HEAVY MESONS IN PENETRATING SHOWERS

It is known that in recent years the so-called $\tau$-mesons have been discovered in photographic emulsions—particles with mass $\sim 1000\,m_e$, decaying into three charged particles (most likely $\pi$-mesons), and $K$-mesons with mass $\sim 1200\,m_e$, whose decay gives rise to one charged particle (apparently a $\mu$-meson) and, probably, several neutral ones*).

) See Butler’s article published in the preceding issue, as well as the abstract in UFN*, 46, no. 1, 118 (1952).

Interesting data on heavy mesons have recently been obtained in the study of penetrating showers in a Wilson chamber placed in a magnetic field of 5000 gauss.

The authors^1 discovered in the chamber particles that decayed in flight into three charged particles—like $\tau$-mesons, or into one particle—like $K$-mesons. These cases attracted attention and were specially studied.

In Fig. 1 one can see the track of a particle crossing a lead plate 2.5 cm thick (without a noticeable change in the direction of motion) and

Fig. 1. Decay of a $\tau$-meson.

decaying in the lower part of the chamber into three particles. Measurement of the curvature of the tracks showed that charge conservation is observed in the decay.

It proved possible to estimate the ionizing power and, from the curvature of the tracks, the momenta of the primary and all the secondary particles. The data obtained are summarized in Table I.

In column 3 of this table are given the masses of the secondary particles, estimated from the impulse and ionization; in the authors’ opinion, it is most probable that all the secondary particles are $\pi$-mesons.

The three $\pi$-mesons are apparently the only decay products, since within the accuracy of the measurements the law of conservation of the particles’ momenta is satisfied; taking for the secondary particles the exact

Table I

the value of the $\pi$-meson mass and, knowing the momentum, it is easy to estimate the mass of the primary particle. It turned out to be equal to $975\,m_e$ *). Thus, with great reliability one may assert that the particle decaying in flight is a $\tau$-meson. One more decay of a $\tau$-meson was observed, but the particle masses were determined less accurately, since in this case the momenta are large and were estimated with a larger error.

The authors’ observation that, before the moment of decay, $\tau$-mesons traveled a greater path in the chamber than $V$-particles is significant, and that their lifetime therefore is probably considerably greater than that of $V$-particles—possibly $10^{-9}$ sec or even more.

Another type of decay is shown in Fig. 2. In the upper part of the chamber a “fork” is visible, formed by the tracks of two particles; one track is thick, while the other is very thin **).

At first glance this case does not differ from an ordinary $V$-decay. However, the authors drew attention to the fact that each of the two particles forming the “fork” traveled a large distance from the possible place of its origin to the vertex of the “fork,” and therefore the lifetime of these particles is apparently greater than that of the known $V$-particles.

It proved possible to measure the momentum and ionization of both particles and thus to estimate their masses. The data obtained are given in Table II.

On the basis of the obtained values of the particle masses, the authors put forward the plausible supposition that in the present case one is probably observing the decay of a $K$-meson in flight ***).

) This value corresponds to a released energy of 75 MeV.
) The directions of the tracks are indicated by arrows.
**) The kinetic energy of the secondary particle (assuming that it is a $\mu$-meson) in the center-of-mass system is equal to 82 MeV.

Table II

It is not excluded, however, that the observed “fork” is due to the decay of a \(V^0\)-particle traveling from below upward. It is interesting that, of more than

Fig. 2. Decay of a \(K\)-meson.

150 observed \(V^0\)-decays, only the one case shown agrees with the assumption that the particle was moving upward.

In addition to decays, the authors investigated strongly ionizing particles in penetrating showers and estimated their masses. By this method they

also found an appreciable number of $\tau$- and $K$-mesons; according to a preliminary estimate, the flux of $\tau$- and $K$-mesons is probably quite large; it is possible that it is considerably larger than the flux of $V$-particles.

The preliminary data also lead the authors to the conclusion that particles with masses of $400$–$600\,m_e$ may exist: three particles were observed with momenta

\[ 180 \pm 20\,\frac{\text{MeV}}{c},\quad 100 \pm 15\,\frac{\text{MeV}}{c},\quad \text{and}\quad 135 \pm 15\,\frac{\text{MeV}}{c} \]

and with ionization respectively $2$–$3$, $3$–$6$, and $4$–$8$ times greater than the minimum.

Accordingly, for the masses of the particles the values $550 \pm 150\,m_e$, $450 \pm 150\,m_e$, and $750 \pm 150\,m_e$ were obtained.

Only with great difficulty, as is pointed out, can one assume that the first two particles are $\pi$- or $\tau$-mesons, and the third a $\tau$-meson.

For the first time Alikhanyan, Alikhanov, and their collaborators, on the basis of a number of experimental works, expressed the assertion that there exist particles with masses $500$–$600\,m_e$ and $900$–$1000\,m_e$. These works are currently (July 1952) being discussed in the pages of the Journal of Experimental and Theoretical Physics.[^2]

For the time being it is premature, in view of the brevity of the communication, to make a quantitative comparison of the data of work[^1] with the works of Alikhanyan and Alikhanov.

It should be noted, however, that the particles observed in work[^1] with mass $\sim 1000\,m_e$ have a comparatively long lifetime and, consequently, can be recorded by an extended apparatus of the Alikhanyan and Alikhanov mass-spectrometer type. It may be assumed that the lifetime of particles to which masses of $400$–$650\,m_e$ have been assigned is also comparatively long; otherwise it would have been impossible to determine their momenta and ionization with the indicated degree of accuracy. However, the note gives no direct indications of the lifetime of such particles.

M. D.

References

1. R. B. Leighton and S. D. Wanlass, Phys. Rev., 86, No. 3, 426 (1952).
2. ZhETF, 21, 1023 (1951); 21, 1062 (1951); 22, 499 (1952).