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PAMELA Measurements of High Energy Cosmic Ray Positrons

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PAMELA Measurements of High Energy Cosmic Ray Positrons PAMELA measurements of high energy cosmic ray positrons LAURA ROSSETTO Doctoral Thesis Stockholm, Sweden 2012 Doctoral Thesis PAMELA measurements of high energy cosmic ray positrons Laura Rossetto Particle and Astroparticle Physics, Department of Physics, Royal Institute of Technology, SE-106 91 Stockholm, Sweden Stockholm, Sweden 2012 Cover illustration: Cassiopeia A supernova remnant located approximately 3.4 kpc away in the Milky Way, in the constellation Cassiopeia. It is a false colour compos- ite image synthesized from observations gathered in different spectral regions by three space-based observatories. Red areas represent infrared data from the Spitzer Space Telescope. Yellow areas represent visible light data collected by the Hubble Space Telescope. Green and blue areas are X-ray data from the Chandra X-ray Observatory. Akademisk avhandling som med tillst˚and av Kungliga Tekniska H¨ogskolan i Stock- holm framl¨agges till offentlig granskning f¨or avl¨aggande av teknologie doktorsexa- men m˚andagen den 11 juni 2012 kl 13:15 i sal FB42, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm. Avhandlingen f¨orsvaras p˚aengelska. ISBN 978-91-7501-362-6 TRITA-FYS 2012:27 ISSN 0280-316X ISRN KTH/FYS/--12:27--SE c Laura Rossetto, May 2012 Printed by Universitetsservice US AB 2012 Abstract PAMELA is a satellite-borne experiment mounted on board of the Russian Resurs DK1 satellite which was launched from the Baikonur cosmodrome in Kazakhstan on June 15th 2006. The satellite orbits around the Earth on a semi-polar and elliptical trajectory and PAMELA has been acquiring data for almost six years. The detector was designed and optimised for the study of the antimatter component in the cosmic radiation. The PAMELA apparatus consists of a time-of-flight system, a permanent magnetic spectrometer, an electromagnetic calorimeter, a neutron detector and an anticoincidence system. Combining information from different detectors and in particular from the calorimeter, positrons can be identified from the significant background due to cosmic ray protons. The interest in cosmic ray positron measurements considerably increased in the last years because of new experimental results. The positron fraction measured by the PAMELA detector clearly increases with energy above 10 GeV. This is not in agreement with a pure secondary positron production, thus indicating a probable primary origin of positrons. In this context, a measurement of the positron fraction and of the positron flux up to the maximum energy permitted by the PAMELA design becomes extremely important. In view of extending positron measurements, a method for positron identifica- tion has been studied. The method uses longitudinal and transverse shower profile variables in the calorimeter for separating electromagnetic and hadronic showers. This method has been first tested on simulated positron and background proton events produced in two different energy ranges (20 100 GeV and 100 300 GeV). Proton contamination can arise in identified positron− events due to the− production of neutral pions which decay electromagnetically. Positron and electron events have been identified from positively and negatively charged particles in flight data, al- lowing the positron fraction and the positron flux to be reconstructed up to an energy of 300 GeV. As a cross-check, a multivariate approach has also been applied to∼ flight data in order to estimate the number of positron and electron events at energies greater than 100 GeV. The positron fraction obtained with these two different methods are in good agreement within statistical uncertainties. The resulting positron flux measurement shows a rise at energies greater than 100 GeV. iii iv Contents Abstract iii Contents v Introduction 1 1 Cosmic rays 5 1.1 Theenergyspectrum.......................... 5 1.2 Accelerationmechanisms. 8 1.3 Propagationmechanisms. 12 1.3.1 Diffusionprocesses . 12 1.3.2 Convectionprocesses. 12 1.3.3 Reaccelerationprocesses . 13 1.4 Solarmodulation ............................ 13 1.5 Cosmicraypositronsandelectrons . 17 1.5.1 PAMELApositronfraction . 19 1.5.2 PAMELAelectronflux. 20 1.5.3 Theoreticalinterpretations . 23 2 The PAMELA experiment 27 2.1 ThePAMELAapparatus ....................... 27 2.1.1 Thetime-of-flightsystem . 30 2.1.2 Themagneticspectrometer . 31 2.1.3 Theelectromagneticcalorimeter . 32 2.1.4 Theneutrondetector . 34 2.1.5 Theanticoincidencesystem . 34 2.2 ThePAMELAdataacquisitionsystem . 37 2.2.1 Thetriggerconfiguration . 38 2.3 PAMELAscientificgoals. 38 v vi Contents 3 Shower development in the PAMELA calorimeter 41 3.1 Electromagneticshowers. 41 3.1.1 Theshowerprofiles....................... 44 3.2 Hadronicshowers............................ 46 3.2.1 Theshowerprofiles....................... 48 3.3 The π0 contaminationofhadronicshowers . 50 3.4 ThePAMELAelectromagnetic calorimeter . 51 3.4.1 Showerprofilevariables . 51 4 Simulation studies of π0 contamination 63 4.1 GEANT3simulations ......................... 63 4.2 “Only-π0 case”simulations ...................... 67 4.3 Simulationanalysis........................... 67 4.4 Positronselectioncriteria . 70 4.4.1 Trackerselections. 70 4.4.2 Time-of-flightselections . 71 4.4.3 Anticoincidenceselections . 71 4.4.4 Calorimeterselections . 72 4.5 The“Natureanalysis”approach . 73 4.6 A new approach for positron identification . 81 4.7 Analysis in the energy range 20 100GeV ............. 85 − 4.7.1 Positronselectionefficiencies . 85 4.7.2 Protoncontamination . 88 4.8 Analysis in the energy range 100 300GeV ............ 90 − 4.9 Positronfraction ............................ 99 4.10Conclusions ............................... 106 5 Multivariate analysis approach 109 5.1 TheToolkitforMultivariateAnalysis . 109 5.2 The MultiLayers Perceptron neural network . 110 5.3 Analysis in the energy range 100 300GeV ............ 111 − 5.4 Positronfraction ............................ 118 5.5 Conclusions ............................... 121 6 Positron flux 123 6.1 Fluxevaluation............................. 123 6.2 Thelivetime .............................. 124 6.3 Thegeometricalfactor . 125 6.4 Selectionefficiencies . 128 6.4.1 Triggerefficiency . 128 6.4.2 Efficiency of selections above the calorimeter . 128 6.4.3 Calorimeterefficiency . 129 6.5 Positronflux .............................. 131 7 Conclusions 135 Contents vii Acknowledgments 137 List of figures 139 List of tables 143 Bibliography 145 viii Introduction Outline of the thesis The work presented in this thesis concerns positron selection studies which have been tested on simulations and then applied to flight data collected by the PAMELA satellite experiment between July 2006 and January 2010. The positron fraction and the positron flux have been evaluated up to 300 GeV. A potential proton contamination in positron identification due to neutral∼ pion production was also studied in a detailed way and is also presented in this thesis. In chapter 1 basic concepts and features of cosmic rays are presented together with a description of acceleration and propagation mechanisms throughout the galaxy. Particular attention is focused on cosmic ray positrons and on the positron fraction first published by the PAMELA Collaboration in 2009. Chapter 2 is ded- icated to a detailed description of the components which constitute the PAMELA experiment. The main scientific goals are briefly summarized together with per- formance for the detection of cosmic ray particles and antiparticles. In chapter 3 electromagnetic and hadronic shower development inside the calorimeter are de- scribed; neutral pion contamination in hadronic showers is also presented. Further- more, a detailed description of the transverse shower profile variables used in the analysis work is presented. Chapter 4 is dedicated to GEANT3 simulation studies of hadronic and electromagnetic showers induced by protons and positrons inside the calorimeter. A new approach for positron identification using shower profile variables in the calorimeter is also described: this method has been tested on sim- ulations in two different energy ranges and up to a maximum energy of 300 GeV. This method was then applied to flight data in order to select positron and electron events. The resulting positron fraction is shown at the end of this chapter. As a cross-check to the results presented in chapter 4, a multivariate approach has also been applied to flight data in order to estimate the number of electron and positron events. This method and the resulting positron fraction are presented in chapter 5. In chapter 6 the procedure followed to evaluate the positron flux up to 300 GeV is described. The obtained result is shown and compared to the positron∼ flux evaluated by other experiments. 1 2 Contents The author’s contribution I joined the PAMELA Collaboration in October 2007 when I started working as a PhD student in the Particle and Astroparticle Physics group at KTH. The first year was mainly focused on learning about the PAMELA experiment. Some months were spent studying the momentum-velocity (pβ) method applied to the PAMELA calorimeter in order to separate antiprotons from negatively charged pi- ons. I produced simulated data sets for this work. This analysis was documented as a PAMELA Collaboration note but is not described further in this thesis. In the second year of my PhD I started working on simulations which focused on the study of neutral pion production in hadronic showers inside the calorimeter. I used the PAMELA Collaboration’s official simulation code and I modified it in order to artificially increase the number of
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