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Strange Particle Theory in the Cosmic Ray Period R STRANGE PARTICLE THEORY IN THE COSMIC RAY PERIOD R. Dalitz To cite this version: R. Dalitz. STRANGE PARTICLE THEORY IN THE COSMIC RAY PERIOD. Journal de Physique Colloques, 1982, 43 (C8), pp.C8-195-C8-205. 10.1051/jphyscol:1982811. jpa-00222371 HAL Id: jpa-00222371 https://hal.archives-ouvertes.fr/jpa-00222371 Submitted on 1 Jan 1982 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C8, suppldment au no 12, Tome 43, ddcembre 1982 page ~8-195 STRANGE PARTICLE THEORY IN THE COSMIC RAY PERIOD R.H. Dalitz Department of Theoretical Physics, 2 KebZe Road, Oxford OX1 3NP, U.K. What role did theoretical physicists play concerning elementary particle physics in the cosmic ray period? The short answer is that it was the nuclear forces which were the central topic of their attention at that time. These were considered to be due primarily to the exchange of pions between nucleons, and the study of all aspects of the pion-nucleon interactions was the most direct contribution they could make to this problem. Of course, this was indeed a most important topic, and much significant understanding of the pion-nucleon interaction resulted from these studies, although the precise nature of the nucleonnucleon force is not settled even to this day. My own entry into the field of strange particle physics came about as a result of working at Bristol University for the academic year 194819 as a research assistant to Prof. N.F. Mott. Much impressed by the fundamental discoveries being made in the attic ('the fourth floor') of his department at Bristol, by Prof. C.F. Powell and his highly international group of co-workers, Mott had formed the intention of wor~ingagain in the field of nuclear physics, as he had done earlier in the 30's. Although not one of them, my contact with many of the younger people on the fourth floor was quite close and I was well informed about the new discoveries being made there. After moving to work with Prof. R.E. Peierls at Birmingham University in 1949, I remained in touch with them and with this work at Bristol in the subsequent years. The contributions from cosmic radiation studies dominated strange particle physics until about 1954. The first strange particles produced in laboratory experiments using accelerator beams were observed in 1953 and the contributions from accelerator experiments then grew rapidly, especially after the Bevatron began operation at the Radiation Laboratory, Berkeley, late in 1954. The Bagneres-de-Bigorre conference of 1953 was the last elementary particle physics conference where the data came entirely from cosmic ray work. Bearing in mind the title of this Session, I have decided to review the major conferences in the field which took place about the years 1953 and 1954, and to discuss what theoretical work concerning new particles was presented ac them. The first conference to consider is the Third Rochester Conference, held 18-20 December 1952. Of its seven sessions, three gave some attention to new particles; session III was on VO Particles, session IVA was on Superheavy Mesons, and the latter half of session V was given to Megalomorphs. It is worth mentioning Amaldi's report on r-meson events, in session IVA, which described two events from Bristol, three events from Imperial College, London, and one each from Padua and from Rome, their mean mass being 500-1_2 MeV. Not all of the secondary particles could be identified but those identifiable were pions, and the consistency between all eight decay events pointed to their most probable identification as There was no theoretical discussion reported, concerning the new particles. There were a number of theoretical sessions, of course, devoted to the nucleon- nucleon interactions and to pionnucleon scattering and related phenomena. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982811 JOURNAL DE PHYSIQUE The next meeting to mention was a Discussion Meeting held on 29 January 1953 at the Royal Society in London, organised by Powell to enable a thorough review to be made of all of the evidence available on new particlesL11. It was primarily a U.K. meeting, most of the papers being from Bristol, London and Manchester, but there were reportsalso from Paris (Ecole Polytechnique) , Milan, Padua and Rome, while Butler presented a report on the M.I.T. work on S particles. The powerful analyses which were made of these events, especially by the emulsion group from Bristol, were very impressive, and it became abundantly clear that the identification of the T-meson decay (1) was on a solid foundation. At last the complete decay mode had become established for at least one new meson. Now there were 11 T meson decay events known and the time was ripe to give some serious consideration to their characteristics, in terms of the T-meson spin and parity. There were also decay events giving only a single a+ secondary, whose energy varied from event to event but lay in the energy range allowed for the decay mode which would necessarily have a close relationship with the decay mode (1). There was no theoretical work presented at this meeting; the time was still too early for that. Its special value lay in the coming together of experimental groups working on a common problem in widely separated places and with different techniques or different approaches, and the importance of this cannot be over- estimated. Questions can be raised and explored, and often settled there and then. The conclusions reached in this way, through open presentation of the data from a wide range of experiments and its comparison and evaluation, have a force much greater than that resulting from the publication of a number of separate papers, each necessarily somewhat tentative and all scattered through a variety of scientific journals. The International Cosmic Ray Conference for 1953 was arranged to concentrate on the new particles, and this was a major event in the lives of all the physicists who took part in it. It was held 6-12 July at Bagnsres-de-Bigorre, in the Basque country on the northern slopes of the Pyrenees. During this conference, it became clear that there was a substantial consensusconcerning the subject matter of all this widespread cosmic ray work. Previously it had seemed as if a new decay mode, or perhaps a known decay mode for a new parent mass, was being reported almost every month, but now it was seen that the most frequent decay nodes were quite limited in number and were associated with fairly definite mass values. Previously, the V: -t pn- particle was the only well established VO particle, known to us as the A(1115) hyperon; now, the pain- stakingly precise work of Thompson had established the existence of the V: -+ a+a- particle, also known as the e0 meson, with mass 496f5 MeV, comparable with that for the meson. Of course, the cloud chamber work still left some further neutral events, labelled v!,v~,. ., and some charged V* events to be sorted out, the latter still to be related with the at-rest decay events reported from the emulsion work. The new techniques for using layered emulsion blocks were being perfected and it was becoming ~ossibleto follow charged secondaries through many layers and so to identify them uniquely and to measure their energies from their range. In committee, rules were drawn up for the formal specification of any new particles or new decay modes. It was an exciting time, as if the mists were lifting and we could at last look ahead. There were five theoretical papers presented. A paper by Cheston and Primakoff, neither of whom are listed among the participants, gave a quantitative discussion of the possibility that a A hyperon bound to nucleons might undergo non-mesonic de-excitation, releasing the full 176 MeV available as kinetic energy for the final nucleons. The first identified A hypernucleus was reported by Danysz and Pniewski 121 only in early 1953, so this was a remarkably quick response to a new phenomenon. Bhabha discussed multiple production of mesons in high energy nuclear collisions, relating it with the localisation of field energy within a nucleon, following the theoretical ideas put forward by Fermi C31 and by Heisenberg C41. Michel gave an important survey of the absolute selection rules governing particle decay processes, as understood at that time. He provided a large table showing all the two-body and three-body decay modes conceivable for the e0 and rf mesons, together with their forbidden- ness (or otherwise) according to the selection rules of angular momentum, parity and charge-conjugation, noting (as an example) that they could be 'corresponding particles' if they had spin-parity (2+). Haber-Schaim, Yasin and Yekutieli reported on a calculation of K-meson production in nucleon-nucleon collisions, based on Fermi's statistical theory L3 1. My own paper was concerned with the analysis of the T-meson decay process (1) in terms of its spin-parity, work which had been done C51 following the Discussion Meeting mentioned above. It was my opinion that the amplitude for the decay mode (1) should be largely calculable in form (although not in magnitude) in terms of angular momentum barrier considerations, apart from a few parameters necessary when the total angular momentum and parity could be apportioned to the internal orbital motions within the three-pion system in more than one comparable way.
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