I feel I should begin by pointing out that in at least two respects I am not qualified to give this talk. The first is that our machine at Liverpool is of course a 400 MeV machine, which only counts as a low-energy one these days, and I have not worked at C. E. R. N. where the real high-energy physics in Europe is now being done; I can only speak about it at second hand. I have, however, been making rather frequent visits to C. E. R. N. recently, thanks to an invitation from Professor Weisskopf, so that I can give some description of the counter experiments on the proton synchrotron there. The description is necessarily from a spectator’s point of view, and to that extent, superficial. The second lack of qualification comes from the fact that Professor Weisskopf has explained all the easy part about the significance of the most interesting counter experiments, so that I have to try and go a little further. Now, that necessarily involves me in the extremely sophisticated and conjectural ideas of the Regge pole analysis, which are not easy to explain to non-specialists. I shall try to convey the spirit if not the substance of that analysis. However, I should like to begin with a description of a different experiment, bearing on the elementary particle spectroscopy to which Professor Weisskopf drew your attention this morning. The main details of elementary-particle spectroscopy have of course come to use from bubble-chamber experiments, and, on the whole, the counter programme has not made a great contribution to it. One experiment, however, that is unusually clear is the counter experiment of Caldwell
et al
. on the production of associated bosons from peripheral collisions. Figure 37 shows the sort of process that is sought in this experiment. A highenergy pion beam is directed at a nucleon and glancing collisions are sought; in other words, collisions that take place at a long range and are probably associated with the exchange of one particle. Of course, the range of the interaction is longer when the mass of the exchange particles is small, so a single pion is most likely to be exchanged. The nucleon emits this pion and may itself break up into a number of particles, which the experiment does not investigate any further. At the other vertex the exchange particle joins the pion and, hopefully, makes a compound particle which later breaks up into associated bosons, either two pions or two kaons. If the particle exchanged is a pion, of course, this short-lived compound particle has strangeness zero, and therefore it can only break up into two pions or two kaons, but not into a kaon and a pion.
Three-Reggeon cuts in QCD amplitudes