Precision measurement of the cosmic-ray electron and positron fluxes as a function of time and energy with the Alpha Magnetic Spectrometer on the International Space Station

Abstract

This thesis presents an analysis of the cosmic-ray electron and positron flux using the AMS-02 detector on the International Space Station as a function of time and energy. The time-averaged flux is integrated over 6.5 years of AMS-02 science data and provides the electron and positron flux with unprecedented accuracy, covering the energy range from 0.5 GeV to 1 TeV. In total 28.39 million events were identified as electrons and 1.95 million as positrons. For each of the 88 Bartels rotation periods (27 days), within the 6.5 years, an individual electron and positron flux is derived spanning the energy range from 1 - 50 GeV. The challenge of the analysis is to extract the small electron and positron signal in the overwhelming proton background present in cosmic rays. A detailed description of the analysis techniques is presented, including a thorough derivation of the systematic uncertainties. The main motivation for measuring the cosmic-ray electron and positron flux in a time-averaged way is to explore the energy dependence up to high energies in detail and search for structures in the spectrum. The traditional understanding is that electrons are primary cosmic rays, whereas positrons are believed to be secondaries, produced by collisions of primary protons with the interstellar medium. A clear deviation from the traditional understanding was discovered: the positron flux cannot be described by a single power law, nor by the sum of two power laws. The secondary production term plus an additional source term, with a finite cut-off energy, is necessary to describe the positron data. Above the cut-off energy, the positron flux is rapidly decreasing. The cut-off is established with a significance of 4σ, providing strong evidence that a new source of cosmic-ray positrons was discovered, which is responsible for the rise of the positron flux, and its decrease at high energies when the source term contribution is vanishing. The origin of the source term remains unclear: both astrophysical sources, such as pulsars, and dark-matter annihilation are candidates to describe the positron flux data. The majority of the electrons is believed to come from one of the several astrophysical sources, each making a power law contribution to the electron flux. The electron flux was found to be well described by the sum of two power laws over the whole energy range, supporting the observation that more than one astrophysical source is responsible for the measured electron flux. For the first time, the charge-sign dependent modulation during solar maximum has been investigated by electrons and positrons alone, using the time-dependent fluxes derived in this thesis. Short-term effects such as Forbush decreases and solar flares were identified simultaneously in the electron and positron flux that cancel in the positron/electron ratio. Long-term effects are revealed in the positron/electron ratio: A smooth transition from one value to another, after the polarity reversal of the solar magnetic field in July 2013. The transition magnitude is decreasing as a function of energy, which was predicated by solar modulation models that incorporate drift effects. This novel dataset allows one to build sophisticated models of solar modulation that can predict the time-dependence of both the electron and positron flux in future. This knowledge will allow a precise modelling of the interstellar electron flux and positron flux from low energies in the GeV regime up to the TeV regime.