Subatomic Physics

314 The Electromagnetic Interaction
10.9 Electron–Positron Collisions and Quarks
In Section 7.6, we mentioned the “1974 November Revolution” in which a new longlived
particle, J/ψ was discovered simultaneously in pp scattering at the Brookhaven
National Laboratory and in e + e − collisions at SLAC.
In 1977, another long-lived
particle, the Υ, was found in
p-nucleus collisions at Fermilab.
The J/ψ is interpreted as
a cc state of a charmed quark
with its antiquark. Similarly,
the upsilon is a bb state and
there is a t(t) state as well.
The detailed investigations of
these particles and of some
closely related states in e + e −
collisions yield strikingly simple
and profound results.
As an example, we show
in Fig. 10.21 the total cross
section for the production of
hadrons in e + e − collisions as
a function of the total c.m.
energy W near 3.1 GeV. (13)
Two features stand out, the
very large cross section and
the narrowness of the resonance
peak.
Figure 10.21: Total hadron production cross section in
e + e − collisions near 3.1 GeV and the J/ψ peak. [From
A. M. Boyarski et al., Phys. Rev. Lett. 34, 1357 (1975).]
Muon pair production is described very well by QED. The cross section for
e + e− → µ + µ − (neglecting the muon mass) is given by an equation similar but not
identical to Eq. 10.82 because here the particles in the final state are distinguishable
from the ones in the initial state:
dσ α2
=
dΩ 4s (�c)2 (1 + cos 2 θ), (10.89)
where s = W 2 =(2Ee) 2 is the square of the c.m. energy, Eq. (10.83), and θ is the
c.m. scattering angle. The total cross section is
σ = 4πα2
3s (�c)2 . (10.90)
13 A. M. Boyarski et al., Phys. Rev. Lett. 34, 1357 (1975); R. F. Schwitters and K. Strauch,
Ann. Rev. Nucl. Sci. 26, 89 (1976).