NEW DATA ON THE DECAY OF $V^0$ PARTICLES

Recently, new data have been published on the decay of $V$ particles observed in Wilson chambers placed in a magnetic field[^1]. Two chambers of rectangular shape were arranged one above the other in a field with an intensity of 5000 gauss. The chambers were controlled by penetrating particles from showers generated in a block of lead located above the apparatus. A layer of lead 2.5 cm thick was also placed between the chambers. The measurements were made at altitudes of 1700 and 220 m above sea level.

FROM CURRENT LITERATURE

In 23,000 photographs, 134 cases of decay of \(V^0\)-particles and 18 decays of charged \(V\)-particles were found. The majority of \(V\)-particles were formed in splittings caused by charged particles, from which one may conclude that not only nucleons but also mesons take part in the generation of \(V\)-particles. Indeed, otherwise one would have to assume different efficiencies for protons and neutrons in the formation of \(V\)-particles, or else a sharp difference in the number of fast protons and neutrons, which is unlikely.

In several photographs the simultaneous decay of two and even three \(V\)-particles was observed; however, in only three such cases (out of 10) was it possible to suppose that these particles were formed in one and the same act of splitting. More precise information on the multiplicity of production of \(V^0\)-particles was obtained by analyzing 37 cases in which \(V^0\)-particles were formed in a lead plate located between the cameras. If one assumes that \(V^0\)-particles are formed in pairs, then simultaneous registration of the decay of two \(V^0\)-particles should have been expected in seven cases, instead of the one case actually observed (taking in the calculation the authors’ experimentally obtained value for the mean lifetime of a \(V^0\)-particle,

\[ \tau=(2.5\pm0.7)\cdot 10^{-10}\ \text{sec.} \]

). Hence the authors conclude that \(V^0\)-particles are not born in pairs.

Of greatest interest are the data relating to the mechanism of the decay of \(V^0\)-particles. A check of the coplanarity of the trajectories of the \(V^0\)-particles and of the products of their decay showed that in the overwhelming majority of cases decay into two particles takes place. Altogether 60 cases of decay were analyzed in which it was possible to determine the point of formation of the \(V^0\)-particles. The accuracy of measuring coplanarity in the present work considerably exceeded the accuracy attained in previous investigations. The angle between the trajectory of the \(V^0\)-particle and the plane in which the decay products lie was measured with an accuracy of \(1\)—\(10^\circ\), more often with an accuracy of \(2\)—\(4^\circ\). An additional comparison of the magnitude of this angle with the smaller of the angles \(\theta_-\) and \(\theta_+\) (Fig. 1) also showed that only a very small fraction of the decay cases can be attributed to decay not into two, but into a larger number of particles. In this connection the authors carried out their principal analysis under the assumption of decay into two particles.

Fig. 1.

We introduce the quantity

\[ \alpha=\frac{P_+^2-P_-^2}{P_0^2} =\frac{\sin(\theta_- - \theta_+)}{\sin(\theta_-+\theta_+)}, \]

the meaning of which is clear from Fig. 1 (\(P\) are the momenta of the particles). For a given scheme of the decay of a \(V^0\)-particle of mass \(M_0\) into two particles of masses \(M_-\) and \(M_+\), with isotropy of the decay in the center-of-inertia system and for a given velocity of the \(V^0\)-particle, the distribution of all decay cases with respect to the quantity \(\alpha\) must be symmetric about the mean value equal to

\[ \frac{M_+^2-M_-^2}{M_0^2}. \]

Thanks to the application of a magnetic field the quantity \(\alpha\) could be determined not only

from the ratio of the angles \(\theta_-\) and \(\theta_+\), but also independently of this, by measuring the momenta of the secondary particles, carried out with great care. Both of these distributions agreed well with one another. From the significant predominance of positive values of \(\tau\) it follows that the decay proceeds mainly into a heavy positive and a lighter negative particle. The position of the maximum of this distribution corresponds to decay into a proton and a negative \(\pi\)-meson. However, the distribution in \(\alpha\) is somewhat asymmetric and is stretched toward negative values of \(\alpha\). Therefore it is possible that some part of the \(V_0\)-particles decays into two particles of equal mass (to which \(\alpha_{\max}=0\) would correspond). However, this share of such a mechanism accounts for no more than 15–20% of all decay cases.

By simultaneous measurement of momenta and ionization it was possible to determine or estimate the mass of a considerable number of secondary particles. Fig. 2 gives the distribution by mass of such secondary particles, whose tracks were characterized by increased ionization. The most remarkable feature of this distribution is a sharp maximum for negative particles, coinciding with the mass of the \(\pi\)-meson. Thus, statistically it can be reliably established whether the decay occurs into \(\pi\)- or into \(\mu\)-mesons. The distribution for positive particles is more diffuse; however, the shift of the maximum to the left from the position corresponding to the proton mass, because of the measurement errors, is not yet proof of decay into a particle lighter than the proton (for example, into a \(\tau\)- or \(K\)-meson), although some number of such cases may have occurred. Indeed, several cases were observed of decay of a \(V_0\)-particle into a positive particle with mass \(600\)–\(1200\,m_e\) and a negative \(\pi\)-meson.

Fig. 2. Distribution by mass \(M(m_e)\); ordinate \(N\) (in arbitrary units). Legend: positive particles (34 cases); negative particles (23 cases). Marked positions: \(\mu\), \(\pi\), \(p\).

Fig. 3. Distribution in \(Q\) (MeV); ordinate \(N\) (in arbitrary units).

In five cases the mass of the negative particle was insufficient for a \(\pi\)-meson, and the decay product was apparently a \(\mu\)-meson. Although decay into a positive \(\pi\)-meson and an antiproton was not observed, the available experimental material does not allow one completely to exclude the possibility of such cases.

Thus, from the totality of the data presented it follows that in the majority of cases the \(V_0\)-particle decays into a proton and a negative-

[[unclear: end of word]] $\pi$-meson. Adopting such a decay mechanism, the authors calculated the energy release $(Q)$ in those cases in which the heavier particle was positively charged and the momenta of the secondary particles were known. Of great interest is the presence of several maxima in the distribution shown in Fig. 3. As the authors indicate, the form of this distribution agrees well with the existence of two discrete values of $Q$: $Q_1 = 35 \pm 3$ MeV and $Q_2 = 75 \pm 5$ MeV, although certain other possible explanations of the form of this distribution are not excluded.

The determination of the mean lifetime for $V^0$ particles whose decay corresponded to a value $Q > 50$ MeV led to the value $\tau = 1.6 \cdot 10^{-10}$ sec.; for $Q < 50$ MeV, to the value $\tau = 2.9 \cdot 10^{-10}$ sec. Thus, in both cases the decay is characterized by practically the same mean lifetime.

The existence of two discrete values of the quantity $Q$ also suggests that the decay of $V^0$ particles does not always proceed according to the simple scheme $V^0 \to p + \pi^-$. In this connection the authors additionally consider various decay schemes into three particles, mixed decay occurring alternatively according to two decay schemes, decay leading to the formation of particles in an excited state, or beginning from an excited state of the $V^0$ particle, and so on.

However, the statistical material available to the authors is insufficient to make a final choice among all the decay variants, since in the majority of cases the decay apparently occurs into a proton and a negative $\pi$-meson, and another type of decay can be responsible only for a small deviation from the distributions due to the principal decay of $V^0$ particles into a proton and a negative meson.

L. E.

References

1. R. B. Leighton, S. D. Wanlass, C. D. Anderson, Phys. Rev. 89, 148—167 (1953).