OBSERVATION OF MESOTRON PAIR PRODUCTION*)

The study of the properties of “half-heavy” particles that are components of cosmic rays is one of the most urgent problems of modern physics. In particular, the question of the production of mesotrons and of their mutual transformations is of great interest.

In the paper under review, A. P. Zhdanov and P. I. Lukirsky apparently report for the first time the observation of a mesotron pair.

The authors investigated mesotron tracks in thick-layer photographic plates exposed to cosmic rays at an altitude of 6000 m above sea level. Two tracks were found which show—

*) A. P. Zhdanov and P. I. Lukirsky, DAN SSSR 69, 785 (1949).

which are of special interest because of the specific distribution of grains in them: the number of grains revealed per unit path length decreases regularly in the direction from both ends toward the middle. This is clearly illustrated by the graph presented in the figure. On the graph, the abscissa gives the distance from one of the ends of the track (the total length of the track was 1274 μ, and it was divided into 26 sections of 49 μ each), and the ordinate gives the number of grains \(n\) revealed in the given section. (The number of grains at end \(A\) is greater than at end \(B\), because the latter is at the boundary of the emulsion and, forming a particle trace, emerges into the air, whereas end \(A\) is in the emulsion.) A similar graph was also constructed for the second trace.

Since the number of grains per unit path length increases as the particle velocity decreases, we must conclude that in both cases

from point \(C\) two particles emerge in opposite directions. Study of the number of grains per unit path length, as well as the characteristic bends of the trace caused by multiple scattering, leads to the conclusion that these particles are mesotrons. Thus, here we have the formation of a mesotron pair.

It is very important that the angle between the directions of the mesotrons is almost \(180^\circ\). This means that no appreciable momentum was transferred to the mesotrons. Consequently, the particle responsible for the creation of the pair is neutral. Indeed, a charged particle with small momentum would necessarily have been detected on the photographic plate, and there is no trace between the three tracks.

Thus, the mesotron pair could have been created by a photon, by a heavy neutral particle, or by a neutral mesotron. However, the “photon” hypothesis is excluded, since, as is easy to calculate on the basis of the conservation laws, the momentum of such a photon would have to exceed by 10 times the momentum of each of the mesotrons, which, in the authors’ opinion, cannot be the case.

The supposition that the mesotron pair was formed in a collision with the nucleus of a heavy neutral particle (a neutron) also falls away. Indeed, in that case both mesotrons would have had to move in the direction close to the direction of motion of the neutron.

Consequently, it remains to suppose that the mesotron pair was formed at the expense of the proper mass of a very slow neutral particle.

Assuming that the observed charged mesotrons belong to the usual, well-known type, and that their mass is equal to \(200\,m_e\) (\(m_e\) being the electron mass), the authors obtain for the mass of the neutral mesotron:

\[ M \sim 416\,m_e \]

(taking into account also the small kinetic energy of the mesotrons).

The formation, first detected in the paper under review, of a mesotron pair by a neutral mesotron is of very great fundamental interest both for cosmic-ray physics and for the theory of nuclear forces.

V. Averbakh