Click here to load reader

Study of a DEPFET Vertex Detector and of Supersymmetric

  • View
    0

  • Download
    0

Embed Size (px)

Text of Study of a DEPFET Vertex Detector and of Supersymmetric

Study of a DEPFET Vertex Detector and of Supersymmetric Smuons at the ILC(Werner-Heisenberg-Institut)
Study of a DEPFET Vertex Detector and of Supersymmetric Smuons at the ILC
Xun Chen
Vollstandiger Abdruck der von der Fakultat fur Physik der Technischen Universitat Munchen
zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigten Dissertation.
Prufer der Dissertation: 1. Hon.-Prof. A. C. Caldwell, PhD
2. Univ.-Prof. Dr. R. Krucken
Die Dissertation wurde am 27.10.2008 bei der Technischen Universitat Munchen eingereicht und
durch die Fakultat fur Physik am 21.01.2009 angenommen.
In memory of my mother and grandmother
Abstract
This thesis is devoted to the study of the performance of a pixel vertex detector based on DEPFET technology at the International Linear Collider (ILC). The ILC is the proposed next generation e+e− collider to explore the physics at the Terascale. At the ILC with its well-defined initial state of collisions, possible discoveries at the Large Hadron Collider can be verified and studied more accurately. It is expected that the precision measurements of the ILC will answer many fundamental questions about the universe, such as the generation of particle masses and the origin of electroweak spontaneous symmetry breaking. The ambitious physics goals present challenges to the ILC detectors. Several detector concepts have been proposed in recent years. A crucial device for all these concepts is the pixel vertex detector. It provides precise impact parameter information of charged particles, jet flavor tagging and improves overall tracking efficiency. To meet the requirements of the ILC environment, the vertex detector will be arranged in a concentric multi-layer array around the interaction point to cover as large a solid angle as possible. Endcap disks are considered in some designs. Silicon pixel sensor technologies must be employed to provide excellent point resolution. The DEPFET technology, which integrates the first level of amplification into a depleted silicon bulk, is one of the promising candidates. The DEPFET sensor is very sensitive with a high signal-to-noise ratio. Power consumption is minimized due to the internal storage of signal charges. The good radiation tolerance makes it capable of working close to the interaction point.
In this thesis, we discuss the detailed simulation of the DEPFET vertex detector, following the general vertex detector layout proposed by the TESLA collaboration. The simulation is used to evaluate the impact parameter resolution. We also discuss the DEPFET test beam analysis on two-track resolution. The whole analysis procedures, ranging from readout of raw data to tracking, are described in detail. At last, we discuss the analysis of right-handed smuon pair production at the ILC with full simulation and reconstruction. The decay channel of smuon, µ±
R → µ±χ0 1 allows the precise determination of mµR
through the energy spectrum of final muons. The ILC vertexing and tracking system is crucial for this analysis. In addition, the scalar nature of the smuon can be revealed by studying its angular distribution. These studies show that the DEPFET vertex detector will well meet the requirement posed by the ILC physics program.
Contents
2.1 Higgs physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Higgs bosons in theories beyond the SM . . . . . . . . . . . . . . . . . 6
2.2 Supersymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1 The Particle Flow Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Detector Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4 The DEPFET Vertex Detector 19
4.1 Principle of DEPFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2 The DEPFET ladder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.3 The steering and readout chips . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.4 Radiation tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5 Software tools 27
5.2 GEAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.3 Mokka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.4 Marlin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.4.1 MarlinReco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.4.2 PandoraPFA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.4.3 PFOID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
6.1 Developing new drivers for the Mokka program . . . . . . . . . . . . . . . . . 37
6.2 Simulation of the DEPFET VXD in LDC . . . . . . . . . . . . . . . . . . . . 38
i
6.2.2 Construction of a single ladder . . . . . . . . . . . . . . . . . . . . . . 40
6.2.3 Placement of ladders . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.2.4 Support structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.2.5 Gear parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.4 Study of the impact parameter resolution . . . . . . . . . . . . . . . . . . . . 48
7 Test Beam Analysis on Two-Track Resolution 53
7.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
7.4 Hits and clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
7.4.1 Blocking of hot pixels . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
7.4.2 Cluster signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
7.4.3 Hit position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
7.5 Track finding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
7.6.1 General procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
7.6.5 Optimization of tracking . . . . . . . . . . . . . . . . . . . . . . . . . . 76
7.6.6 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . 76
8 Production of right-handed smuon pairs 79
8.1 Detection of right-handed smuons . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.2 Event generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
8.5.3 Estimation of systematic errors . . . . . . . . . . . . . . . . . . . . . . 99
8.5.4 Polar angle distribution of µR . . . . . . . . . . . . . . . . . . . . . . . 101
9 Summary 105
CONTENTS iii
B Using ISAJET in a PYTHIA based event generator 111 B.1 Compilation of ISAJET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 B.2 Compilation of PYTHIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 B.3 Compilation of user’s own generator . . . . . . . . . . . . . . . . . . . . . . . 112 B.4 Using ISAJET in the program . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
C Intersection of cones 113 C.1 Basic Formulas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 C.2 Intersection of two cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Bibliography 117
Introduction
The standard model of elementary particle physics provides the best description of nature at current stage. However, some very fundamental questions remain unanswered. The incom- pleteness of the standard model itself and observations of recent astrophysical experiments indicate the existence of new physics beyond the standard model. A number of theories have been proposed to describe the new physics. Most of their predictions can be tested by the high energy experiments at the Terascale, i.e. at energies around and above 1 TeV (1012 eV). The Large Hadron Collider (LHC), that has just started operation at CERN, will explore the physics at the Terascale for the first time. To complement the discoveries of the LHC, the International Linear Collider (ILC), an electron-positron collider, has been proposed. Bene- fitting from the well-defined initial beam energies, luminosity and polarization, and the low backgrounds, the ILC experiments will achieve unprecedented accuracy to verify and extend the discoveries of the LHC and give us a deeper insight into nature.
A next generation of linear electron-positron collider operated at the center-of-mass energy of a few hundred GeV to 1 TeV has been proposed since the 1980s. In 2004, the ILC project has started officially, based on three existing regional linear collider projects, i.e. the Next Linear Collider (NLC) [1], the Global Linear Collider (GLC) [2] and the Teraelectronvolt En- ergy Superconducting Linear Accelerator (TESLA) [3]. The superconducting radio-frequency (SCRF) technology has been chosen by the International Technology Recommendation Panel (ITRP) for the acceleration of electrons and positrons [4]. The ILC accelerator has been de- signed to meet the basic parameters required for the planned physics program [5]. The ILC will be operated up to a maximum center-of-mass energy of
√ s = 500 GeV and a peak lumi-
nosity of 2 × 1034cm−2s−1 in the first stage. The integrated luminosity is required to be 500 fb−1 within the first four years during the phase of operation at 500 GeV. Both electron and positron beams are polarized. Beam energy and polarization must be stable. Subsequently, an energy upgrade to
√ s ∼ 1 TeV is foreseen.
The ILC baseline configuration has been summarized in the ILC Reference Design Report (RDR) [6] released in 2007. The overall length of the ILC complex is around 31 km. One interaction point is designed to be shared by two detectors via a “push-pull” system.
1
2 CHAPTER 1. INTRODUCTION
The physics goals present a challenge to the designs and technologies of detectors at the ILC. Three detector concepts have been proposed and studied so far [7,8,9], applying different structures and technologies. A pixel vertex detector is required by all these concepts. The vertex detector is used to provide accurate impact parameter determination, excellent vertex resolution, jet flavor tagging and to improve the overall tracking performance. The physics program requires the vertex detector efficiency, angular coverage and impact parameter reso- lution to be raised beyond the current state of the art. To meet the requirements, new pixel sensor technologies are proposed and investigated. The concept of DEPFET (DEPleted Field Effect Transistor) pixel [10] is one of them. By integrating the first stage of amplification into a fully depleted bulk, the DEPFET sensor has a high signal-to-noise ratio with mini- mum power consumption. Also a high readout rate can be achieved for the DEPFET sensor matrix. These features make DEPFET an ideal candidate for the ILC vertex detector.
In this thesis, we study the performance of a pixel vertex detector based on DEPFET technology at the ILC, with the detailed simulation of the DEPFET detector, the analysis of DEPFET test beam data, and the analysis of smuon production at the ILC with full simulation.
The thesis is organized as follows: In chapter 2 a brief overview of the physics to be explored at the ILC is given. In chapter 3, we introduce the widely accepted particle flow algorithm and three detector concepts for ILC. Chapter 4 is devoted to the DEPFET technology. The principle of DEPFET, design of DEPFET ladders for the application at the ILC, steering and readout electronics and radiation tolerance are introduced. An overview of the software tools used in the simulation and optimization of a DEPFET pixel vertex detector at the ILC is given in chapter 5. In chapter 6, the detailed simulation of the DEPFET pixel vertex detector in two detector concepts is discussed. A study on the impact parameter resolution is performed. The DEPFET test beam analysis on two-track resolution is discussed in chapter 7. The analysis starts from the processing of raw data. Hits are found after the subtraction of pedestals and calculation of noise. Tracks are reconstructed by using a rough model after the pre-alignment. Reconstructed track candidates are used in the final analysis of two-track resolution. As one possible application of vertexing and tracking at the ILC, the production of right-handed scalar muon is analyzed and discussed in chapter 8 based on full simulation. The masses of right-handed smuons and neutralinos are determined by the muon energy spectrum. The angular distribution of smuons is studied.
The detailed parameters used for the simulation of the DEPFET vertex detector is given in appendix A. Appendix B is a “How-to” about using Isajet in a Pythia based event generator. We discuss the solution of the intersection of two cones in detail in appendix C, which are required in the analysis of angular distribution of smuons.
Chapter 2
Physics at the ILC
Currently, the Standard Model (SM) is the most successful theory in particle physics. It incorporates all the elementary particles observed so far, and describes their electromagnetic, strong and weak interactions. The predictions of the SM have been tested by a large vari- ety of high energy experiments to an unprecedented accuracy. But the hypothetical Higgs boson [11], which plays an important role in the spontaneous electroweak symmetry break- ing (EWSB) and the generation of masses, is not observed yet. The SM also leaves some fundamental questions unanswered. For instance, it does not incorporate gravity. It does not explain what the dark matter is, which dominates over visible matter in the universe and was confirmed by recent astrophysical observations. It also lacks an explanation of the masses of neutrinos. In addition, the huge hierarchy between the electroweak scale and the Planck scale cannot be explained within the framework of the SM [12]. All these problems indicate the existence of new physics beyond the SM. Such new physics usually introduces new interactions, new particles or new symmetries, which are supposed to come into play at the Terascale, a scale that will be probed by the Large Hadron Collider at CERN for the first time. The emphasis of the LHC is on discoveries of new particles due to its high center- of-mass energy. The ILC, with its precision measurements, is essential for understanding the new physics at the Terascale.
Possible discovery scenarios at the ILC have been discussed in detail in the ILC Reference Design Report (RDR) [13]. The interplay between the LHC and ILC will be important for the discovery of new physics and, in particular, the disentangling of different models [14]. For instance, the study of the hypothetical Higgs boson is one of the most important tasks for both colliders. It is believed that the LHC is very likely to discover the Higgs boson if it exists within the energy range of a few hundred GeV. If a Higgs-like particle is discovered by the LHC, the ILC will measure its properties precisely to verify its true identity. The measurements of ILC will determine its mass, spin, parity, decay channels, and couplings to other particles. The precise determination of the Higgs properties is crucial to reveal the mechanism that generates the particle masses. If no Higgs-like particles are observed at the LHC, this might be due to one of the following two reasons: 1. The Higgs…