Measurement of the DeepInelastic ep Scattering Cross Section using the Backward Silicon Tracker at the H Detector at HERA DISSERTATION zur Erlangung des akademischen Grades do ctor rerum naturalium Dr rer nat im Fach Physik eingereicht an der MathematischNaturwissenschaftlichen Fakultat I der Humb oldtUniversitat zu Berlin von Vladimir Vladimirovich Arkadov geb oren am Mai in Novomoskovsk UdSSR Prasident der Humb oldtUniversitat zu Berlin Prof Dr Dr hc H Meyer der MathematischNaturwissenschaftlichen Fakultat I Dekan Prof Dr B Ronacher Gutachterinnen Prof Dr Th Lohse Prof Dr H Kolanoski Prof Dr C Kiesling Tag der mundlichen Prufung Abstract A measurements of the inclusive cross section for the deepinelastic scattering of p ositr onsoprotons is presented at four momentum transfers squared Q GeV in the Bjorken x range between and The analysis is based on data collected by the H exp eriment at HERA in the year taken during a sp ecial minimum bias run for low Q values ABackward Silicon Tracker BST has b een used in this measurement for the rst time Use of this new detector comp onent as presented in this thesis led to an extension of the kinematical region of the measurementtowards high values of inelasticity y at low Q as well as towards the low y region whichoverlaps exp eriments The precision of the measurement is for the rst time with xed target dominated by the systematic uncertainties These are of the order of as compared to typically statistical accuracy Part of the results presented in this analysis has been included into an H publication Zusammenfassung In der vorliegenden Arb eit wird die Messung des inklusiven tiefinelastischen Streuquer schnitts von Positronen an Protonen fur Viererimpulsub ertrage Q zwischen und GeV und Werte der Bjorkenx Variable zwischen und b eschrieb en Die Messung b eruht auf sp eziellen Daten die im Herbst des Jahres mit dem H Exp eriment am Beschleuniger HERA aufgezeichnet wurden Zum ersten Mal wird der ruckwartige SiliziumStreifendetektor BST des HExp erimentes fur diese Messung verwendet Die hier dargestellten Un tersuchungen mit dieser neuen Detektorkomp o nente fuhren zu einer Erweiterung des kinematischen Meb ereiches der nun sowohl groe Inelastizitatswerte von y bei niedrigen Q als auch eine Ub erlappung mit Daten von Fixed Target MuonProton Streuexp erimenten erreicht Erstmalig wird die Prazision dieser Messung von den systematischen Fehlern von etwa dominiert im Vergleich zu statistischen Fehlern von Die Resultate der Analyse wurden teilweise in eine Publikation der HKollab oration aufgenommen Contents Intro duction Theoretical Basics of Deep Inelastic Scattering A Bit of History Cross Section Calculation The DIS Kinematics Cross Section in Born Approximation Quark Parton Mo del Quantum Chromo dynamics and DIS The H Detector at HERA The HERA Accelerator Detector Overview Luminosity System Track Detectors Liquid Argon Calorimeter Backward Detectors Trigger System Kinematics Reconstruction Event and Detector Simulation The Data Sample Trigger and DAQ Run Selection and Time Stability CONTENTS The Backward Silicon Tracker Purp ose Design Data Acquisition Oine Reconstruction Internal Eciencies Coherent losses Simulation of the BST Alignment Internal Alignment External Alignment Alignment of SpaCal and BDC Energy Calibration Backward Calorimeter SpaCal Calibration Calibration of the Liquid Argon Calorimeter LAr Calibration with the Lagrangian Metho d Cross Check with the Iterative Metho d Measurement of the DIS Cross Section Cross Section Determination Bin Selection Event Selection Cluster Radius Cut Hadronic Energy Fraction Cut E P Cut z P Balance Cut t BST Related Cuts Vertex Position Cut Estimation of the Photopro duction Background Kinematic Distributions Systematic Errors Results for the DIS Cross Section Main Kinematic Region Cross Section at high y Extraction of F Concluding Remarks CONTENTS Summary A BST Electron Finder A Intro duction A Ob jectives of the pro cedure A General Description of the Routine A Case central vertex exists A Case central vertex do es not exist A Results Chapter Intro duction A measurement of the inclusive deepinelastic electronproton scattering DIS dier ential cross section is presented using the H detector at HERA and for the rst time the Backward Silicon Tracker BST The BST was fabricated by the H group in Zeuthen and installed in Its installation completed the upgrade of the backward apparatus of the H detector which consisted of replacing a PbScintillator calorime ter BEMC by a Pbb er calorimeter SpaCal of high granularity and a multiwire prop ortional chamber BPC by a planar drift chamber BDC The analysis of the DIS cross section b enets from the high resolution of the angular measurement with the silicon tracker allowing also for electron identication and thus suppression of the neutral comp onent of the photopro duction background The latter is essential to extend the cross section measurement towards the kinematic region of large inelastic ity y which p ermits the longitudinal structure function F to be accessed At low y L the event vertex is reconstructed using the scattered electron track measured with the BST Thus DIS events are identied and the kinematic region of the DIS xed target exp eriments is reached The data used here were collected in Septemb er and Octob er during a sp ecial data taking p erio d dedicated to the region of low momentum transfers Q The data set corresp onds to an integrated luminosityofaboutpb resulting in typically statistical errors The high statistics of the data and new backward instrumentation allowed to understand systematic eects b etter than in previous DIS cross section analyses at low Q This thesis is organized as follows The second chapter gives an intro duction to the theory of deep inelastic scatter ing Theoretical predictions for the evolution of the DIS cross section and the proton structure functions are discussed The third chapter overviews the HERA accelerator and the H detector The chapter pro ceeds with a description of metho ds for reconstruction of the kine matic variables with the H detector A presentation of the analyzed data and their selection concludes the chapter CHAPTER INTRODUCTION Chapter intro duces the Backward Silicon Tracker the presented work is es sentially based on The systematic study of the detector resp onse as well as its alignment with resp ect to the H co ordinate system are discussed The calibration of the electromagnetic and hadronic energy scales of the backward calorimeter is shortly describ ed in the fth chapter Here the main fo cus is on asp ects of the hadronic energy scale calibration of the liquid argon calorimeter The sixth chapter incorp orates all necessary asp ects of the double dierential DIS cross section measurement Sp ecial attention is paid to the role of the BST event validation in the cross section measurement The double dierential DIS cross section is measured with two dierent kinematic reconstruction metho ds whichallowed to cross check the correctness of the systematic uncertainties The chapter concludes with the results for the DIS cross section combined from the two metho ds dep ending on their resolutions Also presented is the structure function F x Q extracted from the measured cross section and a discussion of the obtained results This work used the BST and the SpaCal calorimeter to derive a cross section mea surement The p oints at high y entered the ocial release of structure function data by the H collab oration The systematic work on the BST has been part of further analyses on the inclusiveDIS and exclusive pro cesses The thesis has one app endix devoted to an electron nding pro cedure develop ed for the DIS cross section measurement with the backward silicon tracker Chapter Theoretical Basics of Deep Inelastic Scattering A Bit of History In Rutherford used particles to resolve the structure of gold atoms by measuring the angles of the scattered particles This exp erimentwas done b efore the Quantum Mechanics