Subatomic Physics

316 The Electromagnetic Interaction
R = �
qi 2 , (10.92)
because all other factors cancel. The sum in Eq. (10.92)
extends over all quark species with mass less than
W/(2c2 ). For W < 6GeV, three quarks can be produced,
u, d, ands, with charges 2/3, −1/3, and −1/3,
respectively (See Table 5.7), so that Eq. (10.92) gives
R = 2/3. However, Fig. 10.22 shows that Rexp = 2!
The discrepancy is explained through the introduction
of color, as stated in Section 5.11. If each quark comes in
three colors, R is given by 3q2 i =2foru, d, andsquarks in agreement with experiment. Above the threshold for
J/ψ production there is a fourth quark of charge 2/3,
and above the upsilon threshold five known quark flavor
u, d, s, c, andbare present; with color, we then expect
R to be
� � �2 � � �
2
2 −1
R =3 2 +3 =
3 3
11
3 .
Above the threshold for ¯tt production, we have to include
the charge of this quark. The data in Fig. 10.23
agree approximately with this prediction. The ratio R
thus provides strong evidence for two crucial properties
of quarks, their point-like nature and their color.
Figure 10.24: The Drell–Yan
process for µ + µ − production
in pion–proton scattering.
i
Figure 10.23: Production of
(a) resonance and (b) of “individual”
quark pair. In (b),
the second qq pair shown is
required to produce observable
mesons.
We have discussed primarily experiments in which
lepton pairs annihilate and produce hadrons. The
reverse experiment is also feasible, and is called
a Drell–Yan reaction. (14) In a typical Drell–Yan
process, a high energy pion collides with a proton.
The antiquark in the pion annihilates a quark in
the proton to produce a virtual photon, which creates
a lepton pair, as illustrated in Fig. 10.24. The
process has proven useful for studies of QCD. (15)
14S.D. Drell and T.M. Yan, Phys. Rev. Lett. 25, 316 (1970); 24, 181 (1970); Ann. Phys. (New
York ) 66, 578 (1971).
15P.N. Harriman, Z. Phys. C55, 449 (1992); P.D. Morley, Phys. Rev. C 39,708 (1989); G.L. Li,
J.P. Shen, J.J. Yang, H.Q. Shen, Phys. Rept. 242, 505 (1994).