Absolute luminosity determination for the ATLAS experiment
Zusammenfassung ATLAS ist eines der vier großen Experimente am Large Hadron Collider (LHC). Präzise Messungen von Wirkungsquerschnitten erfordern eine genaue Kenntnis der integrierten Luminosität. Die relative Luminosität wird mit verschiedenen Detektoren und Algorithmen gemessen. Letztere wandeln die von den Detektoren gemessenen Raten in eine zur Luminosität proportionale Größe um. In dieser Arbeit werden drei Algorithmen kalibriert, die auf den zwei primären Luminositäts-Detektoren basieren:
... BCMH EventOR, BCMV EventOR und LUCID EventOR. Die Kalibrierung beruht auf Van der Meer (VdM) Scans, die in den Monaten Juli und November 2012 durchgeführt wurden. Die statistischen Fehler dieser Methode sind klein und die Genauigkeit ist durch systematische Unsicherheiten begrenzt, welche abgeschätzt werden. Die Kalibrierungskonstanten der Juli VdM Scans haben eine Unsicherheit von 5.40 %, die der November Scans eine von 2.50 %. Die November-Kalibrierung ist die Grundlage zur Bestimmung der integrierten Luminosität, deren Unsicherheit auf 3.30 % geschätzt wird. Die vorläufige o zielle ATLAS Luminosität und deren Unsicherheit für Proton-Proton Kollisionen bei p s = 8 TeV im Jahr 2012 basieren auf dieser Arbeit. Summary ATLAS is one of the four big experiments at the Large Hadron Collider (LHC). In order to accurately measure cross sections, the precise knowledge of the integrated luminosity is a prerequisite. The relative luminosity is measured with various detectors and algorithms. The purpose of the algorithms is to convert raw rates measured by a detector into a quantity which is proportional to the luminosity. In this work, three algorithms linked to the two main ATLAS luminosity detectors are absolutely calibrated: BCMH EventOR, BCMV EventOR, and LUCID EventOR. The determination of the calibration constants is based on Van der Meer (VdM) scans, which were carried out in July and November 2012. The statistical errors of this method are negligible and the precision is limited by systematic uncertainties. The di↵erent uncertainty sources are quantitatively estimated. The overall uncertainty on the calibration constants is estimated to be 5.40 % for the July VdM scans and 2.50 % for the November VdM scans. The November calibration is used to determine the integrated luminosity in 2012, its overall uncertainty is evaluated to be 3.30 %. The preliminary o cial ATLAS luminosity and its uncertainty for proton-proton collisions at p s = 8 TeV in the year 2012 are based on this work. Bibliography 133 Acknowledgements 147 viii 1. Introduction sured cross sections to the ones predicted by the SM. The sensitivity of these searches depends directly on the precision of the integrated luminosity. Furthermore, accurate experimental cross sections decrease the theoretical uncertainties on cross section calculations via confining the uncertainties on the input parameters of the calculations . In the ATLAS experiment, the relative luminosity is measured with various detectors and algorithms. The purpose of luminosity algorithms is to convert raw rates measured by a detector into a quantity which is proportional to the luminosity. The redundancy from having multiple independent measurements allows detailed studies of systematic uncertainties. The topic of this thesis is the absolute luminosity calibration of three algorithms linked to the two main luminosity detectors: BCM 3 and LUCID 4 . The former was built primarily for monitoring the beam background, while the latter is a dedicated luminosity detector. The calibration procedure is based on Van der Meer (VdM) scans. An integral part of the scans is to measure the dependency of the interaction rate on the transverse beam separation. Chapter 2 of this thesis gives a brief overview of the LHC and the four main experiments. Chapter 3 introduces the ATLAS detector, the main subdetectors and the trigger and data acquisition system. Special focus is given to the detectors employed for luminosity measurements. The concept of luminosity is introduced in chapter 4. Di↵erent ways to measure luminosity are presented. The theory of VdM scans is derived in chapter 5. Chapter 6 highlights the instrumentation of importance for VdM scans. The analysis of the 2012 VdM scans is carried out in chapter 7. The central calibration constants are determined and the systematic uncertainties impacting the calibration precision are estimated. Chapter 8 presents a partially complementary method to the common VdM scan procedure which is based on reconstructed interaction vertices. Chapter 9 covers the determination of the integrated luminosity in ATLAS and its uncertainty. The main results are summarised in chapter 10. 3 Beam Condition Monitor 4 LUminosity measurement using Cerenkov Integrating Detector 2 • CMS 4 pursues the same goals as ATLAS. It shall give independent measurements and enables crosschecks between both experiments . • LHCb 5 aims at studying decays containing b-quarks in order to precisely measure the nature of CP violation . 2 A Large Ion Collider Experiment 3 A Toroidal LHC ApparatuS 4 Compact Muon Solenoid 5 Large Hadron Collider beauty 5 The ATLAS experiment This chapter gives a general overview of the ATLAS detector, its main subdetectors and the performance requirements motivated by its physics programme. The trigger and data acquisition system is introduced, followed by a discussion of the main luminosity detectors. The ATLAS detector The ATLAS detector is a general purpose detector optimised for studying proton-proton collisions at instantaneous luminosities of 10 34 cm 2 s 1 . The detector was designed and built in a collaboration of several thousand people over a period of fifteen years. The design was driven by the goal to observe new physics phenomena and to probe the predictions of the Standard Model of particle physics in the TeV energy regime. The main performance benchmark was the search for the Standard Model Higgs boson. Coordinate system and nomenclature The origin of the right-handed ATLAS coordinate system is given by the nominal interaction point. The beam axis defines the z axis and its positive part is pointing to the A-side; consequently, the C-side has negative z coordinates. The x-y plane is the plane orthogonal to the beam axis. The positive x axis is pointing from the nominal interaction point to the centre of the LHC ring. The positive y axis is pointing from the interaction point upwards. The azimuthal angle defines the direction in the x-y plane and the polar angle ✓ is given with respect to the beam axis. The definition of the pseudorapidity ⌘ is ln tan (✓/2). Within the ATLAS experiment, the 3564 di↵erent LHC bunch positions are labelled consecutively by integer values called BCIDs 1 .