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Baryon Pro duction in Z Decay Christopher G Tully A DISSERTATION PRESENTED TO THE FACULTY OF PRINCETON UNIVERSITY IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY RECOMMENDED FOR ACCEPTANCE BY THE DEPARTMENT OF PHYSICS January ABSTRACT How quarks b ecome conned into ordinary matter is a dicult theoretical question The p ointatwhich this o ccurs cannot b e measured by exp eriment but other prop erties related to connement can Baryon pro duction is a situation where one typ e of connement is chosen over another In this thesis pro duction rate measurements have b een p erformed for and baryons pro duced in hadronic decays of the Z The rates are low compared to meson pro duction but the particular prop erties of and decay allow these particles to be identied amongst the many mesons accompanying them The average number of and per hadronic Z decay was found to be D E N stat syst and D E N stat syst resp ectively These measurements take advantage of the unique photon detection capa bility of the L Exp eriment The rates are in agreement with those measured by other LEP detectors iii CONTENTS Abstract iii Acknowledgements v Intro duction Particle Creation and Detection Searching for Particle Decays Going Back in Time The History Behind SubNuclear Particles Quarks Discovery of the Pion The Quark Constituent Mo del Deep Inelastic Scattering Quantum Chromo dynamics eak Interaction Quarks and the Electrow Gluon Radiation from Quarks The Standard Mo del and Its Help ers Fundamental Interactions The GSW Standard Mo del of Electroweak Interactions The Prop erties of Quarks from Z Decay Quantum Chromo dynamics Hadronization Phenomenology Hyp eron Decay Particle Acceleration and Collision The Large ElectronPositronLEP Collider LEP Injector Chain LEP Main Ring The L Exp eriment SubDetectors of the L Exp eriment vii viii Contents Trigger and DataAcquisition System Tracking and Calorimetry Tracking Ideal Tra jectories The Geometry of the Tracking Detectors Tracking Measurements Track Fitting and Error Propagation Calculation of the Primary Vertex Calorimetry Electromagnetic Showers The Geometry of the Electromagnetic Calorimeter The Electromagnetic Shower Shap e Statistically Determined Errors on Energy and Angu lar Measurements Gain and Oset Calibration of the Electronics Energy Resolution and Linearity Measurements Baryon Pro duction Measurements Secondary Vertices Kinematic Constraints Measurement of the Baryon Measurement of the Baryon Pro duction Rates Comparison with Other LEP Measurements Polarization Summary and Outlo ok A BGO Resolution Functions B BGO Noise DeCorrelation Algorithm C Constrained Fit Details C Denitions C Fit C Calculation C Convergence C Directed Steps INTRODUCTION Reaching higher energies probing smaller distances revisiting earlier times in the evolution of the universe these are all ways of discussing the direction which particle physicists pursue to uncover the elementary building blo cks and interactions of matter When relativity and quantum mechanics were merged in the late s to form a more complete description of particle dynamics it b ecame understo o d that the number and typ es of particles in a system did not have to remain unchanged in time The freedom to annihilate and create new particles was linked to the relativistic quantum description of elementary forces In this thesis a study is made of the rate of pro duction of dierenttyp es of baryons heavier versions of ordinary nuclear matter Baryons are par ticularly interesting to study because they are conserved This means that for every baryon pro duced an antibaryon will also be pro duced Baryons are heavier than other particles hence the origin of their name Their rate of pro duction is related to their mass their conservation law and prop er ties of the system from which they arise In particular the pro duction rate dep endence on baryon mass is a measure of the temp erature at which quarks b ecome conned within ordinary matter Longlived but unstable baryons known as hyp erons have a signature whichallows them to b e iden tied amongst other particle decays That is whythe and baryons in particular are measured in this thesis and not others The creation of heavy matter is achieved by the op eration of largescale particle colliders Charged particles intro duced into the nearly p erfect vac uum of a b eam pip e can be accelerated and steered with electromagnetic elds The Large ElectronPositronLEP Collider lo cated near Geneva Switzerland is to day the highest energy electronp ositron collider in the world LEP was designed to have four principal collision poin ts equally spaced along its km circumference The L Exp eriment led bySamuel C C Ting occupies one of these interaction regions and is the source of data used in this thesis Intro duction Particle Creation and Detection At the centralmost part of the L Exp eriment highenergy b eams of elec trons and p ositrons are broughtinto collision The particles created in these collisions are detected by the surrounding apparatus The apparatus consists of two main comp onents tracking and calorimetry The goal of the inner part of the detector the tracking is to measure the tra jectories of the particles coming from the center of the detector while aecting the ideal unmeasured tra jectories as little as p ossible High energy particles can travel through thin light materials and gaseous volumes without losing a large fraction of their initial momentum However the small amountthey do lose can be used to track their tra jectories The function of the outer part of the detector the calorimetry is to stop and measure the energies of all the particles which have tra velled through the tracking region This task is achieved by using tons of heavy materials intersp ersed or instrumented with devices that gauge the total number of material interactions which o ccur during the stopping pro cess The number and distribution of material interactions can then b e related to initial energies and typ es of particles which hit the calorimetry Searching for Particle Decays Measuring the prop erties of shortlived particles requires a bit of detective work Only a handful of particles pro duced in beam collisions live long evacuated b eam pip e Therefore the bulk of enough to make it out of the the particles that enter the tracking and calorimetry are longlived particles which come from particle decays But in fact how do es one know that the shortlived particles even existed if they cannot be measured directly When a particle decays the particles coming from this decay will satisfy a set of kinematic relationships Although the kinematics of the longlived particles are measured with the tracking and calorimetry there are measurement errors This intro duces the concept of satisfying kinematic relationships within errors When searching for the evidence of a particular particle decay among the decay pro ducts of several dierent particles there is no smoking gun The analysis requires that distributions of candidate particle decays sho w statistical evidence for the presence of a particular particle typ e The most Going Back in Time telling distribution to identify a particle is the mass distribution In addition to a set of discrete symmetry and angular momentum quantum numb ers particles are distinguished by the way they interact and by their mass Going Back in Time As was indicated heavy particles are formed in the vacuum of the b eam pip e when highenergy electrons and p ositrons are collided This is not a nor mal everyday exp erience The evolution of interaction energies into particle states froze out as the universe co oled down Typical interaction energies in everyday life are well b elow that needed to generate nuclear matter The central story is ab out how particle interactions will b ehavewhenthe interaction energies b ecome closer and closer to those b elieved to b e present in the early universe The LEP Collider energies were designed for studying a particular p erio d of history called the Electroweak Era During this p erio d the interaction resp onsible for radioactivity in heavy atomic nuclei takes on a new form This form is called the Z particle and it is created when an electron and p ositron collide with an energy which is equal to or within close vicinityofthephysical mass of the Z particle
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