higgs - page 25

D. Weak interactions
The electromagnetic interaction is described in terms of
photon exchange. The quantum electrodynamics of photons
and electrons initially encountered divergent quantities
tamed in the 1940s through renormalization, leading to suc-
cessful estimates of the anomalous magnetic moment of the
electron and the Lamb shift in hydrogen.86 By contrast, the
weak interactions as formulated up to the mid-1960s in-
volved the pointlike interactions of two currents. This inter-
action is very singular and cannot be renormalized. The weak
currents in this theory were purely charge-changing. As a
result of work by Gershtein and Zel’dovich ~who suggested
that the weak vector current is of universal strength!,161 Lee
and Yang,162–164 Feynman and Gell-Mann,165 and Sudarshan
and Marshak,166 the weak currents were identified as having
~vector! –~ axial! or ‘‘V – A ’’ form.
161. ‘‘Meson Corrections in the Theory of Beta Decay,’’ S. S. Gershtein
and Ia. B. Zel’dovich, Zh. E´ ksp. Teor. Fiz. 29, 698–699 ~1955! @Sov.
Phys.—JETP 2, 576–578 ~1956!#. ~A!
162. ‘‘Question of Parity Conservation in Weak Interactions,’’ T. D. Lee
and C. N. Yang, Phys. Rev. 104, 254–258 ~1956!. ~A!
163. ‘‘Parity Nonconservation and a Two Component Theory of the Neu-
trino,’’ T. D. Lee and C. N. Yang, Phys. Rev. 105, 1671–1675 ~1957!.
~A!
164. ‘‘Remarks on Possible Noninvariance Under Time Reversal and
Charge Conjugation,’’ T. D. Lee, R. Oehme, and C. N. Yang, Phys.
Rev. 106, 340– 345 ~1957!. ~A!
165. ‘‘Theory of the Fermi Interaction,’’ R. P. Feynman and M. Gell-
Mann, Phys. Rev. 109, 193–198 ~1958!. ~I!
166. ‘‘Chirality Invariance and the Universal Fermi Interaction,’’ E. C. G.
Sudarshan and R. E. Marshak, Phys. Rev. 109, 1860–1862 ~1958!.
~A!
E. Electroweak unification
Yukawa167 and Klein168 proposed early boson-exchange
models for the charge-changing weak interactions. Klein’s
model had self-interacting bosons, thus anticipating the
theory of Yang and Mills.147 Schwinger and others studied
such models in the 1950s, but Glashow169 realized that a new
neutral heavy boson Z, in addition to the massless photon
and massive charged bosons, was needed to successfully
unify the weak and electromagnetic interactions. The use of
the Higgs170–173 mechanism to break the electroweak sym-
metry by Weinberg174 and Salam175 converted this phenom-
enological theory into one suitable for higher-order calcula-
tions.
The charge-changing weak currents could be viewed as
members of an SU~2! algebra.176,143 However, the neutral
member of this multiplet could not be identified with electric
charge. Charged W 6 bosons couple only to left-handed fer-
mions, while the photon couples to both left and right-
handed fermions. Moreover, a theory with only photons and
charged weak bosons leads to unacceptable divergences in
higher-order processes.18 The neutral heavy Z boson can be
arranged to cancel these divergences. It leads to neutral cur-
rent interactions, in which ~for example! an incident neutrino
scatters inelastically on a hadronic target without changing
its charge. The discovery of neutral-current interactions of
neutrinos177–180 and other manifestations of the Z strikingly
confirmed the new theory.
A key stumbling block to the construction of an elec-
troweak theory applying to the quarks known at the time (u ,
d, and s ) was the presence of flavor-changing neutral cur-
rents. The hypothesis of a fourth ‘‘charmed’’ quark c was an
elegant way to avoid this problem.181 The charmed quark
also was crucial in avoiding ‘‘anomalies,’’ effects due to tri-
angle diagrams involving internal fermions and three exter-
nal gauge bosons.182–184 Evidence for charm was first found
in 1974 in the form of the J / c particle,185,186 a bound state of
c and c¯. An earlier Resource Letter75 deals with events lead-
ing up to this discovery, as well as early evidence for the fifth
( b ) quark to be mentioned below. The whole topic of elec-
troweak unification is dealt with at an intermediate level in
several references mentioned earlier ~e.g., Refs. 14, 18, 24! .
167. ‘‘On the Interaction of Elementary Particles,’’ H. Yukawa, Proc. Phys.
Math. Soc. Japan 17, 48– 57 ~1935!. ~A!
168. ‘‘Sur la The´orie des Champs Associe´s a` des Particules Charge´es,’’ O.
Klein, in Les Nouvelles The´ories de la Physique, Paris, Inst. de
Coo¨ peration Intellectuelle ~1939!, pp. 81–98, translation ‘‘On the
Theory of Charged Fields,’’ reprinted in Oskar Klein Memorial
Lectures, Vol. 1, edited by G. Ekspong ~World Scientific, Singapore,
1991!, and in Surveys in High Energy Phys. 5, 269–285 ~1986!. ~A!
169. ‘‘Partial Symmetries of Weak Interactions,’’ S. L. Glashow, Nucl.
Phys. 22, 579–588 ~1961!. ~A!
170. ‘‘Broken Symmetries, Massless Particles, and Gauge Fields,’’ P. W.
Higgs, Phys. Lett. 12, 132–133 ~1964!. ~A!
171. ‘‘Broken Symmetries and the Masses of Gauge Bosons,’’ P. W. Higgs,
Phys. Rev. Lett. 13, 508–509 ~1964!. ~A!
172. ‘‘Broken Symmetry and the Mass of Gauge Vector Mesons,’’ F. En-
glert and R. Brout, Phys. Rev. Lett. 13, 321–322 ~1964!. ~A!
173. ‘‘Global Conservation Laws and Massless Particles,’’ G. S. Guralnik,
C. R. Hagen, and T. W. B. Kibble, Phys. Rev. Lett. 13, 585–587
~1964!. ~A!
174. ‘‘A Model of Leptons,’’ S. Weinberg, Phys. Rev. Lett. 19, 1264–1266
~1967!. ~A!
175. ‘‘Weak and Electromagnetic Interactions,’’ A. Salam, in Proceedings
of the Eighth Nobel Symposium, edited by N. Svartholm ~Almqvist
and Wiksell, Stockholm, 1968; Wiley, New York, 1978!, pp. 367–
377. ~A!
176. ‘‘The Axial Vector Current in Beta Decay,’’ M. Gell-Mann and M.
Le´vy, Nuovo Cim. 16, 705–726 ~1960!. ~I!
177. ‘‘Search for Elastic Muon Neutrino Electron Scattering,’’ F. J. Hasert
et al., Phys. Lett. 46B , 121–124 ~1973!. ~I!
178. ‘‘Observation of Neutrino-Like Interactions Without Muon or Elec-
tron in the Gargamelle Neutrino Experiment,’’ F. J. Hasert et al.,
Phys. Lett. 46B, 138–140 ~1973!. ~I!
179. ‘‘Observation of Neutrino-Like Interactions Without Muon or Elec-
tron in the Gargamelle Neutrino Experiment,’’ F. J. Hasert et al.,
Nucl. Phys. B73 , 1–22 ~1974!. ~I!
180. ‘‘Observation of Muonless Neutrino Induced Inelastic Interactions,’’
A. C. Benvenuti et al., Phys. Rev. Lett. 32, 800–803 ~1974!. ~I!
181. ‘‘Weak Interactions with Lepton–Hadron Symmetry,’’ S. L. Glashow,
J. Ilipoulos, and L. Maiani, Phys. Rev. D 2, 1285–1292 ~1970!. ~I!
182. ‘‘An Anomaly Free Version of Weinberg’s Model,’’ C. Bouchiat, J.
Iliopoulos, and P. Meyer, Phys. Lett. 38B , 519–523 ~1972!. ~I!
183. ‘‘Gauge Theories Without Anomalies,’’ H. Georgi and S. L. Glashow,
Phys. Rev. D 6, 429–431 ~1972!. ~I!
184. ‘‘Effect of Anomalies on Quasi-Renormalizable Theories,’’ D. J.
Gross and R. Jackiw, Phys. Rev. D 6, 477–493 ~1972!. ~A!
185. ‘‘Experimental Observation of a Heavy Particle J,’’ J. J. Aubert et al.,
Phys. Rev. Lett. 33, 1404–1406 ~1974!. ~I!
186. ‘‘Discovery of a Narrow Resonance in e1e2 Annihilation,’’ J. E.
Augustin et al., Phys. Rev. Lett. 33, 1406–1408 ~1974!. ~I!
F. CP violation
The symmetries of time reversal ~ T!, charge conjugation
~C!, and space inversion or parity ~ P! have provided both
clues and puzzles in our understanding of the fundamental
interactions. The realization that the charge-changing weak
interactions violated P and C maximally was central to the
formulation of the V – A theory. The theory was constructed
in 1957 to conserve the product CP, but the discovery in
310 310 Am. J. Phys., Vol. 71, No. 4, April 2003 Jonathan L. Rosner