Subatomic Physics

6.6. Nucleon Elastic Form Factors 153
Figure 6.9: Strong and weak correction terms that appear in the interaction of a charged lepton
with photons.
quantum electrodynamics predicts all observed phenomena correctly if proper theoretical
corrections, such as the strong vacuum polarization shown in Fig. 6.9, are
carried out. Measurements in colliding beam experiments, in particular,
e − e + −→ e − e + , e − e + −→ µ − µ + , and e − e + −→ τ − τ +
show that QED holds to distances smaller than about 10 −18 m. (28) We consequently
cannot yet answer the question raised by the incredible success of QED: Will the
theory break down, and if so, at what scale?
6.6 Nucleon Elastic Form Factors
By 1932 it was well known that electrons have spin 1
2 and a magnetic moment of
1µB, (Bohr magneton), as predicted by the Dirac equation. Two other spin- 1
2 particles
were also known to exist, the proton and the neutron. It was firmly believed
that these would also have magnetic moments as predicted by the Dirac equation,
one nuclear magneton for the proton and zero moment for the neutron. Enter Otto
Stern. Stern had principles in selecting his experiments: “Try only crucial experiments.
Crucial experiments are those that test universally accepted principles.”
When he started setting up equipment to measure the magnetic moment of the
proton, his friends teased him and told him that he should not waste his time on an
experiment whose outcome was foreordained. The surprise was great when Stern
and his collaborators found a magnetic moment of about 2.5 µN for the proton and
about −2 µN for the neutron. (29)
How can the departure of the magnetic moments of the proton and the neutron
from the “Dirac values” be understood? Before quarks were introduced, the explanation
of the anomalous magnetic moments of the nucleons was based on virtual
28K.G. Gan and M.L. Perl, Int. J. Mod. Phys. A3, 531 (1988).
29I. Estermann, R. Frisch, and O. Stern, Nature 132, 169 (1933); R. Frisch and O. Stern, Z.
Physik 85, 4 (1933).