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Development of a Transition Radiation Detector and Reconstruction of Photon Conversions in the CBM Experiment

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Development of a Transition Radiation Detector and Reconstruction of Photon Conversions in the CBM Experiment Melanie Klein-Bösing Development of a Transition Radiation Detector and Reconstruction of Photon Conversions in the CBM Experiment — 2009 — Experimentelle Physik Development of a Transition Radiation Detector and Reconstruction of Photon Conversions in the CBM Experiment Inauguraldissertation zur Erlangung des Doktorgrades der Naturwissenschaften im Fachbereich Physik der Mathematisch-Naturwissenschaftlichen Fakultät der Westfälischen Wilhelms-Universität Münster vorgelegt von Melanie Klein-Bösing aus Telgte — 2009 — Dekan: Prof. Dr. J. P. Wessels Erster Gutachter: Prof. Dr. J. P. Wessels Zweiter Gutachter: PD Dr. A. Khoukaz Tag der Disputation: Tag der Promotion: Contents Introduction 5 1 Theoretical Overview 9 1.1 ParticlesandForces............................. 9 1.1.1 LeptonsandQuarks .. .. .. .. .. .. .. 10 1.1.2 Hadrons............................... 12 1.1.3 Interactions............................. 12 1.2 Quark-GluonPlasmaandHadronGas . 15 1.2.1 The Phase Diagram of StronglyInteractingMatter . .... 15 1.2.2 Chiral-SymmetryRestoration . 17 1.3 RelativisticHeavy-IonCollisions. ..... 18 1.3.1 ExplorationoftheQCDPhaseDiagram . 20 1.3.2 SignaturesforaPhaseTransition. 23 2 ProbingHeavy-IonCollisionswithDileptonsandPhotons 29 2.1 Dileptons .................................. 29 2.1.1 PreviousExperimentalResultsonDileptons . .... 32 2.2 DirectPhotons ............................... 37 2.2.1 ThermalPhotons .......................... 38 2.2.2 Non-ThermalPhotons . 41 2.2.3 Previous Experimental Results on Direct Photons . ..... 41 3 Interaction of Charged Particles and Photons with Matter 47 3.1 EnergyLossofChargedParticles. .. 47 3.2 Additional Mechanisms for Radiative Energy Loss of Charged Particles . 52 3.2.1 CherenkovRadiation . 52 1 2 Contents 3.2.2 TransitionRadiation . 54 3.3 InteractionofPhotonswithMatter . ... 59 4 The CBM Experiment 65 4.1 The Future Facility for Antiproton and Ion Research (FAIR) ....... 65 4.2 Physics Goals and Observables of the CBM Experiment . ...... 68 4.3 DetectorConcept .............................. 70 4.3.1 The Silicon Tracking and Vertex Detection System . .... 71 4.3.2 TheRingImagingCherenkovDetector . 73 4.3.3 TheTransitionRadiationDetector . 74 4.3.4 TheTime-of-FlightDetector . 75 4.3.5 TheElectromagneticCalorimeter . 76 4.3.6 TheProjectileSpectatorDetector . 77 4.3.7 DataAcquisition .......................... 77 4.3.8 TheMuonChamberSystem . 77 4.4 DileptonSpectroscopyinCBM. 79 5 The Transition Radiation Detector 83 5.1 RadiatorMaterial .............................. 84 5.2 MultiwireProportionalChambers . .. 85 5.2.1 SignalsInducedonCathodePads . 89 5.2.2 GasMixture ............................ 92 5.2.3 SignalGeneration. 92 5.3 DesignoftheMB-TRDPrototype . 93 5.3.1 AbsorptionEfficiencyofTRintheGasVolume . 95 5.3.2 Double-sidedPadReadoutElectrode . 97 5.3.3 PrototypeAssembly . 100 6 Performance of the MB-TRD Prototypes 103 6.1 55FeSourceMeasurements . 103 6.2 In-BeamTestatGSI ............................ 106 6.3 PulseHeightDistribution . 109 6.4 Electron/PionSeparation . 110 6.4.1 PionEfficiency ........................... 113 Contents 3 6.5 SimulationofElectron/PionSeparation . .. 119 6.5.1 Simulation of Energy Loss by Ionization and Transition Radiation 119 6.5.2 ImplementationoftheNewDetectorDesign . 120 6.5.3 ParticleSimulation . 120 6.5.4 Comparison of Simulated and Measured Energy-Loss Spectra . 120 6.5.5 PionEfficiencyinSimulation . 124 6.6 PositionResolution ............................. 125 6.6.1 ThePadResponseFunction . 126 6.6.2 ClusterReconstruction . 131 6.6.3 PositionResolutionAlongWireDirection . 131 6.6.4 Position Resolution Perpendicular to the Wire Direction . 138 6.6.5 TheNextGenerationofCBMTRDPrototypes . 141 7 Dielectron Reconstruction in CBM by Using the MB-TRD Geometry 147 7.1 TheCbmRootSimulationFramework . 147 7.2 TrackandVertexReconstruction . 148 7.3 ElectronIdentification . 149 7.3.1 ElectronIdentificationintheRICH . 150 7.3.2 ElectronIdentificationintheTRD . 151 7.3.3 ElectronIdentificationintheTOF . 154 + 7.4 Feasibility Study of Measuring Direct Photons via Conversion into e e− Pairs.....................................154 7.4.1 PhotonMeasurementinCBM . 154 7.4.2 ReconstructionofPhotonConversions . 157 7.5 MeasurementofNeutralPions . 171 7.5.1 Extraction of the π0-Signal..................... 173 Summary 189 Zusammenfassung 193 A Runsummaryofthetestbeamin2006atGSI 199 B Technical Drawings 205 Danksagung 221 Introduction The focus of this thesis is the development of a Transition Radiation Detector (TRD) for the Compressed Baryonic Matter (CBM) experiment at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt. In addition, the usage of the TRD in the measure- ment of direct photons is investigated. CBM will be a fixed-target heavy-ion experiment, which investigates collisions in the beam energy range of 5–35 AGeV. The nature of nuclear matter at very high temperatures and/or densities is the focus of relativistic heavy-ion physics. In particular, the experimental program aims to study ef- fects of a new phase of matter, the quark-gluon plasma (QGP). The QGP is a deconfined and thermalized form of strongly interacting matter, where individual quarks are liberated from the nucleons. Up to now the phase diagram of strongly interacting matter has not been explored in detail at high net baryon densities. Theoretically, at high baryon densi- ties and low temperatures, a first-order phase transition between hadronic matter and the QGP is expected, but has not yet been experimentally confirmed. In addition, new inter- est is sparked since recent lattice quantum chromodynamics (QCD) calculations predict this phase boundary ending in a critical endpoint whose existence and exact position in the phase diagram is currently being debated. A large area of the phase diagram can be covered in nucleus-nucleus collisions in the energy range that will be provided at FAIR. The CBM experiment aims to investigate the regime of high baryon densities where the phase transition is expected to be of first order. Moreover, the discovery of the critical endpoint would be a breakthrough in high-energy heavy-ion research since this observa- tion would be the first measurement on the deconfinement phase-transition line. The goal of the CBM experiment is to look in particular for rare probes by taking advantage of high intensity heavy-ion beams delivered by FAIR. CBM will be a multipurpose experiment with the ability to measure leptons, hadrons, and photons. Therein, a TRD will provide the electron identification and – together with a Silicon Tracking System (STS) – the track- ing of charged particles. In conjunction with a ring imaging Cherenkov (RICH) detector and a time-of-flight (TOF) measurement, the TRD is to provide a sufficient electron- identification for the measurements of charmonium and low-mass vector mesons. For the 5 6 Introduction TRD, the required pion suppression is a factor of about 100 at 90% electron efficiency, and the position resolution has to be of the order of 300 to 500 µm. Moreover, the material budget in terms of radiation length has to be kept at a minimum in order to minimize mul- tiple scattering and conversions which would limit the precise measurement in following TRD stations and other detectors. The largest and up to now unrivaled challenge for the TRD design is that both (PID and tracking) have to be fulfilled in the context of very high particle rates (event rates of up to 10 MHz are envisaged) and at the same time large charged-particle multiplicities of up to 600 per event in the CBM detector acceptance. For the most central part of the TRD, this leads to particle rates of up to 100kHz/cm2. Currently, the TRD system is envisaged to be subdivided into three stations consisting of three or four layers each. In order to fulfill the CBM requirements, small prototypes of the TRD based on multiwire proportional chambers (MWPC) with pad readout were developed and tested. The development and construction of the prototypes was performed in collaboration with the group of Prof. M. Petrovici from IFIN-HH, Bucharest, Romania. By using secondary beams of electrons, pions, and protons the tracking performance and the electron-pion separation were determined for particle rates of up to 200kHz/cm2. The TRD layout and the detector responses measured in the test beam were imple- mented in the existing CBM simulation framework in order to perform realistic physics simulations. The implementation of the TRD layout as well as an analysis of photon conversions into dielectrons are further aspects of the work presented here. A feasibility study for the measurement of photons and neutral pions, decaying into photons, via pho- ton conversions in the target is performed in simulations using the TRD layout developed in hardware. For more than 20 years photons have been considered as one of the most valuable probes of the dynamics and the properties of matter formed in heavy-ion collisions. In contrast to hadronic particles, which have large interaction cross sections in nuclear mat- ter, photons interact only electromagnetically and have a long mean free path. This path length is typically much larger than the transverse size of the hot dense matter created in nuclear collisions. Since photons can pass through the matter almost undistorted by final- state interactions, they carry information on the entire collision history, and thus provide an undistorted insight into the hot and dense phase in which they are produced. In CBM, the ECAL is designated to provide photon identification.
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