The urge and the interest to find those ultimate elements in terms of which everything else is made of are almost as old as the human civilization. However, as our knowledge increases what were thought to be elementary may turn out to be composites. Consequently, the class of these supposedly fundamental elements changes with time. Such was, for example, the periodic table of atoms in the last century. Today we know that all different molecules, atoms, and nuclei are complexes resulting from the existence and the interactions of some thirty particles which are called “elementary particles”.

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J. J.
Thomson
,
Phil. Mag.
46
,
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(
1898
).
An interesting and personal account of this discovery was given by Thomson in his book Recollections and Reflections (G. Bell and Sons, Ltd., 1936).
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A.
Einstein
,
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17
,
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(
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P. A. M.
Dirac
,
Proc. Roy. Soc.
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1929
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J. R.
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,
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35
,
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(
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H. Weyl, Gruppentheorie und Quantenmechanik (1931), 2nd ed., p. 234.
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I.
Tamm
,
Z. Physik
62
,
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(
1930
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See also
P. A. M.
Dirac
,
Proc. Cambridge Phil. Soc.
26
,
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(
1930
);
P. A. M.
Dirac
,
Proc. Roy. Soc.
A133
,
60
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1931
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C. D.
Anderson
,
Science
76
,
238
(
1932
);
C. D.
Anderson
,
Phys. Rev.
43
,
491
(
1933
).
Compare also Blackett and Occhialini,
Proc. Roy. Soc.
A139
,
699
(
1933
).
8.
J.
Chadwick
,
Proc. Roy. Soc.
A136
,
692
(
1932
).
9.
The antiproton was first observed by
Chamberlain
,
Segrè
,
Wiegand
, and
Ypsilantis
,
Phys. Rev.
100
,
947
(
1955
).
The antineutron was first observed by
Cork
,
Lambertson
,
Piccioni
, and
Wenzel
,
Phys. Rev.
104
,
1193
(
1956
).
10.
Proceedings of Solvay Congress, Brussels (1933). While the possible existence of a neutral particle in beta decay was first suggested by Pauli at the American Physical Society Meeting in Pasadena in 1931, serious discussions of its existence and its properties did not appear in any literature until 1933.
11.
E.
Fermi
,
Z. Physik
88
,
161
(
1934
).
12.
For a detailed discussion of these experiments see, e.g., C. S. Wu, The Neutrino, Memorial Volume to Wolfgang Pauli (Pergamon Press, 1960)
and J. S. Allen, The Neutrino (Princeton University Press, 1958).
13.
Cowan
,
Reines
,
Harrison
,
Kruse
, and
McGuire
,
Science
124
,
103
(
1956
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H.
Yukawa
,
Proc. Phys.‐Math. Soc. Japan
17
,
48
(
1935
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Anderson
and
Neddermeyer
,
Phys. Rev.
51
,
884
(
1937
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16.
Street
and
Stevenson
,
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51
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1937
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17.
Convincing evidence of this was given by the experiment done by
Conversi
,
Pancini
, and
Piccioni
,
Phys. Rev.
71
,
209
(
1947
);
and by the theoretical analysis made by
Fermi
,
Teller
, and
Weisskopf
,
Phys. Rev.
71
,
314
(
1947
)
and by
Fermi
and
Teller
,
Phys. Rev.
72
,
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1947
).
18.
Lattes
,
Occhialini
, and
Powell
,
Nature
160
,
453
(
1947
).
19.
Such possibilities have been discussed theoretically, prior to the discovery of π meson, by
Sakata
and
Ionue
,
Progr. Theoret. Physics (Kyoto)
1
,
143
(
1946
)
and by
Marshak
and
Bethe
,
Phys. Rev.
72
,
506
(
1947
).
20.
Bjorklund
,
Crandall
,
Moyer
, and
York
,
Phys. Rev.
77
,
213
(
1950
);
Steinberger
,
Panofsky
, and
Steller
,
Phys. Rev.
78
,
802
(
1950
).
21.
Rochester
and
Butler
,
Nature
160
,
855
(
1947
).
22.
Brown
,
Camerini
,
Fowler
,
Muirhead
,
Powell
, and
Ritson
,
Nature
163
,
47
(
1949
).
23.
For a detailed list of references of these discoveries see e.g.,
Franzinetti
and
Morpurgo
,
Nuovo cimento Supplement
6
, No.
2
,
469
(
1957
).
24.
A.
Pais
,
Phys. Rev.
86
,
633
(
1952
).
25.
M.
Gell‐Mann
,
Phys. Rev.
93
,
933
(
1953
).
26.
Nakano
and
Nishijima
,
Progr. Theoret. Phys. (Kyoto)
10
,
581
(
1953
).
27.
D. A.
Glaser
,
Phys. Rev.
87
,
665
(
1952
).
28.
Alvarez
,
Eberhard
,
Good
,
Grazino
,
Ticko
, and
Wojcicki
,
Phys. Rev. Letters
2
,
215
(
1959
).
29.
The λ̄ particle was first observed by
Prowse
and
Baldo Ceolin
,
Nuovo cimento
10
,
635
(
1958
).
30.
In Fig. 3 (and also in Table 2) it is shown that, unlike the neutral π meson, the K ° particle is different from its antiparticle K̄ °. This was first predicted by
Gell‐Mann
and
Pais
,
Phys. Rev.
97
,
1387
(
1955
).
Furthermore, it can be shown that these two neutral particles, K ° and K̄ °, possess some extremely interesting properties. For example, a K ° particle once produced may later change automatically into its antiparticle K̄ °. Such unusual conversions have recently been observed by
Muller
,
Birge
,
Fowler
,
Good
,
Hirsch
,
Matsen
,
Oswald
,
Powell
,
White
, and
Piccioni
,
Phys. Rev. Letters
4
,
418
(
1960
).
31.
R. H.
Dalitz
,
Phil. Mag.
44
,
1068
(
1953
);
R. H.
Dalitz
,
Phys. Rev.
94
,
1046
(
1954
).
E.
Fabri
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Nuovo cimento
11
,
479
(
1954
).
32.
Lee
and
Yang
,
Phys. Rev.
104
,
254
(
1956
).
33.
Wu
,
Ambler
,
Hayward
,
Hoppes
, and
Hudson
,
Phys. Rev.
105
,
1413
(
1957
).
34.
Garwin
,
Lederman
, and
Weinrich
,
Phys. Rev.
105
,
1415
(
1957
).
35.
Friedman
and
Telegdi
,
Phys. Rev.
105
,
1681
(
1957
).
36.
The possibility of a two‐component relativistic theory of a spin 12 particle was first discussed by
H.
Weyl
,
Z. Physik
56
,
330
(
1929
).
However, in such a theory parity is not manifestly conserved; therefore it was always rejected before the discovery of right‐left asymmetry. (Cf. W. Pauli, Handbuch der Physik, Verlag Julius Springer, Berlin, 1933, Vol. 24, pp. 226–7.)
The possible use of this two‐component theory for expressing the nonconservation property of parity in neutrino processes was independently considered by
Lee
and
Yang
,
Phys. Rev.
105
,
1671
(
1957
);
A.
Salam
,
Nuovo cimento
5
,
299
(
1957
);
and
L.
Landau
,
Nuclear Phys.
3
,
127
(
1957
).
37.
The first conclusive proof that neutrino behayes like a left‐hand screw (and not like a right‐hand screw) was given by
Goldhaber
,
Grodzins
, and
Sunyar
,
Phys. Rev.
109
,
1015
(
1958
).
38.
Lee
,
Oehme
, and
Yang
,
Phys. Rev.
106
,
340
(
1957
).
39.
C. N.
Yang
, International Congress on Theoretical Physics, Seattle, 1956,
Revs. Modern Phys.
29
,
231
(
1957
);
Lee
and
Yang
,
Phys. Rev.
105
,
1671
(
1957
).
This possibility was also considered by
L.
Landau
,
Nuclear Phys.
3
,
127
(
1957
),
and by
E. P.
Wigner
,
Revs. Modern Phys.
29
,
255
(
1957
).
See also footnote (9) of an earlier paper by
Wick
,
Wightman
, and
Wigner
,
Phys. Rev.
88
,
101
(
1952
).
40.
G.
Puppi
,
Nuovo cimento
5
,
587
(
1948
);
O.
Klein
,
Nature
161
,
897
(
1948
);
Lee
,
Rosenbluth
, and
Yang
,
Phys. Rev.
75
,
905
(
1949
);
Tiomno
and
Wheeler
,
Revs. Modern Phys.
21
,
144
(
1949
).
See also the discussions by Serber and Oppenheimer in the Proceedings of Solvay Congress, Brussels (1948).
41.
More recent analysis on Universal Fermi Interactions were made by
Feynman
and
Gell‐Mann
,
Phys. Rev.
109
,
193
(
1958
);
Sudarshan
and
Marshak
,
Phys. Rev.
109
,
1860
(
1958
);
J. J.
Sakurai
,
Nuovo cimento
7
,
649
(
1958
).
42.
See, specially
Feynman
and
Gell‐Mann
,
Phys. Rev.
109
,
193
(
1958
).
43.
Laotse, Tao Té Chin, p. 1 (about 550 B. C.).
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