Limits on the masses of supersymmetric particles from the UA1 experiment at the CERN proteK=as±ipr©ten. collider ?? Afzal Khan Imperial College L on don A thesis submitted to the University of London for the degree of Doctor of Philosophy ABSTRACT An analysis is presented of a data sample with large missing transverse energy, recorded at the CERN proton-antiproton collider. The data were collected at the UA1 experiment and correspond to, an integrated luminosity of 715 nb"1. A description of the Monte-Carlo calculations is given, for processes contributing to this large E>j.miss data sample. The very good agreement between the data and the simulated Standard Model physics processes is then used to set mass limits on the existence of the supersymmetric partners of the intermediate vector bosons, quarks and gluons - with the assumption of a massless lightest supersymmetric particle(taken to be a photino). Variation of the mass limits with a massive photino is also investigated. ACKNOWLEDGEMENTS Before I embark upon the impossible task of recognising all those who have contributed to the fabric of my existence as a postgraduate student, I must apologise to them, who, partly through the exigencies of space and mostly through the deficiency of my recollection, have remained unmentioned. I doubt, however, if such neglect should cause them too much uneasiness of mind. I should like to give thanks to the Science and Engineering Research Council for the financial support with which I have been provided. Many thanks are due to Dave Binnie for allowing me to join the HEP group and for providing the opportunity to spend a most fruitful time at CERN. To Jim Virdee, whose tenacious assiduity was surpassed only by his effusive criticism, I doff the proverbial hat. Without him the inclination of the path may have been more sorely felt. I should also like to express my unbounded gratitude to Chris Seez, who through a combination of eccentric humour and unquestionable competence, created an atmosphere not lacking of spice. His help with that of Tony Wildish, in the early days, was vital. My thanks go to Dave Robinson for his tranquillity^to Christos Markou for his companionship, to Roger Forty for revealing to me the possibility of a future in squash, to Lucas Taylor for his help with some histograms, to Steve Macmahon for providing me with the opportunity to lose his key and to Shaun Roe for sharing in the masochism of excessive physical exercise. I thank also Alan Watson, Marco Cattaneo, Gary Taylor, Julian Shulman, Sylvie Dugaey, James Gillies, Rob Edgecock, Nav Bains and Dave Francis for their camaraderie. I am grateful to Steve Haywood for sharing with me the last bus and to Dave Charlton for rendering it unnecessary to take an axe to the machine. I am indebted to Mohammad Mohammadi, Felicitas Pauss and Richard Batley, whose help with the missing energy analysis was invaluable. Page 2 Special thanks are due to Linda Jones, in whose foot I have been a constant thorn. Perhaps it was the use of this afflicted appendage that caused her to miss the penalty, which was as unlikely an event as the rising of the sun in the middle of the night, and thereby ensure that we achieved national notoriety by losing everything yet again. Thanks also to Betty Moynihan for fussing over the inexorable increase of entropy in my office, To Jim Gibb for ensuring that the greater part of humanity is acquainted with the ’rubber king' anecdote, to Diane, to Ash and also to Jane Ratcliffe for enduring when most had fallen by the wayside(part-timers). I thank, also, John Hassard for checking the spelling. To Jeremy Dodd I express my gratitude for being such a work of art and for showing me that something does turn up. I thank Miles Blencowe for making the effort to descend, occasionally, into our four dimensions. I thank Leslie Marshall for being an affectionate friend to me and quite probably to the rest of the world, Mike Huricane for giving me an appreciation of speed and Maria for possessing a caustic wit. Davie Norman, that one-man show, is surely above appreciation. Watching him pursue a ball to the boundary is truly a sight to behold (beheld all to often, I may add, when playing for CERN). My thanks to Julian and Sian for providing the most delightful society and to Steve Hancock, to whom my advice must surely be to stay on the front foot. Finally, I have the greatest pleasure to express my eternal gratitude to Alison Fowler, the gang and to my mother, Gulzar Begum. Without their love, support and unqualified commitment, the flowers would not blossom, the birds would not sing, the sun would not shine and, discord would reign supreme. London September 1989. Page 3 To my mother and the loving memory of my dear father. CONTENTS ABSTRACT 1 ACKNOWLEDGEMENTS 2 CONTENTS 5 1. THEORETICAL INTRODUCTION 8 1.1 Introduction 8 12 Standard Model 9 13 Supersymmetry 12 2. EXPERIMENTAL INTRODUCTION 19 21 The CERN proton-antiproton project 19 22 Beam cooling 21 23 Physics on UA1 22 24 Missing transverse energy 23 3. THE UA1 APPARATUS 27 3.1 Introduction 27 3.2 The central detector 28 3.3 The electromagnetic calorimeters 30 Page 5 3.4 The hadron calorimeters 32 35 The muon chambers 33 3.6 The forward chambers 33 3.7 The trigger 34 3.8 Data acquisition and reconstruction 35 4. JET FINDING AND THE MISSING ENERGY TECHNIQUE 45 4.1 Introduction 45 4.2 The UA1 jet algorithm 46 4.3 The jet energy 47 4.4 The missing transverse energy technique 48 4.5 Other sources of missing transverse energy 50 4.6 The jet-fluctuation Monte Carlo 51 5. THE MISSING TRANSVERSE ENERGY DATA SAMPLE AND THE STANDARD MODEL CONTRIBUTIONS 57 5.1 Introduction 57 5.2 The Hardware triggers 58 5.3 The on-line selection 59 5.4 The off-line selection 59 55 Standard Model contributions 62 55.1 Monte Carlo results 63 5.5.2 Separation of the W—>rv signal 64 5.55 Systematic errors 66 55.4 Non-tau sample 66 6. MASS LIMITS ON THE SUPERSYMMETRIC PARTICLES ' 83 6.1 Introduction 83 6.2 The supersymmetric model 84 6.2.1 Production of the gauginos 85 6.2.2 Decay of the gauginos 88 Page 6 6.3 Simulation of the supersymmetric processes 89 6.4 Results for the massless photino scenario 90 6.4.1 Properites of the massless photino events 91 6.4.2 The wino mass limit for the massless photino scenario 92 6.5 Results for the massive photino scenario 93 65.1 Properties of the massive photino events 94 65.2 The wino mass limit for the massive photino scenario 94 6.6 Limits on the masses of the supersymmetric partners of the quarks and gluons 95 6.6.1 The data sample 96 6.6.2 The Standard Model background 96 6.6.3 The supersymmetric model 97 6.6.4 Results 98 7. CONCLUSIONS 122 Appendix A : The specific form of the gaugino lagrangian and the couplings 124 A ppendix B : The statistical method used to derive the mass limits 129 REFERENCES 132 Page 7 CHAPTER 1 THEORETICAL INTRODUCTION 1.1 INTRODUCTION The current theoretical description of the physical world, which is often referred to as the Standard Model [14], has been very successful in its description of nearly all available data pertaining to the strong, weak and electromagnetic phenomena, and as yet there is no evidence of behaviour that violates this scheme. The Standard Model does not, however, represent a complete unified theory of all the interactions in nature. There are theoretical and aesthetic arguments which suggest that an extension of the Standard Model will be needed. Supersymmetry is a recent theoretical idea which represents a potentially significant addition to our current understanding of the physical world. It offers many promising avenues in the attempt to construct a unified field theory of physics and alleviates some of the shortcomings of the Standard Model. This chapter contains a brief history of particle physics and a qualitative description of the Standard Model. It ends with a discussion of the theoretical motivation for and the phenomenology of, supersymmetry. Page 8 1.2 STANDARD MODEL The eighteenth century saw the maturation of classical mechanics, and the advent of the theory of classical electromagnetism. This was followed early in the twentieth century by two major new conceptual frameworks, quantum mechanics and relativity theory. These were brought together in 1928 by Dirac [15] and together with Maxwell's theory of electromagnetism, represent a body of knowledge sufficient to understand the interactions of atoms and molecules, which form the basis for all chemistry and ultimately biology. During the 1940’s theorists such as Schwinger, Feynman and Tomonaga [16] created a complete quantum field theory of electrodynamics, QED. The central mathematical idea was that of a lagrangian, which together with Hamilton's least action principle yields the field equations of motion (from the Euler-Lagrange equations). By the 1940’s the existence of the four forces of nature, gravitation, electromagnetism, weak and strong, was known. These four forces are still considered fundamental today, although a connection between the weak and the electromagnetic forces has now been recognised. The 1940’s also saw the set of known particles increase from the simple set of five known in the 1930’s , i.e. protons, neutrons, electrons, photons and neutrinos. The proton, neutron and electron are known to be constituents of atoms, while the photon is known to transmit the electromagnetic force.