The Alpha Magnetic Spectrometer on the International Space Station Ignacio Sevilla 1 on behalf of the AMS Collaboration CIEMAT, Centro de Investigaciones Energéticas Medioambientales y Tecnológicas, Av. Complutense 22-28040 Madrid ignacio.sevilla@ciemat.es Summary. The Alpha Magnetic Spectrometer (AMS) [1] is a particle physics detector designed to measure charged cosmic ray spectra up to TeV energies, with high energy photon detection capability up to a few hundred GeV. Its construction is due to be completed in 2008. With its large acceptance, long duration (3 years) and state of the art particle identification techniques, AMS will provide the most sensitive search for the existence of anti-matter nuclei and dark matter to date. This contribution will describe the overall detector and its physics goals. 1 Introduction The Alpha Magnetic Spectrometer is a high energy particle physics experiment to be installed on the International Space Station (ISS) in 2008-9 for at least three years of operations. The advantages of using this platform are two-fold: an atmosphere-free environment to detect primary cosmic rays and the long-term support of a space station providing power, data transmission and a stable platform for an extended period of time. At this altitude, an unprecedentedly large amount of cosmic rays will be collected providing high sensitivity to fine studies of their spectra, including antimatter searches, indirect hints to non-baryonic dark matter and isotopic ratio measurements. A prototype of the experiment (AMS-01) was successfully built and flown on the STS-91 mission of the Space Shuttle in 1998 during 10 days. The detector was a scaled-down version of the final one (AMS-02) at the ISS. AMS- 01 had a permanent dipole magnet with a bending power of 0.15 Tm 2 in a ring configuration. In the midst of this magnetic field, a partially equipped 6-plane silicon tracker measured the rigidity (R p/z), the absolute charge of the particle and its sign. A two double layer scintillator time-of-flight system, an aerogel threshold Cerenkov counter and lateral veto counters completed the experiment, providing also charge and particle velocity measurements. The success of the flight and the valuable physics results [2] obtained validated the experiment concept and provided important information for the future deployment of AMS-02.
2 Ignacio Sevilla on behalf of the AMS Collaboration 2 Physics goals 2.1 Antimatter Current baryogenesis theory based on Sakharov s [3] model are not fully supported experimentally by the lack of observations of large levels of CPviolation or baryon number non-conservation. This leaves room for not yet excluded models where small pockets of antimatter remain within our Galaxy [4] or where domains of antimatter exist at large (tens of Mpc) distances [5]. The observational constraints to these models can come indirectly from gamma ray and microwave background observations. However, direct evidence can only be provided by the detection of anti-nuclei in cosmic rays. Current limits, including AMS-01 measurements, set the He/He ratio at 10 6 (95% CL). After three years of AMS-02 data, this limit is expected to go down to 10 9 (95% CL) (figure 1), excluding most of the possibilities for macroscopic antimatter domains. To achieve this goal, excellent particle identification capabilities are required. Fig. 1. Results for the anti-helium/helium ratio for past experiments (d. corresponds to AMS-01) and future perspectives for AMS-02 (left). The positron/lepton spectrum where a specific super-symmetric dark matter signal [6] has been added to the expected background, following the HEAT experiment signal [7] found at these energies. AMS-02 expected measurements have been superimposed (right). 2.2 Dark matter An increasing amount of evidence from different fields of astrophysics and cosmology suggest that a large amount of dark matter of unknown nature is present in the Universe. Observations also point that, in any case, it should be composed of non-baryonic weakly interacting massive particles (WIMPs)
The Alpha Magnetic Spectrometer on the International Space Station 3 beyond the Standard Model of particle physics [8]. Some of the most observationally promising scenarios, like super-symmetry or unified extra-dimensions, predict the existence of WIMPs by different mechanisms in an independent way, providing interesting candidates. The co-annihilation of these particles in the galactic halo yield secondary gamma rays, positrons, deuterons and anti-protons among other ordinary products. With a sufficiently sensitive detector and large enough statistics, these small contributions to the standard cosmic ray spectra could be detected (figure 1). Many of the previous measurements of these spectra are unknown or dominated by systematic errors in the GeV/TeV range. 2.3 Galactic cosmic rays High energy cosmic ray nuclei traverse the interstellar medium where they undergo various interactions and re-accelerations until they reach the Earth. Secondary fragmentation products also contribute to the observed spectrum. In the GeV/nucleon to TeV/nucleon energy range, many aspects of cosmic ray physics can be investigated: Proton and helium spectra are fundamental for predictions of the antiproton spectrum from conventional origin. This way deviations coming from exotic sources become easier to identify. See figure 2 for some predictions. Proton spectrum is also very relevant for background calculations for atmospheric neutrino experiments. Deuterium and 3 He abundances can be used for cosmology-related studies, as well as the D/p ratio. Due to their low mass, electrons undergo severe energy losses in their propagation. Their spectrum can give information about cosmic ray source distribution. Measurements in the GeV region are currently very inaccurate. Boron is a species which only appears in cosmic rays as a secondary product. Therefore, by obtaining ratios such as B/C, we can have information on how much material do cosmic rays traverse. The confinement volume can be obtained as well, under the hypothesis of the diffusion model. The 10 Be/ 9 Be ratio is an important measurement of the confinement time of cosmic rays within the Galaxy, as 10 Be is unstable. Figure 3 show the predictions for AMS-02 for the last two points, together with current measurements. 2.4 Gamma ray astrophysics Even though the detector is not optimized for neutral particle detection, it has been proven [10] that high energy gamma rays could be detectable with high resolution and reasonable efficiency. Gamma rays can be detected by pair
4 Ignacio Sevilla on behalf of the AMS Collaboration Fig. 2. Estimations for the proton (left) and helium (right) fluxes, together with past measurements. This precision is attained in the time scale shown above. Fig. 3. Current measurements and AMS-02 predictions on the B/C (left) and 10 Be/ 9 Be (right) ratios assuming the Strong and Moskalenko model [9]. conversion of the photon in the upper layers of the detector, reconstructing the electron and positron tracks. The ECAL can also absorb unconverted gamma rays which enter its acceptance. AMS can contribute to high energy GRB spectra and complementing multi-channel dark matter searches. 3 Detector description The detector strategy is based on redundant measurements which ensure a proper background rejection for rare signal searches. The design must also meet the constraints imposed by safety, launch and operations in a low orbit environment. The main components of the detector are:
The Alpha Magnetic Spectrometer on the International Space Station 5 A cryogenic super-conducting magnet consisting of two Hemholz coils and two series of six racetrack coils cooled by super-fluid helium at 1.8 K (B = 0.86 Tm 2 ). A transition radiation detector (TRD) which detects the transition radiation light produced by ultra-relativistic particles in a set of polypropylene fiber radiators by means of 5248 straw tubes filled with Xe/CO 2 mixture. It provides lepton-hadron separation up to 300 GeV ( 10 2 ). A time-of-flight system (TOF) consisting of two double planes of scintillator counters which is able to reach a precision in time of about 120 ps. This instrument provides the trigger of the experiment and is capable of velocity measurements with a 3.5% resolution, as well as charge up to Z 20. A silicon tracker detector (STD) made of eight double-sided micro-strip silicon sensor layers, six of them located inside the magnet, to record the particle s trajectory as it bends in the magnetic field. Particle rigidity is measured with an accuracy better than 2% up to 10 GV, for singly charged particles. Absolute charge and sign can also be measured. A ring imaging Cerenkov counter (RICH) which measures the velocity of relativistic particles from the opening angle of the Cerenkov cone. It consists of a dual radiator (silica aerogel and sodium fluoride) and a detection plane instrumented with multi-anode photo-multipliers. Typical accuracy is σ(β)/β 10 3 for Z = 1. Charge will also be measured up to Z 26 jointly with the TOF and STD measurements, providing very small charge confusion. An electromagnetic calorimeter consisting of 9 layers of lead foils with glued scintillating fibers resulting into a total radiation depth of 16X 0 for shower development. It provides further ( 10 3 ) lepton discrimination power and an energy resolution for electromagnetic particles better than 3% above 10 GeV. The combined ECAL/TRD separation power thus reaches 10 5. Auxiliary components such as veto counters for laterally incident particles, a CCD star tracker for gamma astronomy and a GPS connected to the trigger complete the detector. A complete view of the detector can be found in [11] in these proceedings. 4 CIEMAT activities The main involvement of the CIEMAT team has been the construction and testing of the RICH detector and its components. This includes the electronics, mechanical assembly, photomultiplier, light guide and aerogel characterization, space qualification (at external sites) and the development of the reconstruction software (velocity and charge). The AMS-CIEMAT team has been one of the main working groups behind the tests of the RICH prototype
6 Ignacio Sevilla on behalf of the AMS Collaboration at CERN [12]. The full assembly of the final RICH is currently (October 2006) taking place at the CIEMAT clean room. Other AMS-CIEMAT activities include the construction of the magnet power supply, providing computing power for the detector simulation, the development of analysis software and tools and a major involvement in photon detection with the STD. 5 Conclusions AMS will be launch-ready some time in 2008, after integration of the different subdetectors during 2007 at a specifically prepared clean room at CERN. The whole detector will be tested in an accelerated beam and will be space-qualified at ESTEC. During three years on the ISS, it will collect 10 10 events with the purpose of acquiring accurate spectra of cosmic rays up to iron. Its physics goals include antimatter, dark matter searches and gamma ray astrophysics. Regarding cosmic ray studies, AMS will become the reference experiment in the GeV-TeV range for years to come. 6 Acknowledgments The author wishes to acknowledge the CIEMAT-AMS team for their review of this summary. References 1. J.Casaus: Nucl.Phys.B (Proc.Suppl.) 114, 259 (2003) 2. M.Aguilar et al.: Phys.Rep. 366, 331 (2002) 3. A.D.Sakharov: ZhETF Pis ma 5, 32 (1966) 4. Y.M.Khlopov: Gravitation & Cosmology 4, 69 (1998) 5. Y.V.Galaktionov: Rep.Prog.Phys. 65, 1243 (2002) and references therein. 6. E.A.Baltz, J.Esdjo: Phys.Rev.D 59 (1999) 7. M.A.DuVernois et al.: ApJ 559, 296 (2001) 8. D.Bertone, J.Hooper, J.Silk: Phys.Rept. 405, 279 (2005) 9. A.W.Strong, I.Moskalenko: ApJ 509, 122 (1998) 10. I.Sevilla: El experimento AMS como observatorio de rayos gamma desde la Estación Espacial Internacional. PhD Thesis, Universidad Complutense de Madrid, Madrid (2006) 11. M.Costado: Astroparticle Physics at the IAC, these proceedings. 12. P.Aguayo et al: NIM A 560, 291 (2002)