NEW DECAYING PARTICLES IN COSMIC RAYS

In a large number of works that have appeared in recent years, there are data on the existence in cosmic rays of unstable particles with various masses, obeying various decay schemes. Thus, in addition to the already well-known decay of the $\mu$-meson, in 1947 a new particle, the $\pi$-meson, was discovered, with a mass equal to $285\,m_e$ and a lifetime of $10^{-8}$ sec., which gives a $\mu$-meson in its decay.^1 In the last two years three cases have been described,^2,3,4 observed in photographic emulsions, which, in the authors’ opinion, permit interpretation as the decay of a heavy meson with mass $\sim 1000\,m_e$. The decay in this case occurs into three charged particles.

In 1947 Rochester and Butler^5 first observed in a Wilson chamber an event which they explained as the decay of a neutral meson with mass $\sim 1000\,m_e$ into two charged particles. Thirty analogous events are described by Anderson et al.^6 in a recently published paper. As in Rochester’s work, a Wilson chamber was used.

pine with a diameter of 30 cm, with a lead plate 2 cm thick inside it, placed in a magnetic field of 6500 gauss. The control was effected by means of a pulse from a group of counters which efficiently selected electron-nuclear showers. In all, 11 thousand pictures were obtained (8000 at an altitude of 3200 m above sea level and 3000 at an altitude of 230 m), on which 10 thousand penetrating particles were observed.

The observed decay is characterized by two tracks making an angle with one another lying in the interval \(3.5\text{--}126^\circ\), and in 13 cases the angle exceeds \(40^\circ\). Most of the particles possess an impulse greater than \(200\,MeV/c\) and produce in the chamber an ionization close to relativistic; only in a few cases of especially large angles did the ionization exceed the relativistic value. In 19 cases, in addition to the charged particles forming the decay “fork,” penetrating particles of an electron-nuclear shower produced in the lead were also observed in the chamber. Twelve charged particles—the decay products—passed through a 2-centimeter lead plate without in any case producing a cascade shower. This fact serves as additional evidence (besides the magnitude of the observed angles*) that the cases described are not electron pairs. At the same time, after these particles had passed through the lead plate, in two cases a nuclear interaction was observed: in one case, spallation; in the other, scattering through an angle of \(35^\circ\).

Thus, at least one of the decay particles interacts strongly with nuclei.

A direct determination of the mass of the secondary particles proved possible in only one case. A value \((150\text{--}350)\,m_e\) was obtained.

If one assumes that neutral particles are not emitted in the decay (there is some experimental indication of their absence), then
\(M_{\max}=\sum M_{\mathrm{mes}}+250\,m_e\), if the decay occurs into two mesons: \(\pi\) and (or) \(\mu\), and
\(M_{\max}=M_{\mathrm{prot}}+M_{\mathrm{mes}}+400\,m_e\), if one of the decay products is a proton.

From the distribution of the points at which the decay occurs over the diameter of the Wilson chamber, the authors estimate, taking into account the corresponding relativistic correction for each case, the lifetime of the neutral meson at rest as
\(\tau_0=(3\pm2)\cdot10^{-10}\) sec. Then the fraction of neutral mesons produced in electron-nuclear showers should amount to \(3\%\) of the total number of shower penetrating particles.

In addition to the decay of neutral mesons, the authors also observed 4 cases of decay of a charged particle with the emission of one charged particle. An analogous case was cited earlier in the already mentioned work of Rochester and Butler, in which there is an estimate of the mass of the decaying particle as \(\sim 1000\,m_e\). In the work of Anderson et al. the masses of the particles were not determined.

The observed number of cases does not contradict the supposition that \(\pi\)-mesons decay in the chamber, which, according to the latest works\(^8\), constitute about half of all penetrating particles in electron-nuclear showers.

N. Birger

*) In the case of electron pairs, the angles would not exceed a few tenths of a degree.

References Cited

1. L. Lattes, G. P. S. Occhialini, C. F. Powell, Nature 160, 453 (1947).

2. R. Brown, U. Camerini, P. H. Fowler, G. Muirhead, C. P. Powell, D. M. Ritson, Nature 163, 47 (1949).

3. J. B. Harding, Phil. Mag. 41, 405 (1950).

4. A. Alikhanyan, D. Samoilovich, I. Gurevich, and Kh. Babayan, JETP 19, 664 (1949).

5. G. D. Rochester, C. C. Butler, Nature 160, 855 (1947).

6. A. J. Seriff, R. B. Leighton, C. Hsiao, E. W. Cowen, C. D. Anderson, Phys. Rev. 78, 290 (1950).

7. N. Birger, V. Veksler, N. Dobrotin, G. Zatsepin, A. Lyubimov, L. Kurnosova, I. Rozental, L. Eidus, JETP 19, 826 (1949).

8. U. Camerini, P. H. Fowler, W. O. Lock, H. Muirhead, Phil. Mag. 41, 413—427 (1950).