Nuclear Capture of Mesons and the Meson Decay

The experiment of Conversi, Pancini and Piccioni [1] indicates that the probability of capture of a meson by nuclei is much smaller than would be expected on the basis of the Yukawa theory [2], [3]. Gamow [4] has suggested that the nuclear forces are due to exclusively to the exchange of neutral mesons, the processes involving charged mesons and the processes having probabilities which are smaller by a factor of about 1012.
We notice that the probability (~106 sec-1) of capture of a bound negative meson is of the order of the probability of ordinary K-capture processes, when allowance is made for the difference in the disintegration energy and the difference in the volumes of the K-shell and of the meson orbit. We assume that this is significant and wish to discuss the possibility of a fundamental analogy between processes and processes of emission or absorption of charged mesons.
An immediate consequence of the experiments of the Rome group [1] is that the usual interpretation of the process as a "two-step" process ("probable" production of virtual meson and subsequent decay of the meson) completely loses its validity, since it would predict too long lifetimes: the meson is no longer the particle responsible for nuclear process, which are to be described according to the original Fermi picture {without mesons). Consequently there is no need to assume that charged mesons have integral spin, as Yukawa explanation of processes required. Once we believe that the ordinary process is not connected in any way with the meson, it is difficult to see strong reasons for the usual assumption that the meson decays with emission of a particle and a neutrino. We shall consider then the hypothesis that the meson has spin 1/2 and that its instability is not a process, in the sense that it does not involve the emission of one neutrino. The meson decay must then be described in a different way: it might consist of the emission of an electron and a photon or of an electron and 2 neutrinos [5] or some other process.
In the hypothesis that the meson decay is not a process (meson of spin 1/2) the process of nuclear absorption or production of a single meson would be accompanied by the emission of a neutrino. This analogy between particles and mesons suggests, in addition, that just as the production of single particles is extremely unlikely, while the production of electron pairs is a very likely phenomenon, so the production of a single charged meson would be very unlikely, while the production of pairs of mesons would be quite probable. The experimental evidence is, in fact [6], that most, if not all, of the meson showers are created in connection with large Auger showers.
The assumption that the emission or absorption of one meson is accompanied by the emission of a neutrino would explain in a natural way a somewhat puzzling experimental result. Among the few pictures of a meson stopping in the gas of a cloud chamber, no "star" has been observed at the end of the meson track [7]. The absence of a star must be due to a process leaving the capturing nucleus in a not too excited state: the mechanism proposed here would explain that the capture of a negative meson from a nucleus Z results in a nucleus Z-1 close to its ground level, since the excess energy could be carried away by the neutrino. Actually, in such a process we should expect that the emission of a neutrino of high energy with consequent production of the nucleus Z-1 in a state of low excitation would be more likely than the emission of a neutrino of low energy with the production of the nucleus Z-1 in a state of high excitation (cf. K-capture process).
The hypothesis that the meson decay is not a process, while the meson absorption is a process, does not require that hypothetical particles such as neutral mesons are invoked to account for nuclear forces. In fact, a heavy electron pair theory of nuclear forces was successfully developed by Marshak [8]. Moreover, a pair theory is capable of accounting, at least in principle, for the existence of processes in which several pairs of mesons are produced in a single act, as suggested by Heisenberg in connection with a different problem [9].
Returning to the actual decay of the meson, an experiment suggests itself which might answer the following question: Is the electron emitted by the meson with a mean life of about 2.2 microseconds accompanied by a photon of about 50 MeV? This experiment is being attempted at the present time, since it is felt that the available analysis [10] of the soft component in equilibrium with its primary meson component is probably insufficient to decide definitely whether the meson decays into either an electron plus neutral particle(s) or electron plus photon.

June 21, 1947

National Research Council
Chalk River Laboratory
Chalk River, Ontario, Canada

References

  1. M.Conversi, E.Pancini and O.Piccioni, Phys.Rev., 71 (1947) 209, see also T.Sigurgeirson and A.Yamakawa, Phys.Rev., 71 (1947) 319.
  2. E.Fermi, E.Teller, and V.Weisskopf, Phys.Rev., 71 (1947) 314.
  3. J.A.Wheeler, Phys.Rev., 71 (1947) 320.
  4. G.Gamov, Phys.Rev., 71 (1947) 550. See also G.Gamov and E.Teller, Phys.Rev., 51 (1937) 289.
  5. W.Nordheim, Phys.Rev., 59 (1941) 544.
  6. G.Cocconi, A.Loverdo and V.Tongiorgi, Phys.Rev., 70 (1946) 852.
  7. See for a critical survey; T.H.Johnson and R.P.Shutt, Phys.Rev., 61 (1942) 380.
  8. R.E.Marshak, Phys.Rev., 57 (1940) 1101.
  9. References can be found in Cosmic Radiation, edited by W.Heisenberg (Dover Publications, New York, 1946), p.127.
  10. See reference 9, pp.84-97.