Generation of Mesons in Penetrating Cosmic Ray Showers

In recent years, investigations have repeatedly been undertaken of the nature of charged penetrating particles that arise in the form of showers, together with electrons and photons, as a result of nuclear processes of high energy. It has become clear that, among these penetrating particles, protons play a significant role; however, in addition to them, mesons are also, as a rule, present, having in any case predominantly masses of \(200\text{--}300\,m_e\).

Therefore the study of penetrating showers is of considerable interest from the standpoint of the question of the origin of cosmic-ray mesons, although the possibility of direct generation of ordinary \(\mu\)-mesons in any nuclear processes appears very unlikely in view of their very weak nuclear interaction.

In the paper under review1, stopped \(\mu\)-mesons were recorded by the method of delayed coincidences, using decay electrons emitted with a delay of \(1.3\text{--}8.5\ \mu\mathrm{sec}\). The apparatus consisted of 3 rows of counters surrounded by lead, and made it possible to distinguish both \(\mu\)-mesons arriving from the air as single particles and \(\mu\)-mesons associated with the production of a penetrating shower (not fewer than two particles) in the lead block of the apparatus itself. The filter in which the recorded \(\mu\)-mesons stopped consisted in one case of graphite, and in the other case of sulfur.

The principal result of the work consists in determining the ratios

\[ \frac{n_c}{n_z}, \]

i.e., of the decay effects in graphite and in sulfur. For single mesons this ratio proved to be

\[ \frac{n_c}{n_z}=1.85\pm0.05, \]

which agrees with the generally known fact of the weak nuclear interaction of stopped \(\mu\)-mesons of negative sign, which are captured by sulfur nuclei but no longer manage to be captured by graphite nuclei, and decay in it along with positive mesons. At the same time, for \(\mu\)-mesons produced in penetrating showers, the ratio obtained was

\[ \frac{n_c}{n_z}=1.1\pm0.1. \]

Piccioni naturally explains this fact by saying that in the present case \(\mu\)-mesons are formed only through the decay of stopped positive \(\pi\)-mesons produced directly in the showers. All negative \(\pi\)-mesons, having been slowed down, undergo nuclear capture both in sulfur and in graphite, and only in the case when the shower is formed in the air do they have time to decay into \(\mu\)-mesons.

Such a process of formation of \(\mu\)-mesons through \(\pi\)-mesons produced in showers must play a very substantial role in the formation of the entire meson component of cosmic rays. Indeed, although the number of \(\mu\)-mesons associated with penetrating showers was, in Piccioni’s experiments, only 6.5% of the number of single stopped \(\mu\)-mesons, extrapolation of both quantities to the upper layers of the atmosphere with the aid of the known altitude dependences for penetrating showers and slow mesons leads to quantities of the same order.

G. B.

References

1. O. Piccioni, Phys. Rev. 77, 1 (1950).
2. N. Birger, G. Zhdanov, V. Polynov, and S. Slavatinsky, ZhETF, No. 5, 1950.
3. G. D. Rochester and C. C. Butler, Proc. Phys. Soc. 61, 307, 535 (1948).

1. Piccioni. ↩