10
1. The Particles and Forces of the Standard Model
in 1977 resulted from the observation of massive mesonic states generally
known as Υ (‘upsilon’) (Herb et al. 1977, Innes et al. 1977), which were
identified as b ¯
b composites. Subsequently, b-carrying mesons were found.
Finally, firm evidence for the expected t quark was obtained by the CDF and
D0 collaborations at Fermilab in 1995 (Abe et al. 1995, Abachi et al. 1995);
see Bettini 2008, section 4.10, for details about the discovery of the top quark.
The full complement of three generations of quark doublets is then
(u, d)
(c, s)
and
(t, b)
(1.10)
together with their antiparticles, in parallel with the three generations of
lepton doublets (1.8).
One particular feature of the t quark requires comment. Its mass is so
large that, although it decays weakly, the energy release is so great that its
lifetime is some two orders of magnitude shorter than typical strong interaction
timescales; this means that it decays before any t-carrying hadrons can be
formed. So when a t quark is produced (in a p-¯ p collision, for example),
it decays as a free (unbound) particle. Its mass can be determined from a
kinematic anaysis of the decay products.
We must now discuss the quantum numbers carried by quarks. First of
all, each quark listed in (1.10) comes in three varieties, distinguished by a
quantum number called ‘colour’. It is precisely this quantum number that
underlies the dynamics of QCD (see section 1.3.6). Colour, in fact, is a kind
of generalized charge, for the strong QCD interactions. We shall denote the
three colours of a quark by ‘red’, ‘blue’, and ‘green’. Thus we have the triplet
(u r , u b , u g ), and similarly for all the other quarks.
Secondly, quarks carry flavour quantum numbers, like the leptons. In the
quark case, they are as follows. The two quarks which are familiar in ordinary
matter, ‘u’ and ‘d’, are an isospin doublet (see chapter 12 in volume 2) with
T 3 = +1/2 for ‘u’ and T 3 = −1/2 for ‘d’. The flavour of ‘s’ is strangeness,
with the value S = −1. The flavour of ‘c’ is charm, with value C = +1, that
of ‘b’ has value B ˜ = −1 (we use B ˜ to distinguish it from baryon number B),
and the flavour of ‘t’ is T = +1. The convention is that the sign of the flavour
number is the same as that of the charge.
The strong and electromagnetic interactions of quarks are independent
of quark flavour, and depend only on the electromagnetic charge and the
strong charge, respectively. This means, in particular, that flavour cannot
change in a strong interaction among hadrons – that is, flavour is conserved
in such interactions. For example, from a zero strangeness initial state, the
strong interaction can only produce pairs of strange particles, with cancelling
strangeness. This is the phenomenon of ‘associated production’, known since
the early days of strange particle physics in the 1950s. Similar rules hold for
the other flavours: for example, the t quark, once produced, cannot decay to
a lighter quark via a strong interaction, since this would violate T -conservation.
1. The Particles and Forces of the Standard Model
in 1977 resulted from the observation of massive mesonic states generally
known as Υ (‘upsilon’) (Herb et al. 1977, Innes et al. 1977), which were
identified as b ¯
b composites. Subsequently, b-carrying mesons were found.
Finally, firm evidence for the expected t quark was obtained by the CDF and
D0 collaborations at Fermilab in 1995 (Abe et al. 1995, Abachi et al. 1995);
see Bettini 2008, section 4.10, for details about the discovery of the top quark.
The full complement of three generations of quark doublets is then
(u, d)
(c, s)
and
(t, b)
(1.10)
together with their antiparticles, in parallel with the three generations of
lepton doublets (1.8).
One particular feature of the t quark requires comment. Its mass is so
large that, although it decays weakly, the energy release is so great that its
lifetime is some two orders of magnitude shorter than typical strong interaction
timescales; this means that it decays before any t-carrying hadrons can be
formed. So when a t quark is produced (in a p-¯ p collision, for example),
it decays as a free (unbound) particle. Its mass can be determined from a
kinematic anaysis of the decay products.
We must now discuss the quantum numbers carried by quarks. First of
all, each quark listed in (1.10) comes in three varieties, distinguished by a
quantum number called ‘colour’. It is precisely this quantum number that
underlies the dynamics of QCD (see section 1.3.6). Colour, in fact, is a kind
of generalized charge, for the strong QCD interactions. We shall denote the
three colours of a quark by ‘red’, ‘blue’, and ‘green’. Thus we have the triplet
(u r , u b , u g ), and similarly for all the other quarks.
Secondly, quarks carry flavour quantum numbers, like the leptons. In the
quark case, they are as follows. The two quarks which are familiar in ordinary
matter, ‘u’ and ‘d’, are an isospin doublet (see chapter 12 in volume 2) with
T 3 = +1/2 for ‘u’ and T 3 = −1/2 for ‘d’. The flavour of ‘s’ is strangeness,
with the value S = −1. The flavour of ‘c’ is charm, with value C = +1, that
of ‘b’ has value B ˜ = −1 (we use B ˜ to distinguish it from baryon number B),
and the flavour of ‘t’ is T = +1. The convention is that the sign of the flavour
number is the same as that of the charge.
The strong and electromagnetic interactions of quarks are independent
of quark flavour, and depend only on the electromagnetic charge and the
strong charge, respectively. This means, in particular, that flavour cannot
change in a strong interaction among hadrons – that is, flavour is conserved
in such interactions. For example, from a zero strangeness initial state, the
strong interaction can only produce pairs of strange particles, with cancelling
strangeness. This is the phenomenon of ‘associated production’, known since
the early days of strange particle physics in the 1950s. Similar rules hold for
the other flavours: for example, the t quark, once produced, cannot decay to
a lighter quark via a strong interaction, since this would violate T -conservation.