Subatomic Physics

308 The Electromagnetic Interaction
suggested that the two hadrons of the pair should be strongly coupled and form a
vector meson, as shown in Fig. 10.13(b). (10) The photon thus would transform into a
vector meson, as already anticipated in Fig. IV.2. Sakurai made his suggestion long
before the vector mesons were discovered experimentally. Theoretical suggestions
can be useful guides for planning experiments, but only the result of experiments can
provide the clues as to the nature of the interaction between the photon and hadrons.
Three types of experiments that can provide information about the photon-hadron
interaction are illustrated by the Feynman diagrams shown in Fig. 10.14. Two of
these involve virtual photons; the third one is performed with real ones. In all three
cases the object of interest is the photon–hadron vertex. In the present section, we
discuss timelike photons in electron–positron scattering; in Section 10.10, real and
spacelike photons will be treated.
Figure 10.14: Diagrams of three experimental possibilities to study the interaction of photons with
hadrons. Details are discussed in the text.
The virtual photon produced in electron–positron collisions is timelike, as follows
from Eqs. (10.86) and (10.87); in the e − − e + c.m., it has energy but no momentum.
The system of hadrons produced by timelike photons must possess quantum
numbers that are determined by those of the photon. Since the electromagnetic
interaction conserves strangeness, parity, and charge conjugation, only final states
with strangeness 0, negative parity, and negative charge parity can be produced. In
addition, angular momentum conservation requires the final state to have angular
momentum unity. Are there such final states that are produced copiously? The
experiments indicate that hadrons satisfying all conditions are indeed produced.
Consider first Fig. 10.15. It shows the number of pion pairs observed at a given
total energy of the colliding electrons, normalized by division by the number of
electrons observed at the same energy. A pronounced peak appears at about 770
MeV, with a width of about 100 MeV.
The reader with a good memory will say “Aha” and will turn back to Fig. 5.12
where a similar peak is shown at the same energy and with the same width. This