The Alpha Magnetic Spectrometer on the International Space Station Mercedes Paniccia, University of Geneva (Switzerland), November 2003 An experiment to search for dark matter, missing matter and antimatter. Manifested on shuttle flight UF 4.1 to the ISS and scheduled to be launched in 2007 AMS The Transition Radiation Detector: Separation between light and heavy particles is provided by the Transition Radiation Detector placed on top of the set-up, which measures the radiation emitted by particles upon changes in the index of refraction of the traversed medium. The detector features 20 layers of fleece radiator 23mm thick and straw tubes, 6mm diameter, filled with a Xe/CO2 mixture, for X-ray detection. It will separate baryons from electrons up to energies of 300 GeV. It will also help in the tracking of charged particles, identification of photon conversion points and energy loss measurement. The Time Of Flight System: Two crossed double layers of scintillators, positioned 1.3m apart, respectively above and below the Tracker System, provide time-of-flight of particles traversing the detector, and then measure the velocity and the direction of particles. It is the main trigger detector for charged particles. Because of the high magnetic stray field of the superconducting magnet, the phototubes have to be aligned more carefully with the local magnetic field. A timing resolution better than 200 ps is nevertheless expected. The Time Of Flight System will also provide a redundant measure of the energy loss with a resolution of about 10% for minimum ionizing particles. The Ring Imaging Cherenkov: A proximity focusing Ring Imaging Cherenkov counter placed below the spectrometer provides further velocity and charge measurement. Light emitted in the combined Aerogel (n = ) and sodium-fluoride (n = 1.33) radiator on the top is detected as a circle of photons by an array of position sensitive photomultipliers on the bottom, occasionally after reflection off a conical mirror. The emission angle measures the velocity with a resolution of 0.2%. The number of photo-electrons measures the energy loss with a resolution of about 8%. Thus it will perform the identification of isotopes of mass A < 25, over its momentum range ( 5-13 GeV/c ) and it will identify chemical elements up to Z = 26 and up to the upper rigidity limit of the spectrometer. The Magnet: The superconducting magnet is cooled by evaporating liquid helium and has a reservoir for about three years of operation without refill. Its dipolar field, normal to the aperture of the magnet, is based on a magic ring configuration of race track coils around a pair of Helmholtz coils. It amounts to 0.8 T close to the center. The Tracker: Trajectories of charged particles are localized by double sided silicon sensors, arranged in eight layers of about one square meter each on five planes of an ultra-light support structure. Close to 2500 sensors provide a tracking resolution of 10  m in the bending direction and of 30  m in the orthogonal direction. Thus the spectrometer measures rigidity up to a few TV. The measurement of specific energy loss, dE/dx, proportional to the square charge of the particle, allows the identification of nuclei. The Tracker measures also the direction and the energy of photons converted in the material above the first Tracker layer, with excellent directional resolution and good energy resolution. The Electromagnetic Calorimeter: At the very bottom an Electromagnetic Calorimeter made of lead scintillating fiber sandwich completes the set-up. The Calorimeter reconstructs the shower with three dimensional sampling using crossed layers. Its energy resolution for electromagnetic showers is expected to be  with E in GeV ). It distinguishes hadronic from electromagnetic showers by the lateral and longitudinal shape, with a protons rejection of 0.01% up to several hundred GeV. The Star Tracker: Recently added to the AMS-02 set-up, it will ensure accurate knowledge about the instrument orientation, since the ISS attitude is rather variable. The apparent lack of antimatter in the Universe, the dark matter problem and the origin, composition and propagation of cosmic rays are still unsolved puzzles in particle physics and cosmology. Theories which could give an answer to these open questions require speculative new physics ingredients and lack experimental data to be confronted with. The Alpha Magnetic Spectrometer project aims at improving this situation by providing a large area, high resolution spectrometer to be exposed to cosmic rays over a long observation period on the International Space Station. For test and space qualification purposes, a preliminary version of the detector (AMS-01), performed a 10 days flight on NASA Space Shuttle mission STS-91 in June 1998, successfully collecting data which already allowed a broad spectrum of physics analysis both concerning cosmic rays and the Earth radiation belts. Based on experience gathered during the first mission, a more ambitious detector, AMS-02, is being built to be installed on the ISS in The AMS-02 detector is based on a spectrometer with a superconducting magnet, providing a rigidity range for charged particles up to few TV. It features sophisticated particle identification devices that will allow to discriminate the small amount of anti-protons, up to high energies. It will also have two ways of measuring high energy photons: by conversion in the Tracker and in the Electromagnetic Calorimeter, which will allow systematic observations of the gamma ray sky. The AMS Collaboration currently includes over 220 physicists from 35 institutions in Europe, the United States and Asian-Pacific countries. The Superconducting Magnet and the Superfluid Helium Vessel High Energy Photons: AMS-02 will constantly monitor the gamma ray sky in the energy interval from 1 GeV to 1 TeV, with rather good acceptance and resolution, both using conversion in the Tracker and the Electromagnetic Calorimeter. Measurements of high energy gamma ray emission from galactic sources like pulsars and extragalactic sources like Active Galactic Nuclei (AGN) will complement the observations in other frequency bands to gain a better understanding of astrophysical particle acceleration mechanism. Cosmic Rays: AMS-02 will collect about one billion nuclei of D, He, Li, Be, B and C. An accurate determination of isotope abundances over a wide range of energies provides crucial information regarding the cosmic rays propagation in the galaxy. Antimatter: The strong evidence supporting the Big Bang origin of the Universe requires matter and antimatter to be equally abundant at the very hot beginning. The absence of sharp annihilation photon peaks excludes the presence of large quantities of antimatter within our cluster of galaxies. Theories which predict either the existence of antimatter in segregated domains or the total absence of antimatter at the present time are highly speculative. The resolution of this important problem requires further data: from the current generation of particle colliders and the B factories at SLAC and KEK to improve our current under- standing of CP-violation; from Tevatron and LHC to provide clues on the correct extension of the Standard Model; from proton decay experiments in Japan and Italy to improve our understanding of baryon stability; and from AMS to improve the observational basis of the matter-antimatter balance in the Universe. Dark Matter: More than 90% of the Universe is made of dark matter. Theory suggests that Supersymmetric particles like the neutralino could be an important contributor to this dominant component of the Universe. Annihilation of these particles in the galactic halo might produce a visible contribution to the anti-particle and photon spectra.