NEW PHOTOGRAPH OF MESON DECAY IN A WILSON CLOUD CHAMBER*

The determination of the products of meson decay and of their energies is at present, in connection with the discovery of a discrete spectrum of meson masses,^1 of especially great interest: if a meson decays into two particles, then discrete spectra of the energy of the decay products of mesons at rest should also be expected. The greater part of the mesons in air have ranges of the order of kilometers; therefore the probability of detecting a meson that has stopped in the small volume of a Wilson chamber is very small, especially at sea level, where there are few slow mesons. Indeed, up to the present time only about 15 cases of meson stopping in the gas of a Wilson chamber have been recorded. In only three of these cases was it possible to observe the decay of a meson stopped in the gas.

The first photograph of meson decay was obtained by Williams and Roberts^2 in 1940, the second by Williams and Evans^3 in the same year, and the third by Shutt, Benedetti, and Johnson^4 in 1942.

The latter authors obtained their photograph of meson decay in a high-pressure chamber (70 atm). The mass of the meson was estimated by them from the magnitude of the scattering which it underwent in the gas of the chamber. It proved to lie within the limits of 40–400 electron masses. Since there was no magnetic field, the energy of the particle formed in the decay could not be determined. Williams and Evans also worked with a high-pressure chamber placed in a magnetic field of strength 2700 oersted. Unfortunately, because the tracks are situated in a poorly illuminated part of the chamber, this photograph does not make it possible to determine the mass of the meson and the energy of the particle formed in the decay. The determination of these quantities can be made only from the first photograph of Williams and Roberts,^2 who used an ordinary chamber with a pressure of 900 mm Hg in a field of 1180 oersted. The mass of the meson stopped in the gas of the chamber, positively charged, determined from the range and radius of curvature of the trajectory, is equal to \((250 \pm 70)\,m_e\). At the end of the meson trajectory there arises the track of a light positively charged particle (positron), whose energy is equal to \(70\ \mathrm{MeV} \pm 50\%\). If a meson at rest decays into an electron and a neutrino (or photon), these particles carry away half the energy of the meson at rest. Therefore, for a meson of mass \(200\,m_e\) \((Mc^2 = 10^8\ \mathrm{eV})\), the energy of the decaying meson should

* Anderson, Adams, Lloyd, and Rau, On the mass and products of meson decay, Phys. Rev. 72, No. 8 (1947).

equals 50 MeV. Thus even this, the best, photograph does not make it possible to assert with certainty whether a meson at rest decays into an electron and a neutrino or into an electron and a photon. It can only be asserted that the obtained values of the meson rest mass and of the energies of the particles formed in the decay do not contradict this hypothesis. The October issue of Physical Review contains a new, fourth in number, photograph of meson decay, obtained by Anderson, Adams, Lloyd, and Rau while raising a Wilson chamber in an airplane to an altitude of 9200 m. In the photograph, the scheme of which is shown in the figure, one clearly sees the track of a positively charged meson (A), with ionization density steadily increasing toward the end, and a faint track of a fast positive particle which arose at the end of the meson track (B).

The energy of this particle is equal to 24 MeV ± 10%. Therefore, if in the given case the meson decayed into a positron and a neutrino, the meson mass has the value \((100\,m_e \pm 10\%)\).

The meson mass can also be determined from the radius of curvature of the trajectory and the range. The usual determinations of the radius of curvature of the trajectory of a meson slowing down in the gas of a Wilson chamber are inaccurate because of the influence of multiple scattering. In the present case, however, the role of multiple scattering is relatively small, since the chamber was placed in a very strong magnetic field (\(H = 7500\) oersteds). The authors indicate that the observed values of the curvature of the trajectory and of the range likewise give, for the meson mass, the value \(100\,m_e\). The role of multiple scattering is such that the observed trajectory could be produced by a meson of mass \(200\,m_e\) with probability 20%, and by a meson of mass \(300\,m_e\) with probability 5%. It may therefore be considered that the photograph presented corresponds to the case of decay of a meson with mass \(100\,m_e\) into an electron and a neutrino or into an electron and a photon. However, the authors themselves do not adhere to this opinion, considering the existence of a meson with mass \(100\,m_e\) improbable.

They suppose that their photograph records a case of decay of a positively charged meson into a neutral meson and a positron. Knowing the mass of the primary meson and the energy of the positron (24 MeV), it is easy, from the laws of conservation of energy and momentum, to calculate the mass and energy of the neutral meson. If one assumes that the mass of the primary meson is \(200\,m_e\), then for the mass and kinetic energy of the secondary meson one obtains values \(140\,m_e\) and 4 MeV. For a primary meson mass of \(300\,m_e\) these quantities are, respectively, \(215\,m_e\) and 2 MeV. In the authors’ opinion, the possibility of such a decay variant is confirmed by the experiments of Lattes, Occhialini, and Powell.^5 The latter found, on thick-layer plates exposed at altitudes of 2800 and 5500 meters, 11 cases of meson decay in which the visible decay product is a charged meson of lower mass. The range of these secondary mesons was the same in all cases and equal, within the scatter of the ranges, to 600 microns of photoemulsion. The masses of the primary and secondary mesons, determined from the range and grain density, have values of about \(200\,m_e\) and \(300\,m_e\), while the kinetic energy of the secondary mesotron is equal to 3–4 MeV. Thus the work of Lattes, Occhialini, and Powell proves the exist—

the existence of charged mesons of decay with a mass of about \(200\,m_e\) and a kinetic energy of several MeV. Obviously, the authors point out that if the decay of a meson into a charged meson and a light neutral particle is possible, then such a decay of a meson is also possible in which a neutral meson is produced, while the charge is transferred to the light particle. The authors consider that their photograph represents precisely such a case of decay.

A. Weissenberg

CITED LITERATURE

1. A. Alikhanian, A. Alikhanov, A. Weissenberg. Doklady Akad. Nauk SSSR, 55, No. 8 (1947).
   A. Alikhanian, A. Alikhanov, V. Morozov, G. Muskhelishvili, A. Khrimian. Doklady Akademii Nauk SSSR, vol. 58, No. 7 (1947).

2. Williams and Roberts, Nature 145, 102, 151 (1940).

3. Williams and Evans, Nature 145, 818 (1910).

4. Shutt, Benedetti and Johnson, Phys. Rev. 62, 558 (1942).

5. Lattes, Occhialini and Powell, Nature 160, 453 (1947); translation in the present issue of UFN.