Last Update: October 2000

AMS Alpha Magnetic Spectrometer

AMS Collaboration:

INFN Bologna, Milano, Perugia, Pisa (Italy); HUT, Turku Univ. (Finlandia); ISN, LAPP (France); RWTH I e III, MEPI (Germany);Accademia Sinica, CALT, IEE, Shandong Univ. (China); ITEP, LIP, MEPI, SRI-RAS (Russia); CIEMAT (Spain); ETH, Geneve Univ. (Svitzerland); Boston Univ.,CALTECH, J.Hopkins Univ., MIT, Maryland Univ. (USA)

Laboratory and beam: Shuttle and International Space Station, Search for dark matter and antimatter in the Cosmic Rays

1. Goal of the experiment

> The disappearence of the antimatter and the presence at all scales in our universeof a non luminous components of matter (dark matter) are two the most intriguing misteries in our current understanding of the structure of the Universe. To study these problemes by measuring with the highest accuracy the composition of Cosmic Rays, a high energy physics experiment, the Alpha Magnetic Spectrometer (AMS) is scheduled for installation on the International Space Station for a three year mission. In preparation for this long duration mission AMS flew a ten days precursion mission on board of the space shuttle Discovery during flight STS-91 in June 1998. This high statistics measurement of the CR in space, enabled for the first time the study of the behaviour of primary CR near Earth in the rigidity interval from 0,1 Gev to 200 GeV, at all longitudes and latitudes up to 51.7o. Search of antimatter requires the capability to identify with the highest degree of confidence, the type of particle traversing the experiment together with the absolute value and the sign of its electric charge.
> This can be achieved through repeated measurements of the particle momentum (solid state spectrometer based on a 6 layer Silicon Tracker located in a permanent magnet), velocity (Time of Flight, Transition Radiation detectors, Cerenkov detectors) and energy deposition (Ionization detectors). AMS consist of a large acceptance magnetic spectrometer (about 0.6 m2sr) surrounding a six layer high precision Silicon Tracker and surrounded by a Time of Flight scintillator system (ToF). A scintillator anticounter system, located on the magnet inner wall, one Transition Radiation Tracker (TRT) located below and above the magnet, and a solid state Ring Imaging Cherenkov detector, complete the experiment. While on the Space Station the complete AMS experiment will operate for three years, a reduced configuration (baseline) has been deployed on the precursor flight. The baseline configuration included a permanent magnet, the Anticounter and the Time of Flight systems, the Silicon Tracker and an Areogel Threshold Cherenkov counter. By combining the various measurement it is possible to determine the type of particle traversing the magnet and/or to distinguish interesting particles from background with an accuracy of one part in ten billions. The search for dark matter will be based on the capability of detecting anomalies in the spectrum of antiprotons, electrons and gamma rays particles. Gamma rays are detected by their conversions in e+e- pairs on the top part of the experiment.

2. Physics achievements

> 1999: The papers published in 1999 refers to the STS-91 mission in june 1998. During the period june 2nd to june 12th, 1998 the Shuttle Discovery has performed 154 orbits at an inclination 51.7o and at an altitude varying between 390 to 350 km. During the mission AMS collected a total of 100 Million triggers, at various Shuttle attitude. Almost all results published in 1999 are obtained with data collected during well defined attitude periods with AMS pointing at 0o, 20o and 40o with respect to zenith (deep space). These data are the first high quality CR data collected with a magnetic spectrometer located outside the atmosphere. The measurement coverall geomagnetic longitudes and most latitudes. These data allow a direct and accurate measurement of the CR composition and spectra, extending the spectra up to 200 GV of rigidity, as well as a systematic study of the effects of the geomagnetic field. As result of the very precise measurement of the C.R. spectrum it has been possible to extend the limit on the existence of antimatter up to 140 GV of rigidity. The measurement of the proton flux as a function of the geomagnetic latitude, clearly suggests that in addition to the primary CR spectrum, visibile above the geomagnetic cutoff, there is a substantial second spectrum, extending to much lower energy and exhibiting some significant latitude dependence close to the equator. These particles cannot come from the deep space, they are on forbidden orbits, but are produced in the interaction of the primary CR with the top layers of the atmosphere. A striking characteristic of the second spectrum is that it is up-down symmetric.

> up to summer 2000:

> Published papers on primary proton spectra, tracker and Time of Flight performances. Ongoing analysis of the properties of second spectra observed during the STS-91 flight. Started work on MC simulation of the interaction of C.R. with the Earth atmosphere. Started the final design of the Calorimeter and Time of Flight systems as well the production of the Silicon Tracker elements for the flight on the ISS scheduled in 2003. Test beam with proton and electrons of the Electromagnetic Calorimeter as well as of the final tracker detector elements and readout electronics.

3. INFN contribution to the experiment in terms of manpower and financial support

> - 1999: three groups belonging to INFN sections (Bologna, Milano, Perugia) including 35 research associates with the support of 20 technicians. During 1999 INFN support has been 2215 Million Lira.

> - 2000: four groups belonging to INFN sections (Bologna, Milano, Perugia, Pisa) including 53 research associates with the support of 25 technicians. During 2000 INFN support has been 2856 Million Lira.

4. Number of publications in refereed journals

> 4 on physics results (AMS Collaboration) 2 on technical results (INFN groups) , 8 as conferences proceedings (INFN only)

5. Number of talks to conferences: 15 (INFN only)

6. Number of undergraduate and doctoral thesis on the experiment: 7 (INFN only)

7. Leadership role in the experiment

> Coordination of the Silicon Tracker, Coordination of the Electromagnetic Calorimeter, Coordination of the Time of Flight system, Coordination of the Ring Imaginc Cherenkov Detector, Co-chairmanship of the AMS Astrophyisics Board.

8. Innovative instruments

> Space qualitification of a several large, state of the art, particle detectors, including: Superconducting magnet, based on a very thin superconducting cable developed by a INFN-ETHZ Collaboration (LASA-Milano). Very large double sided Silicon Tracker (6 m^2), 1 cubic meter, 10 um resolution in the bending plane. Solid state RICH detector based on high density multichannel photomultipliers. High accuracy Time of Flight (120 ps). Low power space qualified electronics. Qualified onboard software.

9. Competing experiments

> Pamela on the Russian Arktica in 2003-4 however with ~200 times smaller detector acceptance and 4 times lower maximum detectable rigidity.

10. International Committee which has reviewed the experiment

> The AMS Experiment has been examined and approved by several international Commettees among which:

> - DoE referee Committee in april 1995 for the Shuttle Mission and in 1998 for AMS on the ISS
> - INFN referee Committee in 1995, Russian Academy of Sciences in 1995.
> - During 1995-96 various referees Commettees have approveed the partecipation to the AMS experiment in various countries/institutes: ETHZ, Swiss National Found , IN2P3, DARA, CIEMAT, TEKES.