8
1. The Particles and Forces of the Standard Model
et al. 2010, do not include the results obtained from analyses of neutrino
oscillations. These oscillations, to which we shall return in chapter 21 in
volume 2, are sensitive to the differences of squared masses of the neutrinos,
not to the absolute scale of mass.
We now turn to the other fermions in the SM.
1.2.2 Quarks
Quarks are the constituents of hadrons, in which they are bound by the strong
1
2
3
2
5
2
QCD forces. Hadrons with spins
, . . . (i.e. fermions) are baryons, those
, ,
with spins 0, 1, 2, . . . (i.e. bosons) are mesons. Examples of baryons are
nucleons (the neutron n and the proton p), and hyperons such as Λ
0 and the
Σ and Ξ states. Evidence for the composite nature of hadrons accumulated
during the 1960s and 1970s. Elastic scattering of electrons from protons by
Hofstadter and co-workers (Hofstadter 1963) showed that the proton was not
pointlike, but had an approximately exponential distribution of charge with a
root mean square radius of about 0.8 fm. Much careful experimentation in the
field of baryon and meson spectroscopy revealed sequences of excited states,
strongly reminiscent of those well-known in atomic and nuclear physics.
The conclusion would now seem irresistible that such spectra should be
interpreted as the energy levels of systems of bound constituents. A specific proposal along these lines was made in 1964 by Gell-Mann (1964) and
Zweig (1964). Though based on somewhat different (and much more fragmentary) evidence, their suggestion has turned out to be essentially correct.
1
2
They proposed that baryons contain three spin- constituents called quarks
(by Gell-Mann), while mesons are quark-antiquark systems. One immediate
consequence is that quarks have fractional electromagnetic charge. For exam2
3
ple, the proton has two quarks of charge + , called ‘up’ (u) quarks, and one
1
3
quark of charge − , the ‘down’ (d) quark. The neutron has the combination
ddu, while the π
+ has one u and one anti-d ( ¯
d ) and so on.
Quite simple quantum-mechanical bound state quark models, based on
these ideas, were remarkably successful in accounting for the observed hadronic
spectra. Nevertheless, many physicists, in the 1960s and early 1970s, continued to regard quarks more as useful devices for systematizing a mass of
complicated data than as genuine items of physical reality. One reason for
this scepticism must now be confronted, for it constitutes a major new twist
in the story of the structure of matter.
Gell-Mann ended his 1964 paper with the remark: ‘A search for stable
1
3
2
3
2
3
1
3
quarks of charge−
and/or stable di-quarks of charge −
or +
or + or
4
3 at the highest energy accelerators would help to reassure us of the non+
existence of real quarks’. Indeed, with one possible exception (La Rue et al.
1977, 1981), this ‘reassurance’ has been handsomely provided! Unlike the
constituents of atoms and nuclei, quarks have not been observed as stable
isolated particles. When hadrons of the highest energies currently available
are smashed into each other, what is observed downstream is only lots more
1. The Particles and Forces of the Standard Model
et al. 2010, do not include the results obtained from analyses of neutrino
oscillations. These oscillations, to which we shall return in chapter 21 in
volume 2, are sensitive to the differences of squared masses of the neutrinos,
not to the absolute scale of mass.
We now turn to the other fermions in the SM.
1.2.2 Quarks
Quarks are the constituents of hadrons, in which they are bound by the strong
1
2
3
2
5
2
QCD forces. Hadrons with spins
, . . . (i.e. fermions) are baryons, those
, ,
with spins 0, 1, 2, . . . (i.e. bosons) are mesons. Examples of baryons are
nucleons (the neutron n and the proton p), and hyperons such as Λ
0 and the
Σ and Ξ states. Evidence for the composite nature of hadrons accumulated
during the 1960s and 1970s. Elastic scattering of electrons from protons by
Hofstadter and co-workers (Hofstadter 1963) showed that the proton was not
pointlike, but had an approximately exponential distribution of charge with a
root mean square radius of about 0.8 fm. Much careful experimentation in the
field of baryon and meson spectroscopy revealed sequences of excited states,
strongly reminiscent of those well-known in atomic and nuclear physics.
The conclusion would now seem irresistible that such spectra should be
interpreted as the energy levels of systems of bound constituents. A specific proposal along these lines was made in 1964 by Gell-Mann (1964) and
Zweig (1964). Though based on somewhat different (and much more fragmentary) evidence, their suggestion has turned out to be essentially correct.
1
2
They proposed that baryons contain three spin- constituents called quarks
(by Gell-Mann), while mesons are quark-antiquark systems. One immediate
consequence is that quarks have fractional electromagnetic charge. For exam2
3
ple, the proton has two quarks of charge + , called ‘up’ (u) quarks, and one
1
3
quark of charge − , the ‘down’ (d) quark. The neutron has the combination
ddu, while the π
+ has one u and one anti-d ( ¯
d ) and so on.
Quite simple quantum-mechanical bound state quark models, based on
these ideas, were remarkably successful in accounting for the observed hadronic
spectra. Nevertheless, many physicists, in the 1960s and early 1970s, continued to regard quarks more as useful devices for systematizing a mass of
complicated data than as genuine items of physical reality. One reason for
this scepticism must now be confronted, for it constitutes a major new twist
in the story of the structure of matter.
Gell-Mann ended his 1964 paper with the remark: ‘A search for stable
1
3
2
3
2
3
1
3
quarks of charge−
and/or stable di-quarks of charge −
or +
or + or
4
3 at the highest energy accelerators would help to reassure us of the non+
existence of real quarks’. Indeed, with one possible exception (La Rue et al.
1977, 1981), this ‘reassurance’ has been handsomely provided! Unlike the
constituents of atoms and nuclei, quarks have not been observed as stable
isolated particles. When hadrons of the highest energies currently available
are smashed into each other, what is observed downstream is only lots more