From Current Literature

Lifetime of Heavy Mesons*

The lifetime of a meson is one of the most important characteristics for explaining many phenomena occurring in nuclei. Until recently only one kind of meson was known, with mass \(\sim 200\,m_e\) (where \(m_e\) is the electron mass) and lifetime \(\sim 10^{-6}\) sec. However, apparently, these “ordinary” mesons are not responsible for nuclear phenomena, since, first, they interact only weakly with the nucleus\(^{1,2}\), and, second, their lifetime is approximately 100 times shorter than follows from Yukawa’s theory. The experiments of Alikhanov and Alikhanyan with collaborators\(^{3}\) have shown that the meson spectrum includes mesons with masses from \(100\,m_e\) to \(25\,000\,m_e\).

In 1948 mesons with mass approximately equal to \(300\,m_e\) were artificially obtained in the Berkeley synchrocyclotron. In the paper under review experiments are described on measuring the lifetime of artificially created mesons produced by \(\alpha\)-particles with energy 350 MeV, which interact strongly with nuclei (i.e. produce “stars”). According to established terminology these mesons are called \(\pi\)-mesons. It is assumed that \(\pi\)-mesons decay into “ordinary” mesons (\(\mu\)-mesons with mass \(\sim 200\,m_e\)), as follows from the experiments of Powell et al.\(^{4}\)

Mesons of different signs, formed in a graphite target (1.5 mm thick) placed at a radius of 1.9 m (magnet radius 2.3 m), were wound by the magnetic field in opposite directions in the cyclotron chamber. Immediately behind the target were placed two sections of a helical channel; the pitch of the screw was 2.54 cm, the inner diameter 11.4 cm, the outer diameter 15.2 cm. The direction of the channels corresponded to the motion of positively and negatively charged mesons, whose horizontal angle corresponded to the initial direction of motion of the \(\alpha\)-particles. One section of the helical channel amounted to only \(180^\circ\) and rose upward by 1.27 cm. The other section was 1.5 times longer—the screw spiral turned through \(540^\circ\) and descended downward by 3.81 cm. If the mesons did not decay, then in a uniform magnetic field three times fewer particles would emerge from the longer channel than from the shorter one. In fact, it is evident that at identical upward and downward angles the same number of mesons emerges, and consequently everything is determined by the angles subtended in the meson beam by the two channels. A favorable factor is the horizontal focusing action of the uniform magnetic field, consisting in the fact that, irrespective of the initial velocity, after a turn of \(360^\circ\) the projection of the particle trajectory onto the horizontal plane passes through the point of meson emission from the target. Therefore, in particular, when mesons pass through the channel the principal role is played by the vertical angle, and the ratio of particles passing through the short and the long channel should be 3, not 9, as might seem at first glance.

* J. R. Richardson, Phys. Rev. 74, 1720 (1948).

LIFETIME OF HEAVY MESONS

A decrease of the magnetic field toward the edge of the magnet will somewhat disturb this picture, but, as calculation and experiment have shown, this influence is small. The check was carried out in the following way. Instead of the target, a plutonium source of α-particles was placed \((1.2 \times 0.3\ \text{cm})\), the height of which corresponded to the height of the target, and the width to that part of the target into which the accelerated α-particles fell. The number of α-particles was determined from tracks in photographic plates (E-1 Ilford) placed at an angle of \(48^\circ\) at the end of each channel. Instead of the ratio 3, corresponding to the ideal case, a ratio of 3.2 was obtained experimentally (with a correction for the different experimental conditions for mesons and α-particles). As will be seen below, the correction obtained changes the value obtained for the meson lifetime only insignificantly.

The subsequent part of the experimental work consisted in measuring the number of mesons emerging from the short \((180^\circ)\) and long \((540^\circ)\) channels. For this purpose, 6 photographic plates were placed at the end of each channel. The plates were developed simultaneously, and the numbers of meson tracks producing “stars” in the plates that appeared during the bombardment in the short and long channels were counted. Forty-eight mesons were found that produced stars and emerged from the long channel. If the mesons did not decay, then, on the basis of measurements of the number of mesons emerging from the short channel, it could be concluded that 92 mesons should emerge from the long channel. Consequently, 44 mesons decayed during one revolution of the meson in the magnetic field. The author assumes that the meson mass is equal to \(286\,m_e\) (this number is not in agreement with the preliminary data\(^5\)) and calculates the meson revolution time, equal to \(7.2 \cdot 10^{-9}\ \text{sec}\). Hence it is easy to obtain the meson lifetime

\[ [7.2 \cdot 10^{-9} \ln(92/48)], \]

equal to

\[ 1.11 \cdot \begin{cases} +0.31\\ -0.22 \end{cases} \cdot 10^{-8}\ \text{sec}. \]

The indicated errors are root-mean-square deviations. In all, 256 plates were measured. The total number of mesons producing stars in this experiment was 302.

M. S. Rabinovich

CITED LITERATURE

1. UFN 33, 129 (1947).
2. UFN 33, 133 (1947).
3. A. Alikhanyan, A. Alikhanov, A. Weisenberg, Dokl. Akad. Nauk SSSR 55, 709 (1947).
4. Lattes, Occhialini, Powell, and Frank, UFN 34, 370 (1948).