FROM CURRENT LITERATURE

$K$-MESONS

In 1951 O’Ceallaigh described four cases of the decay[^1] of a heavy meson with mass about $1100$–$1300\,m_e$. As a result of the decay, each time one charged secondary particle was produced. The energy of the secondary particles lay within wide limits. In one case a decay of the secondary particle into an electron was observed. All these data led one to suppose the existence of a special type of mesons, decaying into a $\mu$- (or $\pi$-) meson and not fewer than two neutral particles, not recorded in the emulsion. The question of the relation between these mesons, which have received the name $K$-mesons, and $\tau$-mesons, decaying into three $\pi$-mesons, or charged $V^{\pm}$-mesons, is at present unclear, and new data on $K$-mesons are therefore of considerable interest.

Trace of a $K$-meson produced in a star. At the end of the meson trace the trace of a secondary particle—a fast $\mu$-meson—is visible. Since the $\mu$-meson trace is very long, only the beginning and the end of the trace are shown in the photograph.

In the work of Leprince-Ringuet and collaborators[^2] six new cases of decay of $K$-mesons are reported, observed in Ilford G5 plates exposed at an altitude of $21$ km. Three of the observed $K$-particles were emitted in stars. A photograph of one of the stars is shown in the figure; since the trace of the $K$-meson has a large length, $4.9$ mm, only the beginning and the end of the trace are shown in the figure.

Let us consider the data on the masses of the $K$-mesons and of the secondary particles arising in their decay.

1. Mass of the $K$-mesons. The longest ranges, allowing an accurate measurement of the mass, are respectively $5$, $6$, and $9$ mm. The masses were measured by two methods: from scattering and range, and from ionization and range. The data obtained are given in Table I.

The values of the masses obtained by both methods, as is seen from the table, are in good agreement.

The mean value of the mass from all 6 measurements is equal to $M=(940\pm40)\,m_e$.

This value does not differ substantially from the best values obtained by other investigators:

\[ (1125\pm150)\quad \text{(track length } 5670\,\mu,\ \text{work }{}^{3}); \]

\[ (870\pm100)\quad \text{(track length } 13\,300\,\mu,\ \text{work }{}^{3}); \]

\[ (1040\pm90)\quad \text{(track length } 5260\,\mu,\ \text{work }{}^{4}); \]

\[ (950\pm50)\quad \text{(track length } 14\,000\,\mu,\ \text{work }{}^{5}). \]

Thus, apparently, all these mesons may be assigned a mass close to \((940\pm40)\, \(m_e\). Let us note that the exact value of the mass of the heavy

Table I

meson, obtained by Alikhanyan’s group \(^{6}\) on a magnetic mass spectrometer, is equal to \((950\pm30)m_e\), which permits the supposition that the mesons recorded by this method are identical with the \(K\)-mesons under consideration.

2. Secondary particles. In all cases the secondary particles produce relativistic ionization. The results of the two most accurate and two less accurate measurements are presented in Table II.

Table II

Joint consideration of the values of \(p\beta c\) and of the grain density in the track leads to the conclusion that the secondary particles are apparently \(\mu\)-mesons. Thus, these data, together with those published earlier, indicate that the mass of the \(K\)-mesons is close to \((940\pm40)\, \(m_e\), and that in their decay a \(\mu\)-meson arises, whose energy is different in all the cases examined. Hence it follows that the \(K\)-mesons decay into at least 3 particles. Let us consider the following decay scheme: \(K\to\mu+2\) neutral particles with mass close to zero. In this case the maximum value of \(p\beta c\) for the \(\mu\)-meson is equal to

\[ p\beta c = 200\ \text{MeV}\quad \text{for } M = 940\,m_e, \]

\[ p\beta c = 220\ \text{MeV}\quad \text{for } M = 1000\,m_e. \]

Meanwhile, the currently measured tracks of secondary particles have given values of \(p\beta c\) of \(250\pm35\) \(^{3}\) and \(290\pm60\) MeV, which somewhat exceed

the indicated limiting values. If one assumes that one of the neutral particles arising in the decay is a $\pi^0$-meson, then the corresponding limiting values of $\beta^2 c$ for the two indicated mass values, equal to 940 and 1000 $m_e$, are respectively 180 and 200 MeV, and the discrepancies become still larger, so that the first decay scheme appears more reliable.

A. V.

References

1. C. O’Ceallaigh, Phil. Mag. 42, 1032 (1952).
2. I. Crussard, L. Leprince-Ringuet, D. Morellet, A. Orkin-Lecourtois and I. Trembley, Phys. Rev. 90, No. 6, 1127 (1953).
3. M. Menon, unpublished data.
4. G. Frier, Bertini and Roberts, Phys. Rev. 85, 426 (1952).
5. G. Harries and W. Davies, Proc. Phys. Soc. (London) A 65, 564 (1952).
6. A. I. Alikhanov, UFN 50, No. 4 (1953).