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Low Energy Electron and Positron Measurements in Space with the PAMELA Experiment

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Low Energy Electron and Positron Measurements in Space with the PAMELA Experiment UNIVERSITA` DEGLI STUDI DI ROMA “TOR VERGATA” DIPARTIMENTO DI FISICA DOTTORATO DI RICERCA IN FISICA PH.D. THESIS Low energy electron and positron measurements in space with the PAMELA experiment CANDIDATE: SUPERVISOR: Valeria Di Felice Prof. Mirko Boezio COORDINATOR: Prof. Piergiorgio Picozza A.A. 2009/10 Author’s address: Dipartimento di Fisica Universita` degli Studi di Roma “TorVergata” Via delle Ricerca Scientifica, 1 00133 Roma Italia Introduction This thesis describes the study of electrons and positrons in the cosmic radiation. New measurements of the positron fraction at the top of the atmosphere between 200 MeV and 3:0 GeV are presented, together with the electron spectrum up to 20 GeV. The measure- ment was conducted with the space borne PAMELA experiment from July 2006 to De- cember 2008, that is a period of minimum solar activity and negative solar magnetic field polarity. PAMELA provides the first long term observation of the low energy positron abundance during this particular solar and heliospheric state. The great amount of col- lected statistics allows to study the variation of the particle intensity at Earth with energy and time, and to investigate effects depending on the particles sign of charge. During ap- proximately 860 days of data collection about 16300 positrons and 178000 electrons were identified. The major sources of background, constituted by cosmic protons and locally produced pions, have been studied. The derived electron spectrum clearly shows the effect of the solar modulation below few GeV. The detected positron fraction has been found to be in accordance with predictions from theoretical models that describe the transport of particles in the heliosphere taking drift effects into account. I.1 Outline of the thesis To put the subject of the thesis into context, an overview of the main topics concerning the study of cosmic ray is given in Chapter 1. Aspects related to the detection of cosmic rays particles and antiparticles and their role in the understanding of our Universe are considered. Particular emphasis is given to the effect of the heliospheric magnetic field on the propagation of charged particles below ∼ 15 GV, and how it changes their energy spectra and intensities respect to the interstellar ones. A review of theoretical models describing the transport of particles in the heliosphere is also given and some of their predictions concerning the expected spectra detectable at Earth and charge sign dependent modulation are presented. Chapter 2 gives a description of the PAMELA instrument. Each subdetector is presented in detail. The remaining chapters describe the physics analysis and the results. Chapter 3 deals with the electron and positron identification. The use of each PAMELA detector and ii Introduction the selections developed to distinguish positrons among the vast quantity of cosmic rays detected by the instrument are described. The calorimeter provides most of the rejection power against the hadron background. Some effort has been put in developing selections that could exploit the different behavior of leptons and hadrons in the detector, even at this low energies where the particle showers are not well developed. Simulations have been used to define the calorimeter selection cuts. The main contamination sources for the selected positron sample are also presented here: an estimate of the residual contamining protons and pions in the final sample is given and taken into account. The possibility of a contamination of reentrant albedo particles in the positron sample has been also considered and excluded. The efficiency of the selection procedure has to be taken into account when reconstructing the number of particles traversing the instrument. Simulation and flight data have been used to estimate the overall instrument efficiency and the entity of possibly introduced biases ; this is described in Chapter 4. All parts of the analysis are put together in Chapter 5 where the number of detected electrons and positrons is corrected for the detection efficiencies and instrumental effects deriving the electron spectrum and the positron fraction. The results are then discussed in comparison with previous measurements and theoretical models. Contents Introductioni I.1 Outline of the thesis.............................i 1 Cosmic ray physics1 1.1 Cosmic ray history.............................2 1.2 Composition and energy spectrum.....................2 1.3 Antimatter in space.............................5 1.3.1 A matter-antimatter asymmetric Universe?............6 1.3.2 The search for antimatter in the cosmic radiation.........7 1.3.3 Dark Matter.............................9 1.3.4 Antiparticle secondary production and propagation........ 12 1.4 The solar environment........................... 16 1.4.1 Charged particles in the heliosphere................ 20 1.4.2 Galactic CRs and solar cycle.................... 22 1.4.3 Experimental evidence of a charge-sign dependent solar modulation 24 1.4.4 Solar modulation models...................... 28 2 The PAMELA Experiment 35 2.1 Mission overview and scientific goals................... 35 2.2 Instruments characteristics......................... 37 2.2.1 Time Of Flight system....................... 39 2.2.2 Magnetic spectrometer....................... 41 2.2.3 Calorimeter............................. 47 2.2.4 Anticoincidence system...................... 48 2.2.5 Bottom scintillator......................... 49 2.2.6 Neutron detector.......................... 49 2.3 PAMELA data acquisition and trigger systems............... 50 2.3.1 Data acquisition system...................... 50 2.3.2 Trigger system........................... 52 2.4 PAMELA first results: p=p¯ and e+=(e+ + e−) ratio............ 54 iv Contents 3 Particle identification 59 3.1 Introduction................................. 59 3.2 Tracker selections.............................. 62 3.3 ToF selections................................ 65 3.3.1 Multiple particle event rejection.................. 65 3.3.2 Lepton selection.......................... 66 3.4 AC selections................................ 66 3.5 Galactic particle selection.......................... 68 3.5.1 Back-tracing in the Earth magnetic field.............. 70 3.6 Calorimeter selections........................... 71 3.6.1 Particle interactions in the calorimeter............... 74 3.6.2 Electromagnetic and hadronic showers............... 76 3.6.3 Double showers........................... 88 3.7 Residual background estimation...................... 90 3.7.1 Proton contamination........................ 91 3.7.2 Pion contamination......................... 94 3.8 The final e± sample............................. 101 4 Selection efficiencies 105 4.1 Efficiency calculation and other instrumental effects............ 105 4.2 The instrument acceptance......................... 107 4.3 The tracker selection efficiency....................... 110 4.4 The Time of Flight system efficiency.................... 114 4.4.1 The ToF selection efficiency.................... 114 4.4.2 The trigger efficiency....................... 116 4.5 The calorimeter efficiency......................... 117 4.6 The total efficiency............................. 118 5 Positron fraction and fluxes 121 5.1 Experimental results............................ 121 5.1.1 The positron fraction........................ 121 5.1.2 The electron spectrum....................... 122 5.2 Experimental and theoretical comparison................. 131 5.2.1 The electron spectrum....................... 131 5.2.2 The positron fraction........................ 136 5.2.3 Comparison with the Ulysses data - a first approach........ 139 Conclusions and perspectives 145 Publications 147 Contents V Bibliography 155 VI Contents Chapter 1 Cosmic ray physics A brief overview of the main topics concerning the study of cosmic rays, subject of in- vestigation of the PAMELA experiment, is given in this chapter. Some emphasis is put on the antiparticle component of cosmic rays and their detection in space. Antiparticles are generally believed to have secondary origin, being produced by spal- lation of primary cosmic rays nuclei (mainly protons) on the interstellar gas. After the production, before reaching the Earth, these antiparticles will travel through the inter- planetary space suffering interaction with the turbulent magnetic fields of the Galaxy, inside the heliosphere and with the Earth magnetic field; this will change their energy distribution depending on time, position and particle charge, species and initial energy distribution. Any signature, possibly pointing out non-standard antiparticle production mechanisms, would appear as a distortion of the expected secondary flux. It is therefore of paramount importance to calculate the expected secondary antiparticle flux and to give a robust es- timation of the associated theoretical uncertainties in order to interpret the experimental data in the correct way. At low energies (≤ tens of GeV) the heliospheric magnetic field affects the propagation of charged particle, changing their spectra in shape and intensity respect to the interstellar one. The study of this effect is essential for a better understanding of the solar envi- ronment, that is involved in modeling the background of possible primary signals, and interesting on its own right. Several aspects related to solar modulation are described in the second half of this chapter. 2 Chapter 1. Cosmic ray physics 1.1 Cosmic ray history Cosmic rays (CRs) consist of energetic charged particles incident on Earth from outer space.
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