D
+ (c ¯
d) carrying the c quark were identified (Goldhaber et al. 1976, Peruzzi
et al. 1976), consolidating this identification.
The second generation of quarks was completed in 1974, with the two
quark doublets (u, d) and (c, s) in parallel with the lepton doublets (ν e , e
− )
and (ν μ , μ
− ). But even before the discovery of the c quark, the possibility that
a completely new third-generation quark doublet might exist was raised in a
remarkable paper by Kobayashi and Maskawa (1973). Their analysis focused
on the problem of incorporating the known violation of CP symmetry (the
product
2 of particle-antiparticle conjugation C and parity P) into the quark
sector of the renormalizable electroweak theory. CP-violation in the decays
of neutral K-mesons had been discovered by Christenson et al. (1964), and
Kobayashi and Maskawa pointed out that it was very difficult to construct a
plausible model of CP-violation in weak transitions of quarks with only two
generations. They suggested, however, that CP-violation could be naturally
accommodated by extending the theory to three generations of quarks. Their
description of CP-violation thus entailed the very bold prediction of two entirely new and undiscovered quarks, the (t, b) doublet, where t (‘top’) has
charge
2 and b (‘bottom’) has charge −
1 .
3
3
In 1975, with the discovery of the τ
− mentioned earlier, there was already
evidence for a third generation of leptons. The discovery of the b quark
1 With the (fleeting) exception of the t quark, as we shall see in a moment.
2 We shall discuss these symmetries in chapter 4.
1.2. The fermions of the Standard Model
hadrons, not fractionally charged quarks. The explanation for this novel behaviour of quarks is now believed to lie in the nature of the interquark force
(QCD). We shall briefly discuss this force in section 1.3.6, and treat it in detail
in volume 2. The consensus at present is that QCD does imply the ‘confinement’ of quarks – that is, they do not exist as isolated single particles
1 , only
as groups confined to hadronic volumes.
When Gell-Mann and Zweig made their proposal, three types of quark
were enough to account for the observed hadrons: in addition to the u and
d quarks, the ‘strange’ quark s was needed to describe the known strange
particles such as the hyperon Λ
0 (uds), and the strange mesons like K
0 (d¯ s).
In 1964, Bjorken and Glashow (1964) discussed the possible existence of a
fourth quark on the basis of quark–lepton symmetry, but a strong theoretical
argument for the existence of the c (‘charm’) quark, within the framework of
gauge theories of electroweak interactions, was given by Glashow, Iliopoulos
and Maiani (1970), as we shall discuss in volume 2. They estimated that
the c quark mass should lie in the range 3–4 GeV. Subsequently, Gaillard
and Lee (1974) performed a full (one-loop) calculation in the then newlydeveloped renormalizable electroweak theory, and predicted m c ≈ 1.5 GeV.
The prediction was spectacularly confirmed in November of the same year with
the discovery (Aubert et al. 1974, Augustin et al. 1974) of the J/ψ system,
which was soon identified as a c¯ c composite (and dubbed ‘charmonium’), with
a mass in the vicinity of 3 GeV. Subsequently, mesons such as D
0 (c¯ u) and
D
+ (c ¯
d) carrying the c quark were identified (Goldhaber et al. 1976, Peruzzi
et al. 1976), consolidating this identification.
The second generation of quarks was completed in 1974, with the two
quark doublets (u, d) and (c, s) in parallel with the lepton doublets (ν e , e
− )
and (ν μ , μ
− ). But even before the discovery of the c quark, the possibility that
a completely new third-generation quark doublet might exist was raised in a
remarkable paper by Kobayashi and Maskawa (1973). Their analysis focused
on the problem of incorporating the known violation of CP symmetry (the
product
2 of particle-antiparticle conjugation C and parity P) into the quark
sector of the renormalizable electroweak theory. CP-violation in the decays
of neutral K-mesons had been discovered by Christenson et al. (1964), and
Kobayashi and Maskawa pointed out that it was very difficult to construct a
plausible model of CP-violation in weak transitions of quarks with only two
generations. They suggested, however, that CP-violation could be naturally
accommodated by extending the theory to three generations of quarks. Their
description of CP-violation thus entailed the very bold prediction of two entirely new and undiscovered quarks, the (t, b) doublet, where t (‘top’) has
charge
2 and b (‘bottom’) has charge −
1 .
3
3
In 1975, with the discovery of the τ
− mentioned earlier, there was already
evidence for a third generation of leptons. The discovery of the b quark
1 With the (fleeting) exception of the t quark, as we shall see in a moment.
2 We shall discuss these symmetries in chapter 4.