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  • Untersuchung zur E/p-Kalibration des Kalorimeter-Systems am ATLAS Detektor

    Bachelorarbeit zur Erlangung des Grades eines Bachelor of Science in Physik

    vorgelegt von

    Carsten D. Burgard aus Denzlingen

    Themenstellung: Prof. Dr. Karl Jakobs

    Fakultät für Mathematik und Physik Albert-Ludwigs-Universität

    Freiburg im Breisgau 2011

  • ii

  • CONTENTS iii

    Contents

    1 Abstract 1

    1.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

    1.2 Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

    2 Introduction 2

    3 Experimental facilities 4

    3.1 The LHC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

    3.2 ALICE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    3.3 LHCb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    3.4 CMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    3.5 ATLAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    4 Measuring jets 9

    4.1 How do particles interact with matter . . . . . . . . . . . . . . . . . . . . . 9

    4.2 The calorimeter system of ATLAS . . . . . . . . . . . . . . . . . . . . . . . 11

    4.3 Jet Energy Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    4.4 E/p measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

    4.5 Background subtraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    5 Improving the background subtraction 18

    5.1 Definition of the terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    5.2 Discussion of the central detector region . . . . . . . . . . . . . . . . . . . 20

    5.3 Discussion of the forward and backward detector regions . . . . . . . . . . 23

    6 The parametric approach 26

    6.1 Derivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

    6.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

    7 Results 39

    7.1 Results on E/p measurements . . . . . . . . . . . . . . . . . . . . . . . . . 39

  • iv CONTENTS

    7.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

    7.3 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

    References v

    List of Figures vii

    A The ATLAS coordinate system 43

    B Monte Carlo Simulation 45

    C Collection of all plots 46

    C.1 Energy deposition density . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

    C.2 Linear approximation of the energy deposition density . . . . . . . . . . . . 55

    C.3 Linear approximation of the energy deposition as a function of track mo- mentum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

    C.4 Comparison of jet energy deposition density for wide and narrow jets (radial RMS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

    C.5 Background estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

    C.6 E/p plots, former correction factor . . . . . . . . . . . . . . . . . . . . . . 88

    C.7 E/p plots, new correction factor . . . . . . . . . . . . . . . . . . . . . . . . 90

    C.8 Comparison of background estimations for both correction factors . . . . . 92

  • 1 ABSTRACT 1

    1 Abstract

    1.1 Abstract

    The accurate knowledge of the Jet Energy Scale is a dominant factor for the vast majority of precision measurements and new physics searches at the LHC. A key component for this is the measurement of the single hadron response of the calorimeter, a problem usually adressed through E/p measurements. In this thesis, we critically review this approach, making use of

    √ s = 7TeV proton-proton collision data collected by the ATLAS

    experiment in 2010. In the past, late showering hadron tracks were successfully used to directly measure the background originating from neutral particles in the periphery of the track, assuming a constant contribution superimposed to the MIP track itself. In this thesis, we extrapolate the background contribution superimposed to the charged hadron track by assuming a linear dependency of the energy deposition density of the distance from the charged track. Concluding that, although the previously used estimation method does not agree with the actual deposition density measured, the assumption used previously underestimates the background on the E/p observable by only 10% due to geometrical reasons.

    1.2 Zusammenfassung

    Die genaue Kenntnis der Jet Energy Scale des Kalorimeter-Systems ist ein bestimmender Faktor für nahezu alle Präzisionsmessungen und Suchen nach neuer Physik an Hadronen- beschleunigern wie dem LHC. Eine Schlüsselrolle hierbei nimmt die Messung der single hadron response des Kalorimeters ein. Hierzu ausgeführte E/p-Messungen werden im Hin- blick auf mögliche Fehler durch systematische Unterschätzung des durch neutrale Teilchen verursachten Untergrunds untersucht. Ereignisse, in denen geladene Hadronen erst spät elektromagnetische Schauer auslösen, werden verwendet, um den peripheren Untergrund direkt zu messen. Die bisher zur Abschätzung des Untergrunds verwendete Annahme eines konstanten Untergrunds im Bereich der geladenen Spur selbst wird untersucht. Als neue Methode der Abschätzung wird eine lineare Extrapolation der Energiedepositionsdichte vorgestellt. Obwohl die bisher verwendete Annahme die wahre Dichte der Energiedeposi- tion nicht gut wiedergibt, liefert auch die neue Extrapolationsmethode nur etwa um 10% höhere Abschätzungen des Untergrundes. Dies kann durch geometrische Überlegungen erklärt werden.

  • 2 2 INTRODUCTION

    2 Introduction

    Ever since, mankind has tried to understand the forces of nature, that drive and hold together the world as we know it. Within the last century, however, this search has advanced vastly, which would not have been possible without the combination and close interplay of both theory and experiment. But the further this search continues, the more complex, elaborate and expensive the experiments become that are necessary to test the theories proposed to explain the nature of matter and interactions.

    One of the greatest successes in the course of this quest was the introduction of the Standard Model of particle physics, providing a consistent theory for particles and inter- actions and making predictions that could be verified experimentally to high precision. There are, however, hints that the standard model, although extremely successful, might not be the full picture. Many theories exist, trying to explain phenomena that cannot be explained within the standard model, among which are fundamental questions like the asymmetry of matter and antimatter observed in our universe, the nature of dark matter and other, more involved phenomena. Experiments have to be planned and per- formed in order to test these theories and provide observations, eventually leading to their verification or falsification.

    State-of-the-art experiments in particle physics, designed to push the limits of our understanding of the fundamental particles and their interactions further, are nowadays no longer projects of single, brilliant scientists, but rather the outcome of large collaborations of scientists and technicians, accurately designed and built over timescales of years and decades.

    Particle physics experiments can roughly be divided into two categories: Astro particle physics experiments and collider experiments, the former looking for high-energy particles from space, the latter trying to produce them with particle accelerators. Each of these fields has its own unique advantages and disadvantages, and again only the combination of all observations from both fields can provide a fully consistent picture. Astro particle experiments benefit from the fact that, in cosmic events such as supernovae, particles with energy ranges vastly exceeding the range of man-made accelerators are produced. The observation of such particles is, however, comparably rare, and since the production does not happen under laboratory conditions, but many lightyears away from the earth, the aquisition of data sufficient to provide experimental proof for theories relies heavily on a long time of observation and sometimes even on luck. Accelerator experiments, on the other hand, have the advantage of producing high energy particles with a sufficient rate to provide large amounts of data. Furthermore, the observations can be made un- der reproducible conditions, ideally with a full coverage and observation of all produced particles for the single event. Then, again, these experiments rely on accelerators, which

  • 2 INTRODUCTION 3

    are costly and limited in the range of accessible energy.

    The ATLAS detector is one of the four main detector experiments at the LHC, the world’s largest particle accelerator to date (2011), providing insight to energy regions previously unaccessible to collider physics. Its calorimeter system is complex, consisting of several layers and different modules. The design was optimized for high performance precision energy measurements of high energy particles emerging the collisions. The proper calibration of this system requires detailed investigation of various effects as well as the calculation and estimation of various backgrounds and correction factors.

    One of the most challenging tasks in the