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Measurements on the Rare Decay ff/' T MEASUREMENTS ON THE RARE DECAY i, WIĽLEM VAN DOESBURG <" "-- t, ,-; •'',, •.(-."V.íľ'-CrTí'í VRIJE UNIVERSITEIT TE AMSTERDAM MEASUREMENTS ON THE RARE DECAY -» e+e- ACADEMISCH PROEFSCHRIFT ter verkrijging van de graad van doctor in de wiskunde en natuurwetenschappen aan de Vrije Universiteit te Amsterdam, op gezag van de rector magnificus dr. H. Verheul, hoogleraar in de faculteit der wiskunde en natuurwetenschappen, in het openbaar te verdedigen op vrijdag 4 december 1981 te 13.30 uur in het hoofdgebouw der universiteit, De Boelelaan HOS door WILLEM VAN DOESBURG . geboren te Amsterdam Serv'Impri AMSTERDAM 1981 O5^.E3iaiíSKB4^ „„ Promotor : Prof. dr. H. Verheul Coreferenten : Dr. NW. Tanner Dr. E.G. Michaeüs 3v S'/ \ - • — VOOR RENÁTE VOOR MIJN OUDERS ? Lay-out : AUTHOR tekstverwerkingsprogramma 1 [- (CERN-Horst von Eicken) i f,, | Type-werk : Renáte van Doesburg 'i "'" | Figuren : A. Pomper/W.C. van Sijpveld 'jf I Drukwerk : Serv'Impri, Amsierdam I ,JĽ^^^.^^ CERN - GENEVE CHAPTER 1. INTRODUCTION AND SURVEY. 1.1. Introductory remarks. In 1935 Yukawa suggested that the nuclear forces were mediated by the virtual exchange of a particle. This so-called meson was finally observed around 1948, and is known as the tT-meson or pion with a mass of about 140 MeV, The pion has isospin one, and thus + o appears in three charge states IT , TT and tľ ; the spin of the pion is zero (it is a boson), and it can therefore be absorbed or created individually in nuclear interactions. The pion is unstable and decays into leptons or gamma rays, with -8 -16 life-times of 2.6*10 sec (charged pion) and 0.8*10 sec (neutral pion). It was recognised that pions are the most important constituents in the nuclear force, and hence an enormous effort was made during past decades to investigate the properties of the pion especially in the pion-nucleus system, both experimentally and theoretically. In its interaction with the nucleon, the pion generates many resonances or excited baryon states. One of these resonances is the (3,3)- or i-resonance which, with a width of about 100 MeV, dominates the pion-nucleon interaction between' 100 and 300 MeV pion energy. Consequently, a lot of research activity in this so-called intermediate energy range is going on to reveal further details of the strong interaction. Pions may be found in cosmic rays; in fact, this was the way they were first discovered. For experimental studies however, accelerators are needed to produce pion beams; accelerators designed for this purpose were put into service in the early fifties. The second generation of these accelerators are the "meson factories", from which at present good pion beams can be obtained. These meson factories include the Los Alamos Meson Physics Facility (LAMPF), the Swiss Institute for Nuclear Research (SIN) and the TRl-University Meson Facility (TRIUMF). Also at the European Organisation for Nuclear Research (CERN) in Geneva, pion beams are produced at the 600 MeV proton-synchrocyclotron. In addition, at the Medium Energy Accelerator (MEA) of NIKHEF-Amsterdam a pion beam is about to be produced mainly by the effort of the Free University of Amsterdam (see e.g. ref.l). w At ^CERN, a general purpose magnetic spectrometer called OM1CR0N" was set up for measuring processes in the domain of intermediate energy physics. While building up and testing the detectors for this spiectrome ter, an experiment was performed on the backward elastic scattering of pions from deuterium (refs.2,3). The first of fieiaf experiment with the spectrometer was on the backward elastic scattering of pions on carbon and oxygen (ref.4). The second experiment, which is the subject of this thesis, was an investigation of one of the decay modes of the neutral pion, namely o + - Tf —*• e e . The decay rate of this rare process is a probe to the "size" of the neutral pion rather than a probe to the pion-nucleus interaction. It o was favourable for the investigation to produce the TT at the A-resonance (which will be explained in ch.3). The aim of the experiment was to establish a value for the branching ratio for the above rare decay; so far from only one experiment a value for this branching ratio was claimed with a large error, and it is hoped that the final results of the experiment will yield a value with a statistical error of the order of 10X. In this thesis the set-up of the experiment will be presented together with its first results, deduced from the first experimental runs performed in 1980. 1.2. Survey of this thesis. In this presentation, apart from various general descriptions, the author has given more detailed accounts of those parts of the experiment where he has mostly been active. In chapters II and IV the OMICRON spectrometer will be presented together with the experimental set-up for the rare-decay measurements; chapter V contains a description of the electronics and data-acquisition. In chapter 111 a theoretical resume is given and, in addition, a discussion of the lay-out chosen in our experiment. The last chapters contain the description of the analysis and the first results, based on the 1980 data. *** In order to gain experience in the field of intermediate energy physics, the Department of Physics of the Free University of Amsterdam decided in 1976 to detach a few of its scientific associates at institutes where experiments with pions in this energy range were envisaged. In this manner the author of this thesis was the first to be stationed at CERN, and the Free University became a member of the OMICRON Collaboration. A considerable time was spent at CERN assisting in the build up of the apparatus and participating in various experiments. During the first two years of the detachment, the author of this thesis also participated in a couple of experiments "r me-onic atom* (refs.5,6). These experiments involved measurements of the ;'„ oub interaction widths and shifts in pionic atoms, which provides ys-fäict-jy^^m^^ij-jy^- important information on the threshold behaviour of the pion-nucleus interaction (see e.g. ref.7). 1.3. Acknowledgement It should be emphasized that the work presented in this thesis represents basically the effort of all the members of the OM1CRON Collaboration who participated in the rare decay experiment. Whenever a member of the Collaboration has been particularly responsible for or involved in some activity, his name has been entered into the text. In addition, the author owes many thanks to those who have remained unnamed. ^^ "'"" " ' References : 1. Pionen als spionnen, H.Verheul, rectorial oration VU-Amsterdam. 20-10-1979 2. A.Stanovnik, G.Kernel, N.W.Tanner. T.Bressani, E.Chiavassa, S.Costa, G.Dellacasa, M.Gallio, A.Musso, M.Panigňini, K.Bos, D.Frame, E.G.Michael is, W.van Doesburg, and J.D.Davies, N.l.M. 177 (19B0) 369 A.Stanovnik., G. Kernel, N.W,Tanner, T.Bressani, E.Chiavassa, S.Costa, G.Dellacasa, M.Gallio, A.Musso, M.Panighini, K.Bos, E.G.Michael is, W.van Doesburg, and J.D.Davies, Phys.Lett. 94B no.3 (1980) 323 E.Bason, K.Bos, T.Bressani, E.Chiavassa, S.Costa, J.D.Davies, G.Dellacasa, M.Gallio, E.G.Michael is, J.V.Jovanovich, G.Kernel, W.Lourens, N.Mirfakhrai, A.Musso, M.Fanighini, S.Playfer, M. Rapetti, F.Sever, A.Stanovnik, N.W.Tanner, R.van Dantzig, C.W.E. van Eijk, W.van Doesburg, and A.G.Zephat, paper presented at the 9th Int.Conf. on High Energy Phys. and Nucl. Structure, Versailles, July 6th-i0th 1981, EP Internal Report (CERN) 81-04 J.Konijn, J.K.Panman, J.H.Koch, W.van Doesburg, G.T.Ewan, T.Johansson, G.Tibell, K.Fransson, and L.Tauscher, Nucl.Phys. A326 (1979) 401 J.Konijn, W.van Doesburg, G.T.Ewan, T.Johansson, and G.Tibell, Nucl.Phys. A360 (1981)187 J.de Kam, Ph.D. thesis VU-Amsterdam, March 1981 CHAPTER 2. THE OMICRON SPECTROMETER. In this chapter, the OMICRON spectrometer will be presented, its configuration at CERN and its magnetic field with .subsequent párame terisat ion. 2.1. General review. In 1973 a working party called "OMICRON working party", formed by various physicists mainly from Oxford (GB) and Turin (I), started to make plans for a general purpose magnetic spectrometer of large solid angle and momentum acceptance, The desirability of such an apparatus arose from the physics interest in certain processes such as low energy pion scattering, rare decay-events and events like pion double charge exchange; a proposal for a spectrometer to enable such physics experiments was submitted to the Physics 111 committee which at that time governed the physics of the CERN 600 MeV-synchrocyclotron or SC (refs. 1 and 2). The. above physics interest was recognised, by the Physics III committee, and work started both at the participating institutes and at CERN for the "manufacturing of wire-detectors, the design of beams and preparation-pf'software and, of course, the preparation and assembly of the magnet. After a short period, an initial physics proposal (ref. 3) was submitted to the Physics ill committee by the thus established "OMICRON collaboration" where a small group of experiments was presented in détail, chosen, from the long list of possible experiments with such a large spectrometer. These experiments were approved in general terms, and the committee decided that approval and machine-time for each specific experiment would be considered in due course. The first of these experiments to be approved was the experiment of backward pion scattering on carbon and oxygen (exp.
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