Measurement of the flux ratio between gamma's and electrons in CR with the AMS-02 experiment

Abstract

The Alpha Magnetic Spectrometer AMS-02 is a particle physics experiment designed to operate for a long time in space. It was installed on the International Space Station ISS during STS-134 NASA mission (May 2011). AMS-02 is a magnetic spectrometer equipped with many sub-detectors, so to have multiple measurements of particles characteristics. AMS-02 principal components are: a Transition Radiation Detector (identication of hadrons and leptons), a Silicon Tracker operating inside the field of a permanent magnet (momentum and charge measurement), a Time of Flight system (particle β and versus), a Ring Imaging Detector (β and Z of ions), an Electromagnetic Calorimeter (energy measurement) and an Anti Coincidence Counters system (background suppression). AMS-02 has the possibility to take data with high statistics and reach energies until 1 TeV, so providing us the opportunity to examine in detail very high energy gamma-rays emission together with an accurate measurement of cosmic rays spectra. The study of gamma-rays emission is important for a better understanding of some modern astrophysics and cosmology topics. One of the most exciting prospects for gamma-rays searches is the possibility of detecting an unambiguous signature of Dark Matter in our galaxy, revealing the photons produced by the annichilation of a WIMP with another WIMP, through reactions as χχ → γγ and χχ → e+ e− . Because the flux of γ 's in CR is much smaller than the electron flux, the γ/e− ratio may be sensible to the contribution of Dark Matter. In AMS-02 the electromagnetic calorimeter (ECAL) is able to measure the energy with good resolution from GeV to TeV energies and its fine granularity allows also an accurate 3D imaging of the shower development. In this thesis is presented the measurement of the flux ratio between gamma's and electrons in CR. First step of the measurement was the identication of photons not converting before reaching the AMS-02 calorimeter. Gamma's are identied by requiring no signals in the front detectors inside a cone defined around the e.m. shower axis, as obtained by ECAL. Then, electrons are identied as electromagnetic showers corresponding to a negative track measured by Tracker. For the γ/e measurement the background substraction represents the most dicult task. The main background in gamma sample are protons (with shower similar to electromagnetic ones) and electrons traversing the fiducial cone but not identified by Tracker; in electron sample the main background are positive tracks recognized as negative by Tracker. After gamma's and electrons identification the γ/e ratio is measured, with an estimation of the residual background and the main systematic errors