Subatomic Physics

5.11. Quarks, Gluons, and Intermediate Bosons 115
Table 5.7: Leptons and Quarks. From PDG † .
Leptons Quarks
Charge Mass Charge Mass ‡
(e) eV/c 2 Flavor (e) MeV/c 2
ν1 0 < 2 u up 2/3 3
e −1 5.1 × 10 5 d down −1/3 6
ν2 0 < 2 c charmed 2/3 1.3 × 10 3
µ −1 1.1 × 10 8 s strange −1/3 110
ν3 0 < 2 t top 2/3 1.8 × 10 5
τ −1 1.8 × 10 9 b bottom −1/3 4.2 × 10 3
† We show the mass eigenstates. As will be shown in Chapter 11,
the weak eigenstates are linear combinations of the latter.
‡ The masses for the quarks are only approximate because they
are deduced from composite states in which their strong interactions
have to be taken into account. All quarks come in three
colors.
Since no free quarks are available, their masses cannot be measured and the
mass estimates in Table 5.7 are based on theoretical arguments. (38)
Quarks have another remarkable property, color! Each quark comes in three
colors, red, green, and blue. Of course, flavor and color have nothing to do with
taste or vision; they are names chosen to describe previously unknown but welldefined
physical properties. While flavor denotes the type of quark (u,d,s,...),
color charge refers to a hadronic “charge.” Just as the electric charge characterizes
the strength of a particle’s interaction with an electromagnetic field [Eq. (5.11)],
color charge represents its interaction with the hadronic field of force. Antiquarks,
like quarks, also have three colors, antired, antigreen, and antiblue. Since no colored
particle has ever been observed, the combinations in (5.55) must be colorless or
white. Consequently, a proton can, for instance, contain a red and a green up
quark and a blue down quark, but not two red u quarks. If you, the reader, at
this point feel you have inadvertently picked up a science fiction story, you are
forgiven. Nature, however, is strange (and charmed) and the concepts introduced
here without justification do make sense. We will justify the concepts later in
more detail. Table 5.8 lists the principal quark composition of some mesons and
baryons.
More particles or quanta emerge when we consider the forces that rule subatomic
physics. In Section 5.8 we told the story of the prediction of the pion as the quantum
mediating the interaction between nucleons. The conviction that no action at a
distance exists and that all forces are transmitted by quanta (24) leads to the quanta
listed in Table 5.9.
38 J. Gasser and H. Leutwyler, Phys. Rep. 87, 77 (1982).