CHS-17 March 1985 STUDIES IN CERN HISTORY High-energy physics from 1945 to 1952/ 53 Ulrike Mersits GENEVA 1985 The Study of CERN History is a project financed by Institutions in several CERN Member Countries. This report presents preliminary findings, and is intended for incorporation into a more comprehensive study of CERN's history. It is distributed primarily to historians and scientists to provoke discussion, and no part of it should be cited or reproduced without written permission from the Team Leader. Comments are welcome and should be sent to: Study Team for CERN History c/oCERN CH-1211 GENEVE23 Switzerland © Copyright Study Team for CERN History, Geneva 1985 CERN-Service d'information scientifique - 300- mars 1985 HIGH-ENERGY PHYSICS from 1945 to 1952/53 I. The scientific situation in 'elementary particle physics' around 1945/46 I.1. Cosmic-ray physics I.2 Nuclear physics II. Institutional changes in nuclear physics due to the war III. The post-war accelerator programmes III.1. The principle of phase stability III.2. The United States III.3. Great Britain - the leading country in Europe III.4. Continental western Europe III.5. AG focusing - another step into higher energy regions IV. Experimental particle physics: developments from 1946 to 1953 IV.1. The leptonic nature of the mesotron and the detection of the pi­ meson (1946/47) IV.2. The artificial production of charged and uncharged pi-mesons (1948/49) IV.3. The complexity of the mass spectrum (1947-1953) IV.3.1.The V-particles IV.3.2.The heavy mesons IV.3.3.The Bagneres-de-Bigorre Conference (1953) V. The theoreticians: from the first Shelter Island Conference to the Rochester Conferences (1947-1952) VI. Concluding remarks HIGH-ENERGY PHYSICS FROH 1945 TO 19521531 High-energy physics (or elementary particle physics) in general, and CERN in particular, have now reached an enormous size. To be able to understand better why in the early fifties such an enterprise was under­ taken, why, having chosen high-energy physics as a topic for investigation, it took the form that it did, we want to pursue the development of this part of physics from the immediate post-war years up to the period when CERN was founded. Thus we will study in particular detail those aspects of the evolution of the field which were of relevance to the setting-up of CERN, although it is D..Q.t. our intention to give a complete description of the development of high-energy physics. Regarding the situation immediately after the war, two rather different aspects are of major interest for our purposes, the scientific situation and the institutional changes which had taken place due to the war. These two aspects will be studied in Chapters I and II. For the period after 1946 we will present the three major avenues of development which were collectively responsible for the shaping of elementary particle physics in the fifties. These were: the development of accelerators, the experimental data collected on these elementary particles, and the theoretical assessment of the events. Each of these parts has its own internal evolution, and we will study them in Chapters III, IV and V respectively. Finally, having done this we will then be in a position to present a more comprehensive picture of the whole situation, showing the points of interaction between the different courses of events. In this way it will be possible for us to see roughly when, how and why elementary particle physics 2 had become an independent discipline using accelerators as its main tools - a discipline of high prestige and considerable attractiveness, which was regarded with fascination by many of the physicists. Before we enter this study, it seemed necessary to us to clarify two notions rather frequently used namely nuclear physics and elementary particle physics (high-energy physics). If we use them in the way we under­ stand them nowadays, nuclear physics means the study of the atomic nuclei, of the reactions occuring when they are bombarded with particles like protons, neutrons or photons, of radioactive decays, etc. The decisive energies would therefore be those given by the binding energy of the protons and neutrons in the nucleus, i.e. several MeV (up to roughly 200-300 MeV). Elementary particle physics means the study of the particles building up the nucleus and those found elsewhere in nature and their interactions with each other. The spatial dimensions of the objects under study are smaller than in the case of nuclear physics so that the energies needed for studying them are much higher than those necessary there. They start at some 300 MeV (threshold for meson production) and have in fact no upper limit.2 Now that we are aware of how these notions are used today we must pay special attention to the way they were used in the different stages of their development, as this is to a certain extent indicative of the stage of evolution of the discipline. I. The scientific situation in 'elementary particle physics' around 1945/46 In the immediate post-war period, notions like elementary particle physics or high-energy physics did not in fact exist. However, if we wish to study the origins of this discipline by the end of the Second World War, we have to take account of all parts of physics which deal with the constitution of matter in general, i.e. with the particles out of which matter is built and the way in which they interact with each other. This leads us to study two different approaches to such investigations, namely cosmic-ray physics on the one hand and nuclear physics on the other. The division is, as we will see, more important for experimental work. For the 3 theoreticians such a division did not exist they were doing nuclear physics in a rather broad sense, taking the basic experimental data from both sources. Almost a year after the end of the Second World War the first big European conference in the field of particle physics took place. It was held from 22-27 July 1946 at the Cavendish Laboratory in Cambridge and was entitled Fundamental Particles and Low Temperature. 3 Its task was twofold to provide a meeting place for the international community of physicists and to help in orientating them in the steadily growing field. It was important as a meeting place because international contacts had become impossible or at least rather difficult during the war. Thus the conference offered a welcome opportunity to establish new contacts or to re-establish old ones. In all some three hundred physicists attended the conference, one third of them from foreign countries. Many European states, excluding Germany for well-known reasons, as well as other countries like the USA, the USSR, China, and India were represented. The conference's designation combined two completely different subjects of which only the first is of interest for our purpose. Even for this section the title Fundamental Particles was rather general. In the preface to the proceedings this was explained by the fact that 'the subject of Fundamental Particles is nowadays so wide that the arrangement of the programme was not easy. The time available for the whole Conference could well have been filled by the discussion of any one of a number of topics which in fact were compressed into a single session or less. But, at the first post-war gathering, one was reluctant to narrow down the field too much.' This conference was meant to give as broad an overview as possible to demonstrate all the possibilities for further investigation. This section of the conference comprised five sessions in all on 'General introduction and survey', 'Mesons and cosmic rays', 'Experimental techniques', 'Nuclear forces and relativistic particles' and 'Theory of Heisenberg's s-matrix' respectively. We now wish to study the different 4 approaches to these topics by both the cosmic-ray physicists and the nuclear physicists. In this way it will be possible for us to see the 'state of the art' in these fields. I ..1 Cosmic-ray. ph ys1cs. 4 Before further use is made of the term 'cosmic-ray physics', we should clarify what this meant in the mid-forties. Basically, one could divide the work of the cosmic-ray researchers into two big categories according to the type of questions they were trying to answer: 1. What are the constituents of cosmic radiation? 2. What is the origin of cosmic radiation and what are its effects on Earth? The investigations dealing with the first question are of primary interest to us. We will not deal with the second question, but it should be mentioned that this question came to represent the main task for the cosmic-ray physicists after the particle aspect of their work had been taken over by physicists working with accelerators. Let us begin by looking at the field from a scientific angle with a summary of the particles known at the time. Altogether eight particles were known by 1945, namely the electron, the positron, the proton, the neutron, the photon, the neutrino, the positive and negative mesotron (named mu-meson by Powell in 1947). It should be mentioned here that, due to the similarity in mass, the aesotron was (wrongly) thought at the time to be the field quantum mediating the strong interaction, as predicted by Hideki Yukawa in 1935. 5 One of the five sessions at the Cambridge Conference was exclusively devoted to mesons and cosmic radiation. Twelve talks were given and physicists like P. M. S. Blackett, G. Bernardini, B. Feretti, L.