Subatomic Physics

6.5. Leptons Are Point Particles 147
Figure 6.5: Probability distribution for 40 Ca and 208 Pb, as obtained from electron scattering.
[From I. Sick. Phys. Lett. 88B, 245 (1979).]
to many excited nuclear states have been examined. (14) We must, however, remember
that the information provided by charged lepton scattering concerns the
nuclear charge and current distributions and that corresponding data on hadronic
structure (matter distribution) require a different probe, such as hadrons (15) or the
weak interaction of electrons. (16)
6.5 Leptons Are Point Particles
We return now to the g factor of the electron. By 1926, the idea of the spinning
electron and its magnetic moment was generally accepted, (17) but the value of the
g factor (Eq. (5.16)),
g(1926) = −2,
had to be taken from experiment. (The minus sign indicates that the magnetic
moment points in the direction opposite to the spin for a negative electron.) It was
exactly twice as large as the g factor for orbital motion, Eq. (5.14). In other words,
even though the electron has spin 1
2
, it carries one Bohr magneton. In 1928, Dirac
14 J.HeisenbergandH.P.Blok,Annu. Rev. Nucl. Part. Sci. 33, 569 (1983).
15 A.W. Thomas, Nucl. Phys. A354, 51c (1981); R. Campi, Nucl. Phys. A374, 435c (1982).
16 C.J. Horowitz, S.J. Pollock, P.A. Souder, and R. Michael, Phys. Rev. C 63, 025501 (2001).
17 A fascinating description of the history of the spin is presented by B.L. Van der Waerden,
in Theoretical Physics of the Twentieth Century (M. Fierz and V.F. Weisskopf, eds.), Wiley-
Interscience, New York, 1960. See also S.A. Goudsmit, Phys. Today 14, 18 (June 1961) and P.
Kusch, Phys. Today 19, 23 (February 1966).