On the Existence of a Neutral Meson with a Mass of About \(550\,m_e\)

In recent investigations of cosmic radiation carried out by workers of Powell’s laboratory[^1], indications were obtained of the existence of neutral mesons with a mass of about \(550\,m_e\). These investigations were performed by means of photographic plates raised into the stratosphere on balloon sondes. The lifetime of the new particle is apparently less than \(10^{-14}\) sec, and in its decay two oppositely charged \(\pi\)-mesons (\(\pi^+\) and \(\pi^-\)) arise.

The first arguments in favor of the existence of neutral mesons of a new type were obtained in considering a star of the form \(10 + 2n\). According to

by the notation accepted in the work of this laboratory, such a star was formed by a neutral particle and contains 10 tracks of strongly ionizing particles and 2 tracks of particles producing minimal ionization. The angle between the latter two tracks was close to \(4^\circ\). The angle between the direction of these two slightly diverging tracks and the vertical direction is approximately \(155^\circ\). At an altitude of 20 km, where the plates were exposed, the overwhelming majority of particles of high energy forming stars move from top to bottom. Therefore the presence of two shower particles moving in almost the opposite direction is a very rare occurrence. The tracks of these particles were carefully studied. Owing to their great length (5.6 and 3.2 mm), it proved possible, by measuring the grain density and the mean scattering angle of the particle along the track, to establish that both tracks belonged to \(\pi\)-mesons, and to determine the kinetic energy of these mesons, which proved to be equal to \(76 \pm 5\) MeV and \(117 \pm 10\) MeV. The probability that both shower particles accidentally have close energy values is very small, and the authors suppose that both \(\pi\)-particles arose from the decay of a neutral particle whose lifetime is sufficiently small (less than \(10^{-14}\) sec) that it did not have time to move appreciably away from the star where it was produced. The authors consider the following simplest decay scheme:

\[ \xi^0 \to \pi^+ + \pi^- + Q, \tag{1} \]

and obtain for the quantity \(Q\), representing the energy liberated in the decay, a value close to 2 MeV. Therefore the mass of the new particle is approximately equal to twice the mass of the \(\pi\)-meson, i.e. \(556\,m_e\).

For further investigations, stars were selected in which two \(\pi\)-particles were observed, emitted in directions making angles greater than \(90^\circ\) with the supposed direction of the primary particle, or stars in which two \(\pi\)-particles with energies less than 30 MeV were detected, irrespective of the direction in which they were emitted. Altogether it was possible to observe 8 such stars. Assuming that these \(\pi\)-particles arose from the decay of a neutral meson \(\xi^0\) according to scheme (1), the authors find that in six cases the values of \(Q\), within the limits of experimental errors, coincide and are close to 3 MeV. In the remaining two cases the values of \(Q\) are equal to 10 and 19 MeV. The coincidence of the values of \(Q\) for the first six cases cannot be accidental and indicates that here, apparently, the elementary decay process (1) is taking place.

The authors point out that, if this interpretation is correct, then it follows from their data that the formation of \(\xi^0\) mesons in stars is a very frequent process. Therefore the occurrence of decay (1) should affect the angular distribution of shower particles arising in stars: among these particles there should be an appreciable number of pairs whose tracks form small angles with one another. To test this supposition, about 200 stars of type \(2p\) were considered, i.e. stars produced by protons, in which only shower particles were formed. For each such star the angle \(\beta\) between the direction of motion of the primary proton and the direction of the shower particle and the angle \(\alpha\) between the tracks of the shower particles were measured. These measurements made it possible to construct the distribution function \(n(\beta)\), showing how the shower particles are distributed over the angles \(\beta\). Having such a distribution \(n(\beta)\), one can calculate the probability that the tracks of two particles emitted at angles close to \(\beta\) form an angle \(\alpha\) with one another. These calculations show that the experimentally observed probability that the angle \(\alpha\) lies in the range from \(5^\circ\) to \(15^\circ\) proves to be considerably greater than the probability calculated on the basis of the distribution \(n(\beta)\), under the assumption that the particles are emitted

in independent directions. This proves the presence of a strong angular correlation between the shower particles in stars of type \(2n\). Within the limits of the values of the angle \(\alpha\), equal to 5 and 15°, the observed and calculated probabilities agree sufficiently well. Other measurements, on which we shall not dwell in detail, show that, besides the angular correlation, there also exists a correlation in energy.

The simplest explanation for the existence of such correlations is the hypothesis that pairs of correlated \(\pi\)-mesons arise in the decay of a neutral meson, proceeding according to scheme (1). From the data available to the authors it follows that, in stars containing from 2 to 6 shower particles, about 10% of all \(\pi\)-mesons appear as a result of the decay of the neutral meson \(\xi^0\).

The result obtained is the first indication that \(\pi\)-mesons arising in high-energy nuclear interactions are not necessarily the direct result of the interactions. It is possible that some of these \(\pi\)-mesons are formed as a result of the decay of short-lived neutral mesons with a mass of about \(550\,m_e\).

It is well known that, along with the neutral \(\pi^0\)-meson, which has a very short lifetime \((\tau_0 \sim 10^{-14}\ \mathrm{sec})\), there exist charged \(\pi\)-mesons that live for a considerably longer time \((\tau_0 \sim 2.5 \times 10^{-8}\ \mathrm{sec})\). These mesons, which have close values of the masses \((M_{\pi0}=264\,m_e;\ M_{\pi-}=M_{\pi+}=275\,m_e)\), form a triad of \(\pi\)-mesons. The question arises whether charged mesons with a mass close to \(550\,m_e\) exist. As is known, evidence for the existence of such charged mesons was given in the work of A. I. Alikhanian and his collaborators.^3 Until recently this work was almost the only detailed investigation proving the existence of charged particles with a mass of about \(550\,m_e\). New data on such particles are presented in a recently published paper,^3 in which, in a study of the nature of particles in penetrating showers, 3 particles with masses of about \(400\text{–}650\,m_e\) were observed. Thus, there are indications that, similarly to the triad of \(\pi\)-mesons, there is a triad of heavier mesons with a mass of about \(550\,m_e\), consisting of two charged mesons and a neutral meson.

A. V.

References

1. Danysz, Lock and Jekutielli, Nature, 169, 364 (1952).
2. A. Alikhanian, A. Dadaian, N. Shostakovich, Reports of the Academy of Sciences of the USSR, 82, 5 (1952).
3. Leighton and Wanlass, Phys. Rev., 86, 3, 426 (1952).