ARTIFICIAL PRODUCTION OF MESONS

V. Lopukhin and V. Ugarov

Quite recently, in this same journal, a report was given on certain remarkable successes of nuclear physics achieved with the aid of the giant synchrocyclotron at Berkeley.^1,2,3

In particular, with the aid of this accelerator it proved possible to accelerate helium nuclei to energies of the order of 400 MeV (artificial $\alpha$-particles).

The energy region from 100 MeV to 1000 MeV is the very energy region whose study is only just beginning and in which it is natural to expect a number of new effects and phenomena. One of such phenomena, characteristic of energies of the order of 100 MeV, was the phenomenon of the glow of relativistic electrons in accelerators, visually observed on the synchrotron at Schenectady.^3

Another major achievement of nuclear physics connected with the use of accelerators is the artificial production (generation) of mesons, information about which has been obtained only very recently.^4 Until now mesons had been known only in cosmic rays.

In the note by Occhialini and Powell^4 it is reported that, when the nuclei of carbon, beryllium, copper, and uranium were bombarded by fast $\alpha$-particles, Gardner and Lattes^10 succeeded in detecting the production of mesons. The mesons were detected by means of a photographic plate with a suitably selected emulsion*). The photographic plate was placed at some distance from the target.

The experiments show that meson generation becomes noticeable after the energy of the $\alpha$-particles reaches 300 MeV. Further, with increasing energy, the yield of mesons begins to grow rapidly.

The question quite naturally arises whether the artificially generated mesons are the very same mesons that are observed in cosmic rays. Let us recall that at present it is accepted

) Photographs of this type are reproduced in the article by Lattes et al., Uspekhi Fizicheskikh Nauk 34, 370 (1948); see also the photographs accompanying the translation of Occhialini and Powell’s book, published in this and the following issue of the journal. Some information about emulsions is given in Uspekhi Fizicheskikh Nauk 34*, 450 (1948).

to regard as a meson any particle with a mass intermediate between the mass of the electron and the mass of the proton. The mass of the proton is known to be about 1840 electron masses.

In cosmic rays several values have been found for particle masses around which the experimental data are grouped and which lie in the indicated interval. In other words, in cosmic rays there are several kinds of mesons. Thus, according to the data of A. Alikhanov, A. Alikhanyan, and Weissenberg⁵, in the indicated interval there are particles with masses \(300—500\,m_e\) and \(700—1000\,m_e\). According to the same authors⁶, one of the reliable values of the meson mass is \(350\,m_e\).

The experiments mentioned were carried out with counters on Mount Alagez at an altitude of \(3250\) m.

English authors⁷ carried out investigations of cosmic rays by means of thick-layer photographic plates at high altitude. (Pic du Midi in the Pyrenees, \(2800\) m; Bolivian Andes, \(5500\) m.) As a result of these investigations a large number of photographs was obtained (644 photographs) in which the tracks of particles subsequently identified with mesons in the above-indicated sense came to an end. The principal results of these investigations are as follows. In cosmic rays there exist two kinds of mesons, differing in mass: heavy and light. For the sake of caution, the authors divide the heavy charged mesons into two types—“\(\pi\)-mesons” and “\(\sigma\)-mesons,” although there are grounds for considering that these particles have the same mass and differ only in the sign of the charge. \(\pi\)- and \(\sigma\)-mesons behave in essentially different ways at the ends of their tracks. Slow \(\sigma\)-mesons, carrying a negative charge, as a rule are captured by the nuclei of the emulsion substance and lead to nuclear disintegrations, with the subsequent emission of heavy charged particles (“stars in cosmic rays”). Experiment shows that \(\sigma\)-mesons are captured by both light and heavy nuclei.

Most of the tracks of \(\pi\)-mesons carrying a positive charge show that at the end of the track the heavy meson decays and gives rise to a meson with a mass smaller than that of the \(\pi\)-meson. The \(\pi\)-mesons do not cause disintegrations of the nuclei of the emulsion substance. The lighter meson appearing at the end of the track of a \(\pi\)-meson is called the \(\mu\)-meson.

It may be considered that heavy \(\pi\)-mesons spontaneously decay with the emission of light \(\mu\)-mesons. It is assumed that in the decay the momentum of the \(\mu\)-meson is balanced by the momentum of an uncharged particle of the same mass, not registered by the photographic plate.

The authors point out that \(\sigma\)- and, possibly, \(\pi\)-mesons arise as a result of nuclear disintegration. It is assumed that the majority of \(\mu\)-mesons observed at sea level are the result of the decay of \(\pi\)-mesons in air. \(\sigma\)- and \(\pi\)-mesons have a short lifetime, lying in the interval from \(10^{-6}\) to \(10^{-11}\) seconds.

Taking into account the formation of \(\pi\)- and \(\sigma\)-mesons in nuclear decay, one may think that these particles interact strongly with nucleons.

In an unpublished work by Goldschmidt et al.⁸, using the method of studying the multiple scattering of mesons in emulsion, the mass of $\pi$- and $\sigma$-mesons was determined. It turned out that for the mass of $\pi$- and $\sigma$-mesons one should take the figure $(270 \pm 40)\,m_e$.

On the other hand, in the work already mentioned above⁷, by calculating the number of emulsion grains along the track of the particle, the ratio of the masses of $\pi$- and $\mu$-mesons was found. It turned out that

\[ \frac{m_\pi}{m_\mu} = (1.65 \pm 0.15). \]

If we assume that the $\mu$-meson arising in the decay of the $\pi$-meson is identical with the light meson in the penetrating component of cosmic rays, i.e. that $m_\mu = 200\,m_e$, then it follows from this that $m_\pi = (330 \pm 30)\,m_e$.

Let us recall here once again that one of the probable values of the meson masses determined in the USSR⁶ is equal to $350\,m_e$. Thus, three independent methods for measuring meson masses in cosmic rays indicate the presence in cosmic rays of charged particles with masses close to the value $300\,m_e$.

Let us return to the artificial generation of mesons carried out on the synchrocyclotron at Berkeley. By observing the deflection of mesons in the magnetic field of the cyclotron, one can determine the sign of the electric charge of the meson and the product of the magnetic-field strength $H$ by the radius of curvature of the meson trajectory $\rho$, i.e., ultimately, the momentum of the particle. Knowing the range of the particle (i.e. its energy) and its momentum, one can determine the mass of the particle. The average value of the mass obtained in this way turned out to be $(313 \pm 16)\,m_e$. The authors assert that, if a larger number of tracks at their disposal is used, it will be possible to bring the accuracy of the measurement to $(313 \pm 7)\,m_e$.

Thus, the mass of heavy mesons produced artificially under laboratory conditions agrees, within the limits of experimental error, with the mass of one of the kinds of mesons encountered in cosmic rays. Moreover, artificial mesons of different signs behave at the ends of their tracks exactly as do the $\pi$- and $\sigma$-mesons of cosmic radiation, i.e. they give, respectively, stars or spontaneous emission of a light meson.

The lifetime of artificial mesons is of the order of $10^{-9}$ sec. Occhialini and Powell, referring to an unpublished work⁹ on the measurement of the half-life of $\pi$- and $\sigma$-mesons in cosmic rays, give for this time the value $4 \cdot 10^{-9}$ sec.

Consequently, one may assume with great confidence that the $\pi$- and $\sigma$-mesons of cosmic rays are identical with artificially generated mesons.

Experiments on the cyclotron at Berkeley also make it possible to conclude that, along with heavy mesons, light mesons are also produced, with a mass of the order of $200\,m_e$.

Let us estimate what energy the bombarding $\alpha$-particles must possess in order that a meson with mass $300\,m_e$ can be produced. Energetically, a mass of $300\,m_e$ is equivalent to 150 MeV. Assuming that only two nucleons directly participate in the production of the meson—one nucleon from the nucleus of the bombarded substance and one nucleon of the $\alpha$-particle—it is natural to expect the onset of generation at $\alpha$-particle energies (containing four nucleons) of the order of 600 MeV. It was therefore somewhat unexpected that meson generation in the Berkeley cyclotron began already at $\alpha$-particle energies of about 300 MeV, which corresponds to an energy of 75 MeV per nucleon.

An attempt was made[^10] to explain this situation within the framework of the notion of pair interactions of nucleons of the bombarding and bombarded substances. This explanation is by no means convincing, but for the present it remains the only one, and therefore we present it.

McMillan and Teller[^11] proposed regarding the production of nucleons as the result of the interaction of two nucleons of the reacting substances. In doing so, the kinetic energy of the intranuclear motion of the nucleons must be taken substantially into account. In order of magnitude this energy may be equal to the binding energy of the nucleons. For carbon and an $\alpha$-particle it may be taken equal to 25 MeV. Then, allowing for the existence of such favorable collisions that the kinetic energies of the nucleons and the relative kinetic energy of the interacting nuclei add completely, one can obtain the required energies. Indeed, suppose that the $\alpha$-particle is accelerated to 300 MeV; then the total (maximum) kinetic energy will be equal to

$$ \frac{1}{2}\left(\sqrt{75}+\sqrt{25}+\sqrt{25}\right)^2 \simeq 190\ \mathrm{MeV}. $$

The latter quantity is more than sufficient for producing a meson with mass equal to $\sim 300\,m_e$.

These considerations make it possible to give an approximately correct threshold energy for meson production (300 MeV).

There are no further details in the paper being reported. However, the artificial generation of mesons attracts so much attention that one may hope to obtain more detailed information in the near future.

Let us sum up. Under laboratory conditions, by means of a powerful accelerator accelerating $\alpha$-particles to energies of the order of 400 MeV, it has proved possible artificially to obtain the mesons discovered ten years ago in cosmic rays. These particles, which interact strongly with nucleons, can and should provide us with new information about the interaction of nucleons, about which we know so little. The production of mesons under laboratory conditions enables us to use controlled beams of these particles of high intensity.

The artificial generation of mesons is an outstanding success of the laboratory technique of physical experiment, all the complexities of which could hardly be mentioned here.

References Cited

1. M. Rabinovich, Uspekhi Fizicheskikh Nauk, 32, 396 (1947).
2. E. Shpolsky, Uspekhi Fizicheskikh Nauk, 34, 440 (1948).
3. V. Lopukhin and V. Ugarov, Uspekhi Fizicheskikh Nauk, 34, 398 (1948).
4. G. P. S. Occhialini and C. F. Powell, Nature 161, 551 (1948).
5. A. Alikhanov, A. Alikhanyan and Vaisenberg, ZhETF, 18, 301 (1947).
6. A. Alikhanov, A. Alikhanyan et al., DAN, 58, 1321 (1947).
7. K. M. G. Lattes, G. P. S. Occhialini, K. F. Powell and F. K. Frank, Uspekhi Fizicheskikh Nauk, 34, 370 (1948).
   K. M. G. Lattes, Muirhead and G. P. S. Occhialini, Nature 159, 694 (1947).
8. Goldschmidt, King, Muirhead and Ritson (in press).
9. K. M. G. Lattes, G. P. S. Occhialini and K. F. Powell (in press).
10. Gardner and Lattes, Science (March 12, 1948).
11. McMillan and Teller, Phys. Rev., 72, 1 (1947).