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

, 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 ‘‘

–

’’ form.

‘‘Meson Corrections in the Theory of Beta Decay,’’ S. S. Gershtein

and Ia. B. Zel’dovich, Zh. E´ ksp. Teor. Fiz.

, 698–699

~

1955

! @

Sov.

Phys.—JETP

, 576–578

~

1956

!#

.

~

A

!

‘‘Question of Parity Conservation in Weak Interactions,’’ T. D. Lee

and C. N. Yang, Phys. Rev.

, 254–258

~

1956

!

.

~

A

!

‘‘Parity Nonconservation and a Two Component Theory of the Neu-

trino,’’ T. D. Lee and C. N. Yang, Phys. Rev.

, 1671–1675

~

1957

!

.

~

A

!

‘‘Remarks on Possible Noninvariance Under Time Reversal and

Charge Conjugation,’’ T. D. Lee, R. Oehme, and C. N. Yang, Phys.

Rev.

, 340– 345

~

1957

!

.

~

A

!

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Mann, Phys. Rev.

, 193–198

~

1958

!

.

~

I

!

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Sudarshan and R. E. Marshak, Phys. Rev.

, 1860–1862

~

1958

!

.

~

A

!

Yukawa

167

and Klein

168

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 Glashow

169

realized that a new

heavy boson

, in addition to the massless photon

and massive charged bosons, was needed to successfully

unify the weak and electromagnetic interactions. The use of

the Higgs

170–173

mechanism to break the electroweak sym-

metry by Weinberg

174

and Salam

175

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

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

boson can be

arranged to cancel these divergences. It leads to

, in which

~

for example

!

an incident neutrino

scatters inelastically on a hadronic target without changing

its charge. The discovery of neutral-current interactions of

neutrinos

177–180

and other manifestations of the

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 (

,

, and

) was the presence of

. The hypothesis of a fourth ‘‘charmed’’ quark

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

/

c

particle,

185,186

a bound state of

and

. An earlier Resource Letter

75

deals with events lead-

ing up to this discovery, as well as early evidence for the fifth

(

) 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

!

.

‘‘On the Interaction of Elementary Particles,’’ H. Yukawa, Proc. Phys.

Math. Soc. Japan

, 48– 57

~

1935

!

.

~

A

!

‘‘Sur la The´orie des Champs Associe´s a` des Particules Charge´es,’’ O.

Klein, in

, Paris, Inst. de

Coo¨ peration Intellectuelle

~

1939

!

, pp. 81–98, translation ‘‘On the

Theory of Charged Fields,’’ reprinted in

, Vol. 1, edited by G. Ekspong

~

World Scientific, Singapore,

1991

!

, and in Surveys in High Energy Phys.

, 269–285

~

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!

.

~

A

!

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Phys.

, 579–588

~

1961

!

.

~

A

!

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Higgs, Phys. Lett.

, 132–133

~

1964

!

.

~

A

!

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Phys. Rev. Lett.

, 508–509

~

1964

!

.

~

A

!

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glert and R. Brout, Phys. Rev. Lett.

, 321–322

~

1964

!

.

~

A

!

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C. R. Hagen, and T. W. B. Kibble, Phys. Rev. Lett.

, 585–587

~

1964

!

.

~

A

!

‘‘A Model of Leptons,’’ S. Weinberg, Phys. Rev. Lett.

, 1264–1266

~

1967

!

.

~

A

!

‘‘Weak and Electromagnetic Interactions,’’ A. Salam, in

, edited by N. Svartholm

~

Almqvist

and Wiksell, Stockholm, 1968; Wiley, New York, 1978

!

, pp. 367–

377.

~

A

!

‘‘The Axial Vector Current in Beta Decay,’’ M. Gell-Mann and M.

Le´vy, Nuovo Cim.

, 705–726

~

1960

!

.

~

I

!

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, Phys. Lett.

, 121–124

~

1973

!

.

~

I

!

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tron in the Gargamelle Neutrino Experiment,’’ F. J. Hasert

,

Phys. Lett.

, 138–140

~

1973

!

.

~

I

!

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tron in the Gargamelle Neutrino Experiment,’’ F. J. Hasert

,

Nucl. Phys.

, 1–22

~

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!

.

~

I

!

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A. C. Benvenuti

, Phys. Rev. Lett.

, 800–803

~

1974

!

.

~

I

!

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J. Ilipoulos, and L. Maiani, Phys. Rev. D

, 1285–1292

~

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!

.

~

I

!

‘‘An Anomaly Free Version of Weinberg’s Model,’’ C. Bouchiat, J.

Iliopoulos, and P. Meyer, Phys. Lett.

, 519–523

~

1972

!

.

~

I

!

‘‘Gauge Theories Without Anomalies,’’ H. Georgi and S. L. Glashow,

Phys. Rev. D

, 429–431

~

1972

!

.

~

I

!

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Gross and R. Jackiw, Phys. Rev. D

, 477–493

~

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!

.

~

A

!

‘‘Experimental Observation of a Heavy Particle

,’’ J. J. Aubert

,

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, 1404–1406

~

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!

.

~

I

!

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1

2

Annihilation,’’ J. E.

Augustin

, Phys. Rev. Lett.

, 1406–1408

~

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!

.

~

I

!

The symmetries of time reversal

~

!

, charge conjugation

~

!

, and space inversion or parity

~

!

have provided both

clues and puzzles in our understanding of the fundamental

interactions. The realization that the charge-changing weak

interactions violated P and

maximally was central to the

formulation of the

–

theory. The theory was constructed

in 1957 to conserve the product

, but the discovery in

310

310

Am. J. Phys., Vol. 71, No. 4, April 2003

Jonathan L. Rosner

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