Elementary Particles 491

Table 13.5Compositions of some hadrons according to the quark model

Quark Baryon

Hadron Content Number Charge, e Spin Strangeness

 ud ^13 ^13  

0 ^23 ^13 

 1 ↑↓ 00  0

 0

K us ^13 ^13  

0 ^23 ^13 

 1 ↑↓ 00  1 

 1

p uud ^13 ^13 ^13 

 1 ^23 ^23 ^13 

 1 ↑ ↑↓ ^12  0  0  0

 0

n^0 ddu ^13 ^13 ^13 

 1 ^13  ^13 ^23  

0 ↓↓ ↑ ^12  0  0  0

 0

 sss ^13 ^13 ^13 

 1 ^13  ^13 ^13 

 1 ↑↑↑ ^32   1  1  1  3

+ 2 –

3

=

Antineutron

=

Antiproton

• 2 –
  3


• 2 –
  3

=

uu

d

Proton

• 1 –

(^3) + 2 –
3

dd
u
Neutron

• 2 –
  (^3) – 1 –
  3

• 1 –
  3

uu

d+ 1 –

3

dd

u– 2 –

(^3) + 1 –
3

• 1 –
  3
  +1
  0 0

• 1

Figure 13.10Quark models of the

proton, antiproton, neutron, and

antineutron. Electric charges are

given in units of e.

Color

A serious problem with the idea that baryons are composed of quarks was that the

presence of two or three quarks of the same kind in a particular particle (for instance,

two uquarks in a proton, three squarks in an baryon) violates the exclusion

principle. Quarks ought to be subject to this principle since they are fermions with

spins of ^12 . To get around this problem, it was suggested that quarks and antiquarks

have an additional property of some kind that can be manifested in a total of six

different ways, rather as electric charge is a property that can be manifested in the

two different ways that have come to be called positive and negative. In the case of

quarks, this property became known as “color,” and its three possibilities were called

red, green, and blue. The antiquark colors are antired, antigreen, and antiblue.

According to the color hypothesis, all three quarks in a baryon have different colors,

which satisfies the exclusion principle since all are then in different states even if two

or three are otherwise identical. Such a combination can be thought of as white by

analogy with the way red, green, and blue light combine to make white light (but

there is no connection whatever except on this metaphorical level between quark

colors and actual visual colors). Similarly, an antibaryon consists of an antired, an

antigreen, and an antiblue quark. A meson consists of a quark of one color and an

antiquark of the corresponding anticolor, which has the effect of canceling out the

color. The result is that

Hadrons and antihadrons are colorless.

Quark color is thus a property that is significant within hadrons but is never directly

observable in the outside world.

The notion of quark color is more than just a way around the exclusion principle.

For one thing, it has turned out to be the key to explaining why the neutral pion has

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