From Current Literature

Production of $\pi^+$ Mesons in Proton—Proton Collisions

Two papers being reviewed present the results of experiments in which the production of $\pi^+$ mesons in proton—proton collisions was investigated. In the first of them$^{1}$ the sought effect was detected from the difference between the results for polyethylene and carbon; in the second$^{2}$ a liquid-hydrogen target was used. The experiments were carried out at an energy of the bombarding protons of 345 MeV, in an extracted proton beam.

Figure 1 shows the scheme of one of the setups used.$^{1}$ The $\pi^+$ and $\pi^-$ mesons produced in the target were deflected by a magnetic field in opposite directions and entered wide channels made in the copper shielding. Protons and other heavy particles could also enter the channel for $\pi^+$ mesons, but they had other momenta, and, besides, their paths were much less similar to those of mesons. Thus, complete separation of the $\pi^+$ mesons from the background of heavy particles was achieved. In observing $\pi^-$ mesons there could be no interference at all, since photographic plates, insensitive to electronic ionization, were used as the meson detectors. Since the threshold for meson production in proton—proton collisions corresponds to 292 MeV in the laboratory system, the maximum kinetic energy of the mesons produced in the center-of-mass system is—

Fig. 1. Scheme of the setup for observing mesons.

Fig. 2. Energy spectrum of mesons (0°).

\(u\)—from the integral result
\(g\)—from the differential results

Fig. 3. Energy spectrum of mesons (30°).

Legend:
\( \circ \) cross section for \(C\)
\( \times \) cross section for \(H\)

Vertical axis: cross section in mb/steradian \((\times 10^{-31})\)
Horizontal axis: meson energy (MeV)

Fig. 4. Diagram of a hydrogen target.

is only 25 MeV; the meson velocity only slightly exceeds the velocity of the center of gravity of the system, and the flux of the mesons formed is distinguished by a strong forward directionality, while the upper limit of the meson energy depends strongly on the angle (74 MeV at 0° and 7 MeV at 90° in the laboratory system). Observations were therefore of interest at angles close to zero with respect to the direction of the primary proton beam. In the reviewed work the observation angle was \(0 \pm 5^\circ\). In all, 231 \(\pi^+\)-mesons from a polyethylene target and 176 \(\pi^+\)-mesons from a carbon target were observed. The intensity of the proton beam was determined with the aid of an ionization chamber calibrated by a Faraday cylinder, and in this way it was possible to determine the differential, in energy, cross sections for meson production on protons. In Fig. 2 the energy spectrum of \(\pi^+\)-mesons is given. A spectrum similar in appearance (Fig. 3) was obtained in the bombardment by protons of a target of liquid hydrogen, shown in Fig. 4. In experiments with a hydrogen target,³ photographic plates also served as meson detectors, but mesons directed at an angle of 30° to the primary proton beam were collected. In these experiments, tracks from 115 \(\pi^+\)-mesons were observed. In the meson energy spectra obtained in both works, of particular interest is the sharp “peak” at an energy close to the upper limit, and the subsequent sharp drop. A sharp peak cannot be satisfactorily explained if it is assumed that the final products of the reaction \(p + p = \pi^+ + p + n\)—the proton and neutron—do not interact (i.e., if one adopts the Born approximation). The results of the experiments rather testify in favor of a strong interaction of the neutron formed with the proton and even, possibly, in favor of deuteron formation. From calculation by energies and momenta it follows that if, in addition to the meson, a proton and a neutron are formed in \(pp\)-collisions, then the meson energy at an angle of 0° should not exceed 70 MeV. If, however, a deuteron is formed, then at 0° the upper limit of the spectrum is 74 MeV. In this case a monochromatic “line” should appear in the meson spectrum at an energy approximately 4 MeV (at 30°) exceeding the limit of the continuous spectrum. The form of the spectra shown in Figs. 2 and 3 near the upper limit would confirm the presence of such a “line” or a displacement of the upper boundary of the spectrum from 70 to 74 MeV. However, the accuracy of the experiments is still insufficient to draw definite conclusions concerning the interaction of the proton with the neutron formed together with the \(\pi^+\)-meson.

Since the Pauli principle limits the possible states of the initial \(pp\)-system, the proof of deuteron formation in a \(pp\)-reaction would have to impose definite restrictions on the assumed type of meson–nucleon interaction. Therefore additional experiments were carried out in which the dimensions of the hydrogen target were reduced in order to improve the angular resolution, and the background of scattered protons was lowered to \(1/500\) of the meson count with the aid of a magnetic deflector. In these experiments another 315 tracks of \(\pi^+\)-mesons were observed, and the initial data were confirmed. But in these experiments the accuracy is still insufficient for a resolution in energy of \(\pm 4\) MeV, if only because the primary proton beam itself has an energy of \(345 \pm 4\) MeV.

G. M.

CITED LITERATURE

1. W. F. Cartwright, C. Richman, M. N. Whitehead, and H. A. Wilcox, Phys. Rev. 78, 823 (1950).
2. V. Z. Peterson, Phys. Rev. 79, 407 (1950).