Reviews of Modern Physics

Abstract

T is really very hard to say something definite and scalar interaction, is correct, and further, we do not~p recise about meson theories. This is partly because know exactly what is meant by the cutoG in the theory. of the strong interaction between mesons and nucleons, A safe way of explaining the cutoff is to accept the as well as between two nucleons. But more than that, view that it is a mere expression of our ignorance. it is so because any simple-minded meson theory Perhaps this is an extreme viewpoint, but it is not easy cannot be a complete and closed theory. Obviously, to express more moderate views clearly. In the case it is much more an open theory than quantum electroof the pseudoscalar coupling, it is mell known that the dynamics. There are two reasons for this: First, the eGect of nucleon pair creation is so enormous that so existenceof newstrangeparticlescompelsustoenlarge far there is no satisfactory way of dealing with it. the scope of the theory so as to include these particles;This question is certainly very intimately connected second, we believe some real fundamental change is with the question of the nucleon-antinucleon internecessary in quantum Geld theory. We do not know action. We do not yet know exactly what the interwhen and how it will come, but the future modification action is; perhaps the complete understanding of the in Geld theory will aGect meson theory substantially antinucleon and nucleon system will come in a later whereas, in the case of quantum electrodynamics, the stage of development in what we may call a comprehenchange will be much more academic. For these reasons, sive theory. itis clear that the validity of any simple meson theory There was another feature in the simple-minded is considerably limited.meson theory from which we started. Twenty years Despite the fact that the present situation is dBBcult, ago, beta decay was a very important and outstanding definite progress certainly has been made in meson problem. Although Fermi's theory worked very nicely, theory in recent years due to Taketani, Levy, Chew, we could think at that time that we didn't need a and many others. This has been partly due to the direct Fermi interaction, but could account for beta increase in our knowledge of the pion-nucleon interdecay equally well through the meson field, by introaction. In the early days we had only limited knowledge ducing some weak interaction between the meson field about the nucleon-nucleon interaction, We actually and electron-neutrino system. Later such attempts knew nothing about mesons themselves. They were became more and more diKcult for two main reasons; just introduced in order to account for nuclear forces. one was that we came to know that the interaction At that time I was a little more ambitious; I wanted to responsible for beta decay is mainly scalar and tensor. incorporate the phenomenon of beta decay into the This is contradictory to the pseudoscalar nature of same scheme, but this was also rather speculative. the pion. Furthermore, the intervention of t...