From the Current Literature

Observation of $K$- and $\tau$-Mesons Generated at Accelerators

The successful development of accelerator technology has recently made it possible to generate heavy mesons under laboratory conditions. In the papers reviewed here, several heavy mesons, produced in beryllium and copper targets bombarded by protons with energies of $2.2$ Bev and $3.0$ Bev, were recorded with the aid of photographic emulsions. These mesons are apparently analogous to the $K$- and $\tau$-mesons observed in cosmic radiation.

In paper $^1$ a beryllium target was bombarded with protons of energy $2.2$ Bev. The experimental arrangement is shown in the figure. Negative particles generated in target $A$ were deflected by the magnetic field of the accelerator and, through an aluminum window of thickness $\frac{1}{16}$ inch and a collimator $C$ in a lead block $B$, entered a stack of photographic emulsions (Ilford G-5, thickness $400\,\mu$ and size $2 \times 3$ inches). The planes of the emulsions were arranged horizontally.

Particles entering the stack left the target at angles of $2$–$17^\circ$ to the proton beam, traveled a distance $AD = 235$ cm, and had momenta in the interval $310$–$350$ Mev$/c$. At such momenta, particles with a mass greater than $700\,m_e$ stopped in the emulsion.

Fig. Experimental arrangement. Top view.

An area of $28.2\ \text{cm}^2$ of emulsion was examined, and about 2000 stars with two or more prongs were found, produced by fast $\pi$-mesons generated in target $A$ by the primary protons.

In one case a star was observed in the emulsion, caused by a slow particle coming from the target. The mass of this particle was determined by two methods—by measuring the angles of multiple scattering and the range along the track

and measurement of the magnitude of the gaps between grains along the track and comparison with the same quantities in the tracks of protons and π-mesons. By these two methods, close values of the particle mass were obtained, equal to \(1050 \pm 150\,m_e\) and \(1200 \pm 300\,m_e\). Independently, the mass of the particle was determined from its momentum and range in the emulsion stack and proved to be equal to \(1080 \pm 220\,m_e\). From the distance from the target to the star, the lifetime of the heavy meson in its rest system was determined. It proved to be equal to \(1.2\cdot 10^{-8}\) sec.

In subsequent works, no special magnetic analysis of the particles emerging from the target was used. Thus, in work \(^{2}\) a copper target \(6\) mm thick was bombarded. Analysis of the products emerging from the target was carried out by means of stacks of emulsions arranged in two positions—(a) at a distance of \(28\) cm from the target, at an angle of \(90^\circ\) to the primary proton beam; (b) at a distance of \(50\) cm from the target, at an angle of \(45^\circ\) to the beam. In both positions, on the path between the target and the emulsion there was the steel wall of the cosmotron (\(1.1\) cm), and in position (b), in addition, \(7.5\) cm of Cu. The investigations were made at two values of the primary-proton energy: \(2.2\) Bev and \(3.0\) Bev.

Proton energy \(2.2\) Bev. An area of \(36.2\ \mathrm{cm}^2\) of emulsion exposed in position (b) was scanned, and 231 stopped π- and μ-mesons were found. In one case, a stopped negative heavy particle (range in emulsion \(31\) mm) formed a star of two rays, very similar in its external characteristics to the star described in the preceding work \(^{1}\), the mass of the primary particle being found to be \(970 \pm 150\,m_e\). The authors identify it with a \(K\)-meson. One of the particles arising in the star is identified (by grain density and scattering) as a π-meson with energy \(\sim 50\) Mev, and the other as a heavy fragment (range \(600\,\mu\)). From the range an estimate was made of the kinetic energy of the heavy meson upon leaving the target; it is equal to \(270\) Mev.

Knowledge of the kinetic energy makes it possible to conclude that, under the conditions of this work, in a nucleon-nucleon collision (taking the maximum value of the Fermi energy as \(25\) Mev) only one \(K\)-meson, or a \(K\)-meson and a hyperon, could be produced. However, the possibility is not excluded of the simultaneous production of two \(K\)-particles with mass of order \(920\,m_e\).

Proton energy \(3.0\) Bev. An area of \(10.8\ \mathrm{cm}^2\) of emulsion in position (a) was scanned “by area” and 386 π- and μ-meson stops were recorded. In three cases the mass values of the stopped mesons (ranges \(19\) mm, \(40\) mm, and \(46\) mm of emulsion) lay within \(1050 \pm 250\,m_e\), and their kinetic energies upon leaving the target were within \(90\)–\(130\) Mev. A characteristic feature of all three cases was the occurrence of a single relativistic particle (π- or μ-meson) when the heavy meson stopped, and the absence of tracks of recoil nuclei and electron tracks. Thus, these cases represent the typical decay of \(K^+\)-mesons produced in the target. The lifetime of the \(K^+\)-mesons, estimated from the distance from the target to the stopping point, was equal to at least \(2\cdot 10^{-9}\) sec.

In work \(^{3}\), which is a continuation of work \(^{2}\), in a stack of emulsions exposed at an angle of \(90^\circ\) to a beam of protons with energy \(3.0\) Bev, one τ-meson was found. Its energy on leaving the target, estimated from the magnitude of the range in the cosmotron wall and in the emulsion (\(3.4\) cm), is equal to less than \(122\) Mev. When the τ-meson stops, three secondary particles arise, the tracks of which at the point of emission are coplanar within \(1^\circ\), the angles between the tracks being \(61.3^\circ\), \(150^\circ\), and \(148.5^\circ\). One of the mesons travels a path of \(4.9\) mm in the emulsion and gives the characteristic \(\pi \to \mu \to e\) decay. The energy of this \(\pi^+\)-meson is \(15.4\) Mev. The other two mesons leave the emulsion. Under the assumption that these are π-mesons, the momenta and angles of emission of the stopped π-meson are calculated on the basis of momentum conservation. The obtained energy values \(16.8\) Mev and \(43.9\) Mev lead to

\(Q = (76 \pm 5)\) MeV, which is in agreement with the value \(Q \simeq 75\) MeV accepted for \(\tau\)-decay. This case of \(\tau\)-decay was used by the authors for a more accurate determination of the masses of the \(K^-\)- and \(K^+\)-particles observed in the same emulsions (work \({}^{2}\)): a comparison was made of the mean scattering angles at equal residual ranges for the \(\tau\)-meson and the \(K^+\)-particles. Since the mass of the \(\tau\)-meson is at present known very accurately (\(m_\tau = 965.5\,m_e\)), from such a comparison the mass of the \(K^\pm\)-particles was determined, and was found to be \(m_{K^-} = (970 \pm 180)m_e\) and \(m_{K^+} = (1040 \pm 200)m_e\).

Similarly to the conclusion on the impossibility of pair production of two \(K\)-mesons under the conditions of work \({}^{2}\), in the present work the authors conclude that, at the given energy of the primary proton beam, the emission of a \(\tau\)-meson with an energy of 122 MeV at an angle of \(90^\circ\) to the proton beam makes implausible the assumption of simultaneous production of the \(\tau\)-meson and a \(K\)-particle in a nucleon–nucleon collision. Under the conditions of this work, with pair production, a \(K\)-particle with a mass not exceeding \(800\,m_e\) could have arisen together with the \(\tau\)-meson.

M. D.

REFERENCES CITED

1. I. Hornbostel and E. O. Salant, Phys. Rev. 93, 902 (1954).
2. R. D. Hill, E. O. Salant and M. Widgoff, Phys. Rev. 94, 1794 (1954).
3. R. D. Hill, E. O. Salant and M. Widgoff, Phys. Rev. 95, 1699 (1954).