STUDY OF SLOW MESONS IN COSMIC RAYS*)

The paper under review is a continuation of a series of works on the study of mesons by means of photographic plates \(^{1,2,3}\). In the preceding papers the following was shown:

1. Mesons of two different masses were investigated: light mesons (\(\mu^\pm\)-mesons) with a mass approximately equal to 200 electron masses (\(m_e\)), and heavy mesons (\(\pi^\pm\)-mesons) with a mass of about \(300\,m_e\).

2. The decay of heavy mesons into \(\mu^\pm\)-mesons (\(\pi \to \mu\)) was established. Here the mesons with mass \(200\,m_e\) (\(\mu^\pm\)-mesons) are the ordinary mesons that make up the penetrating component of cosmic rays. Apparently, they are products of the decay of heavy mesons (\(\pi^\pm\)-mesons) formed directly in the material surrounding the photographic plate.

3. The authors proposed a phenomenological classification of mesons according to their action on the photographic plate:

a) \(\pi\)-mesons — heavy mesons which, upon stopping in the emulsion, give the track of a light \(\mu\)-meson;

b) \(\rho\)-mesons, which stop in the emulsion and give a single track;

c) \(\sigma\)-mesons, which give at the end of their path a “star” of heavy particles (nuclear disintegrations);

d) \(\mu\)-mesons (with mass approximately \(200\,m_e\)), produced in the decay of \(\pi\)-mesons.

The \(\rho\)-mesons include mainly light mesons of both signs (\(\mu^\pm\)-mesons).

The \(\sigma\)-mesons are attributed chiefly to negative heavy mesons (\(\pi^-\)-mesons) and, in small numbers, to negative light mesons (\(\mu^-\)-mesons).

The \(\pi\)-mesons are heavy positive mesons (\(\pi^+\)-mesons). They cannot give a “star” at the end of their path, since owing to Coulomb repulsion they are not captured by a nucleus and therefore do not produce nuclear disintegrations.

The same applies also to positive light mesons (\(\mu^+\)-mesons).

An estimate of the flux of \(\sigma\)- and \(\pi\)-mesons shows that they are produced in approximately equal numbers. It is of great interest to measure the lifetime of heavy mesons. In the preceding article a lower limit was given for the lifetime of such mesons (of the order of \(10^{-8}\)–\(10^{-9}\) sec). By radio-technical means (as for light mesons) it is difficult to determine such a short lifetime. Therefore the method of determining the time of life of heavy mesons from the number decaying over a definite segment of path is the simplest. This method was used in the present work. For this purpose the apparatus shown in the figure was employed.

) Catterini, C. Powell, H. Muirhead and Ritson, Nature 162*, 438 (1948).

The photographic plates were arranged vertically in a box of thin iron (0.5 mm thick) and were located at a distance of 2 m from the floor and 60 cm below a ceiling made of light material. On all lateral sides they were surrounded by lead 5 cm thick. On the plates, tracks of various types of mesons stopping in the emulsion were recorded. Mesons of different types and their angular distribution were studied with the apparatus surrounded by lead and without it.

Ilford C-2 plates (containing boron) were used.

The experiments led to the following results:

There exists a sharply expressed vertical flux of $\rho$-mesons. From their angular distribution and from the magnitude of the flux it is evident that these $\rho$-mesons correspond to the ordinary mesons, of mass $200\,m_e$, of the penetrating component of cosmic rays, whose ranges end near the apparatus.

Taking into account the scattering of $\rho$-mesons in the photographic plate makes the picture of the angular distribution still sharper (the predominance of the vertical and opposite flux in the angular distribution is strengthened).

Along with the vertical flux of $\rho$-mesons there exists an opposite flux of the same particles.

Experiments with lead show that very few $\rho$-mesons recorded by the photographic plate are formed in lead.

For $\sigma$-mesons and $\pi$-mesons, when the apparatus is surrounded by lead, on the contrary, a large increase in their number is observed.

If one assumes that in light and heavy substances heavy mesons are formed to an equal extent (as is shown by experiments in which plates were placed directly on lead of great thickness), then, on the basis of the above, one may assert that heavy mesons are formed directly in the substance and have a short lifetime.

Indeed, when the lead is close to the plates, the heavy mesons do not have time to decay; without lead their number is considerably smaller, since they have already had time to decay in the air.

Hence the authors conclude that all or almost all $\rho$-mesons are $\mu^{\pm}$-mesons formed in the decay of $\pi^{\pm}$-mesons.

Considering the $\rho$-mesons of the opposite flux to be secondary light mesons ($\mu^{\pm}$), formed in the decay in flight of heavy $\pi^{\pm}$-mesons, one can determine the lifetime of heavy mesons. Comparing the number of $\pi^{\pm}$ and $\mu^{\pm}$ mesons of the opposite flux, the lifetime of heavy mesons was obtained as

$$ \tau_{\pi}=6\pm3\cdot10^{-9}\ \text{sec}. $$

If one rejects the assumption that $\mu^{\pm}$-mesons are not produced directly in the substance surrounding the photographic plate, then for $\tau_{\pi}$ one obtains two limits—

$$ 3\cdot10^{-9}\ \text{sec.}<\tau_{\pi}<1.5\cdot10^{-8}\ \text{sec.} $$

G. Guro

CITED LITERATURE

1. C. Powell, H. Muirhead and C. Lattes, Nature 159, 694 (1947).
2. C. Lattes, G. Occhialini, C. Powell, Nature 160, 453 (1947).
3. G. Occhialini and C. Powell, Nature 162, 168 (1948).