A STUDY OF 4 Gev/c % -p INTERACTIONS USING A BUBBLE CHAMBER. by 'LIU YAO SUN A thesis presented for the degree of Doctor of Philosophy of The University of London and for the Diploma of the Imperial College, Department of Physics, Imperial College of Science & Technology, London, S.W.7. =============== June 1963. CONTENTS Page Abstract AS 00 SO 0* ts 1. Preface .• .. .. .. PO 2. Introduction Os 400 OP ** 3. CHAPTER I. Pion Resonances. 1.1'. The Chew-Low Extrapolation Method. 5. 1.2. The Pion-Pion Interaction and the p-particle. 8. 1.3. Other Pion Resonances. 10. CHAPTER II. Contribution of the P -particle to g.-P Scattering. 2.1. One Pion Production. 14. 2.2. Low Energy Elastic Scattering. 16. A PENDIX 2.A. .. 19. APPENDIX 2.B. 27. CHAPTER III. Apyaratus and Experimental Procedure. 3.1. The Hydrogen Bubble Chamber. 33. 3.2. The Pion Beam. 34• 3.3. Method of Scanning. • • 34. 3.4. Measurement of Events. 38. 3.5. Data Processing. 40. 3.6. The Maximum Detectable Momentum. 43. 3.7. Measurement of Beam Momentum and Related Quantities. 45. CHAPTER IV. Interpretation of Events. 4.1. Interpretation of Events by -Pitting Prododure 50 412i Identification of Track by Ionization Measurement. 52 CHAPTER IV. -contd. 4.3. Scanning Efficiency and Cross-sactions 53 CHAPTER V Analysis of Four-prong Events. 111•• 5.1. The Reaction + p p + + 63. (A) Angular and Momentum Distributions. (B) Productional channels. (C) The 3 -particle channel. (D) The charged 3/2, 3/2 Isobar channel. (E) The Direct Channel. 5.2. The Reactions t)r +p-p+r;,- + +cir 71. + - and fir + p n + (IT + + rx + qr (A) Anestilnx. tea. momentum Distributions. (13) Production of N (A) and ni . 5.3. Events with More than one Neutral Secondary Particles. 74 (A) Angular and Momentum Distribution. (B) The Effective Mass Distributions. CHAPTER VI Discussion. 6.1. The Effect of Increasing Multiplicity 114 6.1. Bipion States 114 (A) The Possible 4q7 Resonances at 395 and 520 Mev. (n) The 1=2 Resonance. (C) The Asymmetry In +ha5°-Deceiy. 6_1, Thd Nucleon-Two-Pion System. 117. 6.4. Three Pion System. 118. ACKNOWLEDGEMENTS. Os 00 REFERENCES. • • •• •• 131. CAPTIONS TO FIGURES, •• 135. 1. ABSTRACT' The interactions of 4-Gevic negative pions with protons have been studied by exposing the 81 cm Hydrogen Bubble chamber of Saclay and Ecole polytechnique to a 91:7-beam originating from the CERN Proton synchrotron. The results of a study of 2128 events with four charged secondaries are presented in this thesis. Chapter 1 contains a brief account of pion resonances, Chapter 2 discusses possible contributions of the p -particle io some 7c7- p reaction channdis. In chapters 3 and 4, apparatus and method of interpreting individual events are presented. Results of various reaction channels are presented and discussed in chapters 5 and 6. Cross-sections for these reactions including p w and N productions are given..Possible evidence for dipion resonances at 400 and 500 Mev ••• are presented and the presence of other resonances examined. 2. PREFACE The work presented in this thesis was undertaken by the author, as a member of the Bubble Chamber Film Analysis Group at Imperial College, under the supervision of Professor C.C. Butler, The author joined the group in 1959. He spent a year studying the basic theory of elementary particles before taking part in film analysis for the 24 Gev/c P-P experiment in which the group was a collaborator. After the completion of measurement and reconstruction of the P-P events, the author took responsibility for organizing the scanning, measuring of some 10 Gev/c 117-p films which were taken in CERN using the 81 cm Hydrogen Bubble Chamber of Saclay and Ecole Polytechnique. The primary beam was then found io have far too large a momentum and angular spread to make good analysis possible. Subsequently, in 1961, a decision was made to carry out a more refined experiment using a 4 Gev/c pion beam in the Bubble Chamber, in collaboration with the Achen, Munich, Hamburg, Bonn and Birmingham groups. Since November 1961, about 100,000 7i7-p pictures have.1►v produced during 3 runs, together with an equivalent number of ie-p pictures. The author took part in two of these runs. The analysis of the results of the /t-p experiment in this thesis was partly made by the author. The work on the p —particle contribution to pion-proton scattering discussed in Chapter II is based on a paper by the author and his colleague Mr. A-U-Zaman, published in Il Nuovo Cimento, 1962. 3. INTRODUCTION Since the existence of the %-meson (or pion) was predicted by Yukawa in 1935 and its subsequent discovery in 1947 by Powell et al, many experiments have been carried out in order to determine the properties of free pions and to explore their interactions with nucleons. The main properties of free pions are now determined whereas the interactions of pions and nucleons are still not well understood, although explored by many experiments on pion-nucleon scattering and pion photo-production: Early experiments on pion-nucleon scattering were at low energy where inelastic scattering was either energetically forbidden or insignificant compared with the elastic channel, and so a relatively straight forward phase-shift analysis was possible. When the primary energy exceeds 300 or 400 Mev, there is enough energy in the centre of mass system of the pion and the nucleon to produce additional particles with high probability. These so called inelastic interactions, which have been extensively investigated both experimentally and theoretically, are an important source for our understanding of nucleon structure. Recent experiments using highly energetic pion beams (i.e. above 1 Gev) with hydrogen bubble chambers or other hydrogen targets, have shown that in most of the collisions, only a small fraction of the available energy is spent on the production of secondary particles. In other words, in the overall centre of mass system the primary particles (the pion and the proton) retain most of their energies L . and momenta after the collision. The same is true for nucleon-nucleon collisions. This implies that the collisions are essentially peripheral with only the meson cloud of the nucleon involved. Recent experiments have shown that in thoce collisions, resonant states between pions are formed. The quantum numbers of these states are in many cases still uncertain. The purposes of the present experiment are to further investigate these resonances, to search for new resonances and to test various models of the collision processes. Attention is concentrated on peripheral interactions i.e. those of low momentum transfer. In previous experiments on pion-proton interactions the 1 #.0 2 Gev/c region has already been explored, but with a primary pion momentum of 4 Gev/c one is able to search for new pion resonances at higher masses. Furthermore,, at this primary momentum peripheral collisions are expected to be more dominant then they are in the 1r•.2 Gev/c region. On the other hand, the primary momentum was not chosen to be higher than 4 GeV° because of the limitation imposed by the error on determining the effective missing mass of neutral particles formed in the reaction. If a maximum limit on one pion mass is fixed for this error, then, with the maximum detectable momentum 300 Gev/c of the 81 cm Hydrogen Bubble Chamber, it was calculated that the primary momentum should not exceed 6 Gev/c, however, when this experiment was started the 4 Gev /c pion beam was the only available one. 5. CHAPTER PION RESONANCES. 1.1 The Chew-Low Extrapolation Method. Direct study of pion-pion interactions is not yet possible and so the study of peripheral collisions where a free incident pion interacts with a pion in the pion cloud of the nucleon, becomes indispensible as a means of gaining an insight into the pion-pion interaction, One importance of this is that one may relate the pion-pion interaction to the nucleon electromagnetic structure through dispersion relations of the nucleon form factor, although in itself the pion-pion interaction is a subject of enormous interest. The. peripheral collision of the following reaction 117 p + p + 9`) (1.1.1) can be represented by the Feynman diagram in Figure 1.1. The theoretical differential cross section corresponding to the (1.1) first approximation theory was first calculated by Chew and Low in the following form. 2 42 - -µ2 a2 a f2 44). G700210)2) a02)a(w2) _ 2n 0?+42)2 kiL (1.1.2) For the sake of clarity in discussion, we shall for most of the time restrict ourselves toi=p interactions. 6. where A2 = -(pf-p)2 is the invariant square of the four momentum transfer, w2 = (qi q2)2 is the square of the total energy of two pions in their centre of mass system, 2 f = 0.08, f being a dimensionless •it -N coupling constant, A = Pion mass, kiL = momentum of the incident pion in the laboratory system, paf,k,q1 and q2 are respectively the four momenta of the target proton, the recoil proton, the incident pion, the scattered Trand the 7[ The assumptions of Chew and Low are that, \ a2 a 2 2 0), „has a pole at A =-11 and that this pole is isolated. a(e)a(of) f 2 2k (2)0 u 1 (0 ) approaches the physical cross--section of the free TK 2 2 pion-pion scattering as A approaches -µ . Here we use the metric 7.