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Volume 82, 1954-55
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4.1.2. Mesons

A list of mesons so far positively identified (Powell, 1953; U.S.A.E.C, 1953) is shown in the following table:—

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Table of Elementary Particles
Particle. Symbol. Mass Me. Lifetime Seconds. Mode of Decay. Remorks.
Electron e- l Stable — 1897
e+ 1 Stable — Annihilation with e-
Neutron ν° < 002 — — Never directly observed
μ meson μ+ 210 22 ×10-6 e++αν0 1936–38
μ- 210 22 ×10-6 e-+αν0 1936–38
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πmeson π+ 276 2 × 10-8 μ++ν° 1947
π- 276 2 × 10-8 μ-+ν° 1947
π° 262 10-14 αhν 1950
Neutron ν2 ν°2 800 10-9 π++π- 1947
τ Meson τ+ 969 10-9 π++π++π- 1949
τ- 969 10-9 π-+π++π- 1949
χMeson χ+- 1100? 10-9 π+-+neutral 1952 Charge Unknown
κMeson κ+ 1300? 10-9 μ++2neutral 1911
Proton p+ 1836 Stable — 1911
Neutron n 1836 720 p+e-+ν° 1932
Neutral ν1 ν1° 2200 10-9 p++ν- Produced with cosmotion 1951

There have been reports of other particles u±, S, V± but the evidence for then existence is not as convincing as for the above particles. (See Le Prince-Renguet, L., and Rossi, B., 1953).

The μ meson is not produced directly from the nucleus and has only a weak reaction with it. It arises from the decay of other mesons—directly from the π and κ and indirectly from the others.

The π, κ, χ and τ mesons interact strongly with the nucleus with approximately geometrical cross sections; they may be regarded as types of quanta of the nuclear field. The Compton wavelength of the π meson h/mu has a value (10-13 m) of the same order as the range of the nuclear field. If we regard the heavier mesons as “heavy quanta” it would be reasonable to assume that the field is made up of a number of components with different smaller ranges defined by the Compton wavelength of the different particles. Certainly these particles play an important part in nuclear reactions of protons in the 5 Bev range. The immediate problem is to establish accurate values for masses and lifetimes, critical energies for production and modes of decay. The Bev accelerating machines are already beginning to provide this information and with their controlled beams offer a more systematic attack than cosmic ray studies except at the highest energies.

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