A New Day for the Weak Interaction

Abstract

weak interaction is one of the least understood topics in particle physics. is one of the four kinds of force that physicists recognize in nature, and, as its name indicates, it is much weaker than two of the others, the strong (nuclear) interaction and electromagnetism. has therefore been difficult to study because its effects are usually masked by the two stronger forces. In a few years the world will have at least two particle accelerators with capacities of hundreds of billions of electron-volts energy, and physicists hope that these machines may allow the beginning of a new era of systematic and vigorous investigation of the weak force. There is a finite chance that in our lifetime we'll see something interesting in the weak David B. Cline of the University of Wisconsin told the recent Coral Gables Conference on Fundamental Interactions at High Energy at the University of Miami. first manifestation of the weak interaction that physicists noticed was nuclear beta decay. In beta decay a neutron inside an atomic nucleus turns into a proton, emitting by the way an electron (called in the old days a beta ray) and an antineutrino. In recent years many other particles have been found that decay radioactively under the influence of the weak interaction. From these decay activities alone it is hard to arrive at a good mathematical description of the weak force or to test rival theories. That can be better done by investigating how the force acts in particle collisions in accelerator experiments. But the two stronger forces blot out the effect of the weak force unless one party to the collision is a neutrino. (Neutrinos are the only known kind of particle that responds to the weak force but to neither of the two stronger ones.) To do experiments with neutrinos requires a copious and energetic beam of them. world's two new accelerators (the National Accelerator Laboratory now in operation at Batavia, Ill., and CERN II beginning construction near Geneva) will provide such beams. They may be used to study the weak force itself and to use it as a means of probing the structure of other particles. The new accelerators may very well provide a new era in neutrino spectroscopy, A. K. Mann of NAL told the Coral Gables Conference. It may be the new optics of the remainder of this century. One of the most important questions to be decided by forthcoming experiments is whether the weak force is a local interaction or a nonlocal one. A local interaction is one in which the two interacting particles affect each other directly, rather like two billiard balls colliding. In a nonlocal interaction, the interacting particles exchange an intermediarly particle, and this particle mediates the effect of one to the other. In the strong interaction, the intermediary particle is a virtual meson; in electromagnetism it is a virtual photon. Formulations of both kinds have been suggested for the weak interaction. In the traditional nonlocal formulation, the intermediary is called the intermediate vector boson. Lately some theorists, most prominently T. D. Lee of Columbia University (SN: 10/9/71, p. 252) and Steven Weinberg of Massachusetts Institute of Technology, have been trying to work out a combined theory of the weak interaction plus electromagnetism. In this case there is a family of intermediaries among which both the vector boson and the photon find a place. local formulations arouse dismay because in them the particles most concerned with the weak interaction, the class called leptons (the electron, the muon, the two kinds of neutrino and their respective antiparticles), are point particles; like a formal geometric point, they have zero dimensions. is difficult to visualize a body with mass that does not occupy space, and, when one tries, all sorts of disturbing paradoxes come forth: particles have infinite mass densities, infinite self-energies, infinite electric charge densities and other infinities, some of which appear to be at variance with known properties of the particles. A nonlocal weak interaction allows the particles to occupy space. also does away with the troublesome infinities. Besides its esthetic qualities, what evidence is now in hand tends to incline some people toward the nonlocal theory. The point theory of the weak interaction cannot be exact, says Mann. He and others expect that the inexactness will become clear in the new range of energies about to be opened up. At energies near 300 billion electron-volts (GeV) something new may happen, he suggests. Some structure may appear in the interaction: maybe a vector boson propagator, maybe new kinds of leptons, maybe new neutrinos, maybe higher~~~~~~~~~~~~~~~~~~~~~~~~~~~~~lr i,.,...,...,.~~~~~~~~~~~~~~~~-,-----------------'>.' s