Subatomic Physics

10.10. The Photon–Hadron Interaction: Real and Spacelike Photons 317
10.10 The Photon–Hadron Interaction: Real and Spacelike Photons
Are there not other original properties of the rays of light, besides those
already described?
Newton, Opticks
The interaction of real photons with hadrons at low and moderate photon energies
(say below 20 MeV) has formed a considerable part of nuclear physics for at least
40 years. One example, multipole radiation, was sketched in Section 10.5. Another
celebrated case is the photodisintegration of the deuteron,
γd −→ pn,
which was discovered in 1934 by Chadwick and Goldhaber (16) and used by them
for a measurement of the neutron mass. A third example is the exploration of the
excited states of nuclei with incident gamma rays. The cross section for gamma-ray
absorption shows the existence of individual excited states and the occurrence of
the giant dipole resonance. (17) The basic features of the resulting cross section have
already been given in Fig. 5.34. Such studies produce a great deal of information
concerning nuclear structure, but they teach us little new about the nature of the
photon–hadron interaction: The photon interacts with the electric charges and
currents in the nucleus. The distributions of the charges and currents are determined
by the strong force. If they are assumed to be given, then the interaction with the
probing photon can be described by the Hamiltonian 10.48. Below, say, 100 MeV
incident photon energy, this behavior can be understood: the (reduced) photon
wavelength is of the order 2 fm or longer, short enough to probe some details of
the nuclear charge and current distributions but not short enough to probe the
photon–nucleon interaction. (18)
The interaction of high-energy photons (E ≥ a few GeV) with hadrons presents
a different picture and new aspects emerge: the photon shows hadron-like properties.
(19) The roots of these properties can be understood with concepts that have
been introduced earlier. In Section 3.3, the production of real electron–positron
pairs by real photons was mentioned. In the previous section, it was found that
timelike photons can produce hadrons, as indicated in Figs. 10.21 and 10.22. To
describe the high-energy behavior of real photons, we now consider such processes in
16J. Chadwick and M. Goldhaber, Proc. Roy. Soc. (London) A151, 479 (1935).
17K.A. Snover, Ann. Rev. Nucl. Part. Sc. 36, 545 (1986); J.J. Gaardhoje, Ann. Rev. Nucl.
Part. Sc. 42, 483 (1992).
18It has been shown by various calculations that the scattering of photons in the limit of zero
photon energy is given entirely by the static particle properties, mass, charge, and higher moments.
The hadron structure dynamics does not enter, and the limit agrees with the classical result. W.
Thirring, Phil. Mag. 41, 1193 (1950); F. E. Low, Phys. Rev. 96, 1428 (1954); M. Gell-Mann and
M.L. Goldberger, Phys. Rev. 96, 1433 (1954).
19L. Stodolsky, Phys. Rev. 18, 135 (1967); S.J. Brodsky and J. Pumplin, Phys. Rev. 182, 1794
(1969); V.N. Gribov, Sov. Phys. JETP 30, 709 (1970); D.R. Yennie, Rev. Mod. Phys. 47, 311
(1975); T.H. Bauer et al., Rev. Mod. Phys. 50, 261 (1978).