KS00206460G R: FI DEOO9O5232O \; «DE009052320* Cover detail; De aardappeleters, Nuenen 1885 Vincent van Gogh Amsterdam, Van Gogh Museum (Vincent van Gogh Stichting) Cover design: Jaap Joziasse Cover photo: Loek van Beers Prepress: Hennie Spermon VOL <n 7 A luminosity measurement at LEP using the L3 detector Een wetenschappelijke proeve op het gebied van de Natuurwetenschappen. Proefschrift ter verkrijging van de graad van doctor aan de Katholieke Universiteit Nijmegen, volgens besluit van het College van Decanen in het openbaar te verdedigen op 25 Juni 1996, des namiddags te 15.00 uur precies, door Elisabeth Nikolaja Koffeman geboren op 3 november 1967 te Nuenen. Promotores: Prof. Dr P. Duinker Prof. Dr F. L. Linde Co-promotor: Dr M. H. M. Merk Manuscriptcommissic: Dr D. J. Schotanus Dr G. J. Bobbink The work described in this thesis is part of the research programme of the "Nationaal Instituut voor Kernfysica en Hoge-Encrgic fysica (NIKHEF)'. The author was financially supported by the 'Stichting voor Fundamenteel Onderzoek der Materie (FOM)'. ISBN: 90-9009549-7 Un savant dans son laboratoire n'est pas settlement un tech- nicien: c'est aussi un enfant place en face des phênomènes naturels qui l''empressionnent comme un conte defées. Nous ne devons pas laisser croire que tout progrès scientifique se reduit a des mécanismes, des machines, des engrenases, qui, d'ailleurs ont aussi leur beauté propre. Marie Curie-Sklodowska (1867-1934) Contents Introduction 1 1 Theory 3 1.1 Introduction 3 1.2 The Standard Model 3 1.2.1 Particles and fields 3 1.2.2 GWS model 4 1.3 The Z boson 5 1.3.1 Decay modes 5 1.3.2 Cross section 7 1.3.3 The number of light neutrino species 8 1.4 Small angle Bhabha scattering 9 1.4.1 Introduction 9 1.4.2 First order QED cross section 10 1.4.3 Corrections to the Born QED cross section 11 2 LEP and L3 15 2.1 Introduction 15 2.2 The Large Electron Positron collider 15 2.2.1 The accelerator 15 2.2.2 The luminosity 16 2.3 The L3 detector 18 2.4 The luminosity monitor 21 3 Silicon strip detectors 25 3.1 Introduction 25 3.2 Measurement of high energy particles in silicon 25 3.2.1 Principle of detection 25 3.2.2 Charged particles 26 3.2.3 Photons 27 3.3 Silicon detector characteristics 28 3.3.1 General properties 28 3.3.2 Sensors used in the luminosity detector 31 3.4 Leakage current 32 3.4.1 Test measurements 33 Contents 3.4.2 Long term behaviour at LEP 37 4 The Silicon Luminosity Tracker 39 4.1 Introduction 39 4.2 Mechanical construction 39 4.2.1 Sensor geometry 41 4.2.2 Detector support 43 4.3 Data Acquisition 44 4.3.1 Layout 44 4.3.2 The AMPLEX chip 44 4.3.3 Digitisation and synchronisation 47 4.3.4 Calibration 51 5 Event reconstruction 55 5.1 Introduction 55 5.2 Trigger and Data Flow 55 5.3 BC.0 56 5.3.1 Energy calibration 56 5.3.2 Shower fit 57 5.4 SLUM 58 5.4.1 Pedestal subtraction 58 5.4.2 Multiple hits 59 5.5 Alignment of the calorimeter and the tracker 64 6 The Luminosity determination 69 6.1 Introduction 69 6.2 Event selection 69 6.2.1 Selection criteria 69 6.2.2 Background subtraction 76 6.2.3 Trigger efficiency 76 6.3 The visible cross section 76 6.3.1 Acceptance 76 6.3.2 Position of the sensors on the support, J"a.ns 79 6.3.3 Beam offset in the .vv-plane, J-\Kam 80 6.3.4 Distance to the vertex, Fwn 82 6.3.5 Temperature correction, .F,on,p 87 6.3.6 Total visible cross section 87 6.4 Luminosity 88 6.4.1 Systematic uncertainties 89 6.5 Lincshape 91 7 Radiative Bhabha Events 95 7.1 Introduction 95 7.2 Generated radiative events 95 7.3 Final state radiation 96 Contents 7.4 Initial state radiation 99 Summary 103 Samenvatting 105 Bibliography 107 Acknowledgements 111 in Contents tv Introduction The present understanding of matter and forces in our universe is the result of an everlasting in- terplay between the development in theory and experiment. The theoretical model describing the interactions between elementary particles to our best present knowledge is the Standard Model. One way to study the interactions experimentally is the use of a particle collider. At CERN near Geneva a large collider is constructed (LEP) where electrons and their anti par- ticles are brought into collision. The electrons and positrons are injected in a circular tunnel with a length of 27 km. At four points the particle beams collide and large detectors have been con- structed to study the results of these collision processes. The LEP machine is specially designed to study the weak interactions. Millions of Z bosons - the boson responsible for the weak neutral current interaction - are produced and its properties are studied in detail and with high precision.. To perform high precision measurements at particle colliders it is crucial to know the exact intensity of the colliding beams. In particle physics this quantity is generally referred to as the luminosity. The determination of the luminosity in one of the experiments (L3) is the topic of this thesis. The implementation and the use of a silicon strip detector in L3, will be described in detail. In chapter one the most important parameters measured at LEP are discussed, preceded by a short introduction to the Standard Model. The process generally used for luminosity measure- ments in electron positron colliders is small angle Bhabha scattering. This process is discussed at the end of chapter one. In chapter two the characteristics of the collider and the L3 experiment are given. Together with the signature of the small angle Bhabha scattering, these experimental conditions determine the specifications for the design of the luminosity monitor. The general features of silicon strip detectors for their application in high energy physics are presented in chapter three. Some special attention is given to the behaviour of the sensors used for the tracking detector in the luminosity monitor. The more specific design details of the luminosity monitor are constricted to chapter four. In chapter five the conversion from detector signals into coordinates relevant for the analysis is explained. The selection of the small angle Bhabha scattering events and the subsequent de- termination of the luminosity, are presented in chapter six. Systematic uncertainties are carefully studied. Important for a good understanding of the Bhabha selection are the events where a pho- ton is produced in the scattering process. These events are separately studied. In chapter seven a comparison is presented between the radiative events observed in the data an their modelling in the Bhlumi Monte Carlo programme. Introduction Chapter 1 Theory 1.1 Introduction The purpose of this chapter is to emphasise the importance of the luminosity measurement and to give a theoretical description of the process used for this measurement: small angle Bhabha scattering. The chapter starts with a short introduction to the Standard Model. Its basic concepts need to be understood in order to put the experimental results in perspective. Subsequently a description is given of the most important parameters measured at the Large Electron Positron collider at CERN. The LEP collider is designed such that the total available en- ergy in a collision can be adjusted to the mass of the neutral mediator of the weak interactions, the Z boson. Under these conditions the probability of the process of electron-positron annihilation into a Z boson is strongly enhanced. The LEP collider therefore provides a unique environment to study the weak interaction processes. In the last part of this chapter the theoretical calculation of the Bhabha scattering cross sec- tion is given. A first order approximation is presented as this leading order behaviour is used to determine most detector design specifications. Higher order predictions are discussed since their precision remains one of the limiting factors in the final luminosity determination. 1.2 The Standard Model 1.2.1 Particles and fields The search for unification of different theories explaining a variety of observable phenomena is one of the driving forces of (particle) physicists. In the past Maxwell achieved an important step in this unification with the description of the electric and magnetic forces in one theory. Many years later a new theory was developed by Glashow, Weinbcrg and Salam [ 1,2,3] which unifies the weak interactions with the electromagnetic force: The Standard Model of clcctrowcak interaction. The model describes the interaction between elementary particles caused by elcc- trowcak forces. Although the strong inlcractions urc also implemented in the Standard model framework they arc not specifically discussed here. The particles with spin = 1/2, the fermions, are Jivided into quarks and leptons. Furthermore 1.2. The Standard Model Theory FERMIONS spin = 1/2 generation quarks leptons Q=2/3 Q==-1/3 Q--1 Q=0 1 u d e Vc 2 c s 3 t b x vt BOSONS spin == 1 mediator field photon (Y) electromagnetic W+, W, Z weak gluon (g) strong Table 1.1: 77ie basic constituents of matter and the mediators of the fundamental forces (Q is the charge in units of the elementary charge c). the quarks and leptons can be subdivided into different generations ordered by increasing mass. Such a generation or family, contains two quarks and a lepton with its corresponding neutrino.