From the Current Literature

On a Possible New Type of $\tau$-Meson Decay

In a systematic examination of a stack of thick-layer photographic emulsions exposed in the stratosphere[^1], the track of a heavy particle was found which, after stopping, emitted a positive $\pi$-meson, identified by the subsequent $\pi \to \mu \to e$ decay. The mass of the heavy particle proved to be equal to $(950 \pm 85)\,m_e$. The range of the $\pi$-meson is $4.3$ mm, which corresponds to a kinetic energy of $13.8 \div 15$ MeV.

A similar case was also found in work[^2].

In a multiprong “star,” two heavy mesons with mass $\sim 1000\,m_e$ were formed simultaneously. One of them (the more precise value of the mass is $1060 \pm 50\,m_e$) stopped inside the emulsion, which led to the appearance of a negative $\pi$-meson, which in turn formed, after stopping, a three-prong “star.” The kinetic energy of the $\pi$-meson is $38.5 \pm 2.5$ MeV, and its range is $23.2$ mm.

It is not excluded that in both cases what is involved is the nuclear capture of negative heavy mesons, leading to the generation of $\pi$-mesons. It is difficult, however, to explain the circumstance that, despite the relatively large energy release, besides the $\pi$-meson in the proposed nuclear disintegration no other charged particle is formed. On the basis of this, the authors of works 1, 2 consider such an interpretation improbable, assuming that what occurs is not nuclear capture, but the decay of the heavy mesons that have stopped.

It cannot, however, be a question either of the decay of $K$-mesons (since in such a decay not a $\pi$-meson but a $\mu$-meson is formed), or of the decay of $\chi$-mesons (since the energy of the resulting $\pi$-mesons considerably exceeds the energy values observed in the cases described). The authors therefore assume that the heavy particles found are $\tau$-mesons decaying according to the scheme

$$ \tau^\pm \to \pi^\pm + \pi^0 + \pi^0. $$

The above decay scheme was theoretically considered in works 3, 4, in which it was shown that the probability of its occurrence may be comparable with the probability of the occurrence of the usually observed decay scheme

$$ \tau^\pm \to \pi^\pm + \pi^+ + \pi^-. $$

If the proposed explanation is correct, then from the observation of the second of the described cases

$$ (\tau^- \to \pi^- + \pi^0 + \pi^0) $$

it follows that slow negative $\tau$-mesons interact weakly with nuclei, for they manage to decay before a nuclear act takes place.

capture. But then, alongside the rather numerous cases of decay of positive $\tau$-mesons, cases of decay of negative $\tau$-mesons according to the scheme

\[ \tau^- \to \pi^- + \pi^- + \pi^+, \]

which have not been found up to now, should also have been observed.

The indicated “contradiction” is apparently only seeming, since it is necessary to take into account the peculiarities of the decay of negative $\tau$-mesons connected with the fact that the $\tau^-$-meson forms, together with the nucleus, a “mesoatom” and therefore decays “in flight.”

In the paper cited in reference 5 it was noted that, under these conditions, the tracks of the secondary $\pi$-mesons are, generally speaking, coplanar. Meanwhile, the basic starting point of the presently accepted system of identification of $\tau$-mesons is precisely the presence of strict coplanarity of the tracks of the $\pi$-mesons of the decay. It therefore seems an interesting problem to search for cases of decay of a $\tau$-meson into three $\pi$-mesons whose tracks do not lie in one plane\(^5\).

M. P.

CITED LITERATURE

1. J. Crussard, M. F. Kaplon, J. Klarmann, J. H. Noon, Phys. Rev. 93, 253 (1954).
2. E. Amaldi, G. Baroni, C. Castagnoli, G. Cortini, C. Franzinetti, A. Manfredini, Nuovo Cimento 11, 207 (1954).
3. V. B. Berestetskii, DAN SSSR 92, 519 (1953).
4. R. H. Dalitz, Proc. Phys. Soc. A 65, 710 (1953).
5. M. I. Podgoretskii, ZhETF 26, 255 (1954).