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particles. Gell-Mann called these particles quarks from the phrase “three quarks for

Muster Mark” that appears in James Joyce’s novel Finnegan’s Wake. The original three

quarks were called up (symbol u), down(d), and strange(s); whereas uand dquarks

have the strangeness number S0, the squark has S1 (Table 13.4).

Because each baryon (B1) is made up of three quarks, the baryon number of a

quark must be B^13 . Antibaryons (B1) are made up of three antiquarks, so the

baryon number of an antiquark must be B^13 . Mesons, for which B0, consist

of a quark and an antiquark. Quarks all have spins of ^12 , which accounts for the observed

half-integral spins of baryons and the 0 or integral spins of mesons.

In order for hadrons to have charges of 0 or integral multiples of e, the various

quarks must have the fractional charges shown in Table 13.4. No other particles in

nature have fractional charges, which made the quark hypothesis hard to accept at first,

but soon the evidence for it proved overwhelming. The most direct experiments that

point to the reality of quarks involved the scattering of high-energy (hence short-

wavelength) electrons by protons, which revealed that there are indeed three pointlike

concentrations of charge inside a proton. Quarks are thought to be elementary in the

same sense as leptons, essentially point particles with no internal structures. Figure 13.9

shows the quark compositions of the hadrons of Fig. 13.5 and Table 13.5 details how

the properties of several hadrons are derived from those of the quarks they contain.

Figure 13.10 illustrates the quark models of nucleons and antinucleons.

490 Chapter Thirteen

Table 13.4 Quarks. All have spin ^12 and baryon number B^13 . Antiquarks have

charges that are the negatives of those shown and baryon number B^13 .

The strange antiquark has a strangeness number of S1.

Quark Symbol Mass, GeV/c^2 Charge, e Strangeness

Up u 0.3 ^23  0

Down d 0.3 ^13  0

Strange s 0.5 ^13   1

Charmed c 1.5 ^23  0

To p t 174 ^23  0

Bottom b 4.3 ^13  0

u

dd

d

uu

s

uu

s

ud

s

ud

u

s s

d

s s

s

− (^1) dd
0
s
− 2
Q −^10 +^1
Figure 13.9Quark compositions of the spin ^12 baryons shown in Fig. 13.5.
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