Pion-nuclear interactions at the SC and beyond

Abstract

The SC at CERN pioneered most of the lines in nuclear pion physics that were later explored in depth at the meson factories at LAMPF, at SIN or PSI and at TRIUMF. The golden age for these investigations was the period 1965—75, while the later programme with a few exceptions was overwhelmed by the superior intensities and beams at the meson factories. This programme at CERN established most of the general ideas which now are routinely exploited although with a far higher degree of sophistication. The impact has been far-reaching well outside of nuclear physics. The reason for this is that pions are the major nuclear constituent in addition to the neutron and the proton. In the early 1960s very little was known about the quantitative and even qualitative consequences of the pion for nuclear structure, except of course the one-pion-exchange nucleon—nucleon potential as a major explanation for the nuclear force. At the time, nuclear physics was mainly dominated by the recent breakthroughs in the understanding of nuclear shell structure and collective motion as well as by the intricate interplay of the two. There was no particular need for exploring those aspects using pions. At present the situation is very different. The pion degree of freedom is integrated in the mainstream of nuclear physics, solidly established as a principal nuclear feature, while nuclear quark physics now is the frontier that then was occupied by pion physics. Indeed, we have now reached a point at which a large part of modern nuclear pion physics is not even carried out by physical pion beams, but by virtual pions in many forms. All of this change is largely due to the pion programme at the SC. The origin of the programme, like the rest of the nuclear physics programme at the SC, was the crucial decision by Yiki Weisskopf in 1963 to broaden the CERN programme by exploiting this machine for another field instead of shutting it down. His far-sighted idea was that of a symbiosis between particle and nuclear physics with each fertilizing the other one. On the one hand one would exploit the properties of nuclei to learn more about elementary particles and their interactions, and on the other one would probe properties of nuclear structure using the high energy nucleons and mesons as probes. It is very interesting to examine the priorities and the perspectives at the time. In order to stimulate this idea Amos de Shalit and Viki Weisskopf made it known in the middle of 1962 that there would be a small conference at CERN in order to discuss these issues. This was enthusiastically received and, in fact, this meeting was the first in what has now become the PANIC conferences on High Energy Physics and Nuclear Structure, of which the 12th was recently held in Cambridge, MA, with over 700 participants. I had the luck as a rather fresh postdoc to be given the task of organizing this meeting at short notice, both scientifically and materially. Interestingly, it was easy to find logical main speakers on muonic atoms (Hans Sens), strange particle interactions