Home page > PhD fellowships > Previous Calls > Proposed PhD subjects 2013-2016 > Indirect search for dark matter and study of primary sources of positrons and electrons coming from astrophysical sources
by- 25 February 2013
Topics : Indirect search for dark matter and study of primary sources of positrons and electrons coming from astrophysical sources (pulsar nebula and supernova remnants)
Proponents : Vincent Poireau
Address : LAPP - 9 chemin de Bellevue - BP110 - 74941 Annecy-le-Vieux cedex
Phone : + 33 4 50 09 16 48
Contact Email : firstname.lastname@example.org
AMS (Alpha Magnetic Spectrometer) is an experiment launched by NASA in May 2011 on the International Space Station (ISS). It is a particle detector in orbit that allows to directly detect cosmic rays before they en-ter the atmosphere. Its main goals are to study the propagation of cosmic rays and search for antimatter and dark matter. To perform these tasks, AMS is measuring with unprecedented precision the spectrum of charged particles from protons to iron, electrons, positrons, and gamma rays, and for energies ranging from GeV to TeV. AMS will allow to constrain models of propagation of cosmic rays in the universe, and will there-fore define a standard physics for these cosmic rays. AMS has already registered more than 26 billion events after a year and a half of data taking.
The visible matter in the universe, such as stars, is less than 5% of the total mass of the universe. The re-maining 95% consists of dark matter, estimated at 23% of the mass of the universe, and dark energy. The ex-act nature of these two components is still unknown. However, if the dark matter turns out to be a particle, annihilation of these particles in the galactic halo could produce excess of charged or neutral particles de-tectable by AMS. The presence of anomalous peaks or structures in the positrons, anti-protons, or gamma ray spectra could indicate the existence of neutralinos or other dark matter candidates.
The AMS collaboration brings together hundreds of scientists from 16 countries on three continents. The LAPP is part of this collaboration for many years, and has actively participated in the construction of the calo-rimeter and the related analysis (simulation, calibration, identification of electrons, positrons and gamma, indirect search for dark matter, astrophysical interpretations). The AMS detector consists of 8 layers of silicon tracker inserted in a permanent magnet providing 0.14 Tm bending power, to measure momentum and charge of particles and to participate to the nuclei identification thanks to dE/dx information. Four layers of time-of-flight scintillating system situated on top and bottom of the tracker provide the main trigger and give the direction of particles. A transition radiation detector and an electromagnetic calorimeter discriminate electrons and positrons from the large proton background. Finally a ring imaging Cherenkov detector identify nuclei in conjunction with the tracker.
The thesis will focus on a search for any deviation compared to the standard model of cosmic rays through the measurement of the ratio Ne+/(Ne+ + Ne-) and the absolute positron spectrum. A deviation of these measurements compared to the expected behavior would indirectly be an evidence of dark matter. Howev-er, a deviation due to near pulsars would also be a possible explanation: precise measurements and better understanding of the propagation of cosmic rays is needed to solve this question. The measurement of the positron spectrum is a flagship measurement of AMS, and is eagerly awaited by the scientific community. The statistics available in AMS in the next three years will make this measurement much more precise than any previous measurements, and the spectrum will be extended to high energies, something that was never done before with such accuracy.
A first task in the thesis is to control the level of backgrounds using the complementarities and the redun-dancy of the AMS detectors; primarily the proton background can be reduced by combining information from different subdetectors. The charge confusion inducing an electron background will need to be precisely quantified. At this stage, the student will be able to measure the ratio of positron over electron fluxes Ne+/(Ne+ + Ne-). In a second step, the more complex task is the determination of the positron flux, where the acceptance needs to be computed precisely. Finally, interpretation of the measurement in terms of an exotic component such as neutralino annihilations will require a good prediction for the conventional backgrounds and a confrontation with different models of new physics. Collaboration with LAPTH will take place in order to confront the model with the data. Besides new physics and dark matter, an interpretation with astrophys-ical sources such as pulsar nebula or supernova remnant will be attempted to see if they could explain the potential deviation.
The PhD student will participate to the data taking in the AMS POCC – Payload Operation Control Center - located on the CERN site.
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