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The Alpha Magnetic Spectrometer (AMS): search for antimatter and dark on the International Station R. Battistona aDipartimento di Fisica e Sezione INFN, Via Pascoli, 06100 Perugia, Italy

The Alpha Magnetic Spectrometer (AMS) is a state of the art detector for the extraterrestrial study of anti- matter, matter and missing matter. After a precursor flight on STS91 in may 1998, AMS will be installed on the International Space Station where it will operate for three years. In this paper the AMS experiment is described and its potential reviewed.

1. Introduction Rotational velocities in spiral galaxies and dy- namical effects in galactic clusters provide us con- The disappearance of cosmological antimatter vincing evidence that, either Newton laws break and the pervasive presence of are two down at scales of galaxies or, more likely, most of the greatest puzzles in the current understand- (up to 99%) of our consists of non- ing of the universe. luminous (dark) matter. There are several dark The model assumes that, at its very matter candidates. They are commonly classified beginning, half of the universe was made out of as ”hot” and ”cold” dark matter, depending on antimatter. The validity of this model is based on their relativistic properties at the of decou- three key experimental observations: the reces- pling from normal matter in the early universe. sion of galaxies (Hubble expansion), the highly As an example, are obvious can- isotropic cosmic microwave background and the didates for ”hot” dark matter while Weakly In- relative abundances of light isotopes. However, teracting Massive (WIMP’s) are often a fourth basic observation, the presence of cos- considered as ”cold” dark matter candidates [4,5]. mological antimatter somewhere in the universe, is missing. Indeed measurements of the inten- sity of flux in the MeV region exclude the presence of a significant amount of antimatter up to the scale of the local of galax- ies (tens of Megaparsecs). It follows that, either antimatter has been destroyed immediately after NASA the Big Bang by some unknown mechanism, or AMS OV-103 matter and antimatter were separated (by some other unknown mechanism) into different region Discovery 12345678910 11 12 13 of space, at scales larger than . All Payloads SM-Spacehab efforts to reconcile the the absence of antimat- ter with cosmological models that do not require new physics failed (see [1–3], for a review of these Figure 1. AMS on STS 91 (Discovery) theories). We are then currently unable to explain the fate of half of the baryonic matter present at the beginning of our universe. In either cases we are currently unable to ex- plain the origin of most of the of our uni- 2

Figure 2. The International Space Station Alpha; AMS will be installed on the left side of the main truss

verse. To address these two fundamental questions in astroparticle physics a state of the art detector, the Alpha Magnetic Spectrometer (AMS) [6] has been recently approved by NASA to operate on the International Space Station Alpha (ISSA). AMS is manifested for a precursor flight with STS91, (Discovery, may 1998, Figure 1), and for a three year long exposure on the International Space Station (ISS) (Figure 2), after its installa- tion during Utilization Flight n.4 (Discovery, jan- uary 2002). AMS has been proposed and is being built by an international collaboration involving China, Finland, France, Germany, Italy, Rume- nia, Russia, Switzerland, Taiwan and US.

Figure 3. Astroparticle physics 3

2. physics and antimatter since there is a long range force associated with baryonic charge, there is no compelling reason Particles and are connected for number to be conserved. Neverthe- through three discrete space-time symmetries less, until experimental data pro- which are the foundations of relativistic field the- vides confirmation of these ideas, the observed ory: charge conjugation (C), parity (P) and time lack of antimatter in our part of universe will be reversal (T). We know today that each of these the strongest evidence for viola- symmetries can be independently violated. In- tion. variance of fundamental interactions under the CP violation has been observed directly in three combined transformations is, however, al- three decays of the KL meson and nowhere else ways maintained (CPT theorem). The best tests in particle phyisics experiments: of the CPT theorem are indeed based on the + − oo comparison of lifetimes and of particle KL → π π ,KL→ππ,KL→πlν (1) and antiparticles, since the a CPT tranformation (l is either an or ). The absence links a particle to its . An asymme- of antimatter in our part of the universe (and try between matter and antimatter in our uni- perhaps in the entire universe), combined with verse would then be strictly related to a violation Sakharov conditions, provides the only other ev- of these discrete symmetries. In 1967, Sakharov idence for CP violation. noted that , which describes the evo- Since CP violation as the origin of baryogenesis occurs at scales much greater and at much greater level than in the in the kaon system, the study of baryon in the universe pro- Low Energy Particle Shield (Germany) vides crucial in attempts to probe beyond the of particle physics. Time of Flight (3) (Italy) Cosmological models that predict baryon symme- try on the scale of the may in- Anti-counters(4) (Germany,China) volve different sorts of CP violating mechanisms than those models which exclude antimatter al- together. Then, the determination by AMS of whether the universe does or does not contain do- NdFeB Magnet (1) (China,USA) mains of antimatter beyond our local supercluster Electronics (Germany, Silicon Tracker (2) of galaxies is of great importance to understand (Italy,FinlandSwitzerland) Italy,Switzerland, Aerogel (8) USA) the fundamental laws of particle physics. (France,Italy,Taiwan,USA) A comment about the study of CP violation on the B-meson system. The B-factories currently being developed to study CP violation in the B- Figure 4. AMS configuration for the may 1998 meson system may provide pertain- Shuttle percursor flight (STS91) ing to . However current un- derstanding suggest that CP violation in the B- system is most likely unrelated to the CP vio- lation necessary to produce cosmological baryon lution from a symmetric universe to an asymmet- asymmetry. In particular, just as CP violation, ric one, requires four ingredients, three of which, present in the kaon system from a in the baryon number violation, C and CP violations Cabibbo-Kobayashi-Maskawa (CKM) matrix, is are fundamental properties of elementary parti- too small to produce the observed cosmic asym- cles [7]. Neither particle acceleration experiments metry, similar conclusions are likely to hold for nor decay experiments have yet provided the B-system. However it is possible that CP evidence for baryon number violation. However, violation beyond the phase of the CKM matrix 4 is operative in the B-system, and B-factories are likely to reveal whether or not this is the case. However, new sources of CP violation which have σ = 103 ps been suggested for the origin of the baryon asym- (t1+t2)/2 2 metry do not typically lead to significant new CP 10 violation in the B-system. AMS will then bring informations on the origin of CP violation which are complementary to those obtained at the ac- celerators (Figure 3). 10 events/50 ps

50 Nd-Fe-B AMS

)] 1 e

G O 40 6 45678

(10 (t1+t2)/2 (ns)

Max Nd-Fe-B 30 [(BxH) Figure 6. AMS Time of Flight detector resolution Rare-Earth 20 as measured in a test beam

10 Alnico The experiment configuration for the precur- Magnetic strength Ferrite son flight is shown in Figure 4. It consist of a 0 large acceptance magnetic spectrometer (about 1960 1970 1980 1990 2000 0.6 m2sr) made of a new type permanent Nd-Fe- B magnet (1), surrounding a six layer high preci- sion silicon tracker (2) and sandwiched between Figure 5. Progress on permanent magnets four planes of the Time of Flight scintillator sys- tem (ToF) (3). A scintillator anticounter system (4), located on the magnet inner wall and a state Cherenkov (8) detector, complete the exper- iment. 3. AMS experiment design principles The magnet is based on recent advancements in permanent magnetic material and technology Search of antimatter requires the capability to (Figure 5) which make it possible to use very high identify with the highest degree of confidence, grade Nd-Fe-B to construct a permanent magnet the type of particle traversing the experiment to- with BL2 =0.15 Tm2 weighting ≤ 2tons.A gether with the absolute value and the sign of traversing the spectrometer will its . This can be achieved through trigger the experiment through the ToF system. repeated measurements of the particle momen- The ToF will measure the particle velocity with a tum (Spectrometer), velocity (Time of Flight, resolution of ∼ 100 ps over a distance of ∼ 1.4 m Cerenkov detectors) and energy deposition (Ion- [8] (Figure 6) . ization detectors). The momentum resolution of the Silicon Spec- 5

4. AMS physics potential 100 The physics objectives of AMS are: C-12 He-4 • searchforAntimatter(He¯ and C¯)inspace 4 5 P with a sensitivity of 10 to 10 better than current limits.

K He-3 The breakdown of the Time-Reversal symmetry in the early universe might have set different sign 10 π for the production of matter and antimatter in different regions of space. Since there are O(108)

Momentum resolution dP/P (%) e super clusters of galaxies and the observational constraints are limited to the scale of the local 0.1 1 10 100 1000 supercluster, the universe can be symmetric on Momentum (GeV/c/) a larger scale. The observed matter-antimatter asymmetry would then be a local phenomenon. Figure 8a,b shows the sensitivity of AMS, after Figure 7. Momentum resolution of the AMS three years on ISSA, in detecting He¯ and C¯ com- Silicon Tracker pared to the current limits. In Figure 8a we have included the prediction of He¯ yield as predicted by a model assuming a matter-antimatter sym- metric universe at the level of super-cluster of galaxies [10]. As seen from the figure, the cur- trometer is given in Figure 7 [9]: at low rigidi- rent limits are not sensitive enough to test the ties, below 8 GV, its resolution is dominated by existence of superclusters of galaxies of antimat- multiple scattering ( ∆p 7%) while the maxi- p ∼ ter and AMS is 103 more sensitive than this mum detectable rigidity is about 500 GV .The prediction based on matter-antimatter symmetric parameters of the Silicon Spectrometer are given universe. in Table 1. Both the ToF scintillators and the dE silicon layers measure dx , allowing a multiple de- • search for dark matter by high statistics termination of the absolute value of the particle precision measurements ofp ¯, e+ and γ spec- charge. tra.

These measurements will allow direct searches for the various and decay products of Table 1 WIMP’s in the galactic halo: AMS silicon tracker parameters χ¯ + χ → p¯ + X, e+ + X, 2γ (2) Number of planes 6 Accuracy (bending plane) 10 µm χ, χ¯ → γν (3) Accuracy (non bending plane) 30 µm Number of channels 172000 Figure 9 shows the simulation of the 100 hour Power consumption 400 W measurement of thep ¯ flux on the precursor flight Weight 130 kg compared to a compilation of existingp ¯ data to- Silicon Area (double sided) 6 m2 gether with a prediction of the effects of an heavy [11]. Similarly Figure 10 shows the simulation of three years of AMS e+ data compared to the ex- isting measurements. The accuracy of the AMS 6

1x10 -4 1x10 -3 Current Limits Current limits Z > 9 -4 1x10 -5 1x10 Z > 2

1x10 -5 1x10 -6 1x10 -6 1x10 -7 -7 1x10 Z > 2 1x10 -8 This experiment 1x10 -8 Z > 2 Antihelium/helium -9 This experiment 1x10 -9 Antinucleus/Nucleus Ratio 1x10

1x10-10 1x10 -10 0.1 1 10 100 0.1 1 10 100 Energy (GeV/amu) (GeV/amu)

Figure 8. Sensitivity of AMS (3 years on ISSA) in a search for (a) He¯ ;(b)Z>2antinuclei (95% C.L.). He¯ sensitivity is compared to a prediction assuming a matter-antimatter symmetric universe at the level of super-cluster of galaxies

e+ 7 measurement of the e− spectrum would allow to ple AMS will measure in three months 5 10 deu- test the existence of an heavy neutralino (mχ ∼ terium events reducing the current uncertainty on 100 GeV ) [12]. Also the measurement of the γ D/p by a factor of 100. Similarly, AMS will be spectrum will test the existence of in able to separe 3He from 4He up to 5 GV of rigid- case of R-parity violating SUSY models, through ity, thus collecting in three years ∼ 41083He and 94 its decay in two particles with Eγ,ν = mχ/2 [13– ∼ 410 He, reducing the existing uncertainties B 15]. by a factor 200. The ratio C , which constrains the parameters regulating the outflowing galactic • astrophysical studies by high statistics pre- wind, can be measured by AMS better than the cision measurements of D, 3He,B,C, 9Be current data within one day and up to 100 GV and 10Be spectra. 10Be of rigidity. On the other side, the ratio 9Be de- termines the CR confinement time; at present, a Precision measurement of isotopes and elemen- dozen of 10Be events have been detected during tal abundances in Cosmic Rays (CR) give very more than forteen years of observations and the important informations about CR origin, their CR galactic confinement time is currently known galactic confinement time and their propagation within a large uncertainty (∼ 40%). With AMS inside and between galaxies. This is deeply re- it will be possible to detect tens of 10Be/day in lated to the search for antinuclei, since antimatter the 1 GV range, thus dramatically reducing the particles, to be detected, should excape a region error on CR confinement time. of the universe dominated by antimatter, prop- In addition, the capability of AMS of detecting agate through the intergalactic void separating high energy γ rays would make it possible to per- superclusters and enter our galaxy. As an exam- 7

Table 2 Physics capabilities of AMS after three years on ISSA Elements Yield/sensitivity (Now) Range (GV) Physics e+ 108 (∼ 103)0.1−100 Dark Matter p¯ 500000 (∼ 30) 0.5 − 100 SUSY γ 0.1 − 300 R-parity ¯ 1 1 He/He 109 ( 105 )0.5−20 Antimatter ¯ 1 1 C/C 108 ( 104 )0.5−20 CP vs GUT , EW 9 D, H2 10 1.0 − 3.0Astrophysics 3He/ 4He 109 1.0 − 3.0 CR propagation 10Be/ 9Be 2% 1.0 − 3.0 CR confinement

-3 0.2 10 Ð p+p → p + x IMAX(5)

p / 0.1 Ð -4 0.09 10 MASS2(9) IMAX(8) 0.08 IMAX(3) 0.07 (E.Diehl P.R. D52, 4223, 1995) 0.06 ÐÐÐ Ð Mχ = 275 GeV BESS(4)

-5 /e 0.05

10 + e 0.04

-6 M χ : 60GeV 0.03 10 G.Jungman & M.Kamionkowski Phys.Rev D 49, 2316(1994) CAPRICE (498) 0.02 HEAT (293)

WIZARD (52) -7 AMS on Shuttle 10 0.01 0.1 1 10 100 Kinetic Energy (GeV) 1 10 100 Energy (GeV)

Figure 9. Simulated 100 hour shuttle flight p¯ mea- surement by AMS, compared with a compilation Figure 10. Simulated 3 years AMS e+ measure- of existing p¯ data ment compared with a compilation of existing e+ data

form very important observations in gamma ray Before the advent of new generation gamma after the turn off of the EGRET ex- ray space facilities like GLAST [18], AMS will periment (within one or two years). Simulations then be the only space born experiment to con- suggest indeed that the performances of AMS in tinue the EGRET mission at the beginning of monitoring the γ-sky in the multi GeV range are the next century, covering the interesting region comparable to these of EGRET (however AMS Eγ =30−300GeV . These data will extend our on the Space Station will not be able to point knowledge in key areas of γ astrophysics like: Ac- to targets of opportunity) [16,17]. A comparison tive Galactic Nuclei (AGN), blazars, γ-bursters. between the two experiments is shown in Table 3. They will also allow a measurement of the ex- 8

Table 3 Comparison of EGRET and AMS γ detection capabilities Parameter EGRET AMS Peak effective area (cm2sr) 1500 900 o Eγ −0.534 o Eγ −0.874 Angular resolution (68%) 1.7 ( 1 GeV ) 1.27 ( 1 GeV ) Mean opening angle ∼ 25o ∼ 30o Total viewing time ∼ 2.5 yr ∼ 3 yr Source flux sensitivity (> 1GeV ) ∼ 10−8cm−2s−1 ∼ 10−8cm−2s−1 Detector energy range (GeV )0.02 to ∼ 20 0.3 to ∼ 200 tragalactic γ background and of the interaction 8. Palmonari, F. : 1996, private communication. of high energy γ rays with the black-body mi- 9. Battiston, R. : 1995, Nucl. Instr. and Meth. crowave background. (Proc. Suppl.) B44, p. 274. The physics capabilities of AMS after three 10. Ahlen, S. et al.: 1982 ,Ap. J. 260, p. 20. years of exposure on the ISSA are summarized 11. Jungman, G., Kamionkowski, M. : 1994, P.R. in Table 2. D49, p. 2316. 12. Diehl, E.: 1995, P.R. D52, p. 4223. 5. Conclusion 13. Bergstrom, L.: 1989, P.L. B225, p. 372. 14. Berezinsky, V., Masiero, A., Valle, J.W.F. : During the past forty years, there have been 1991, P.L. B266, p. 382. many fondamental discoveries in astrophysics 15. Stecker, F.W.: 2nd Int. Symp. on ”Sources measuring UV, X-ray and γ-ray . There and Detection of Dark. Matter in the Uni- has never been a sensitive magnetic spectrometer verse”, S. Monica, CA, February 1996, in space, due to the extreme difficulty and very Nucl.Phys. B. Proc. Suppl. 51B, 299. high cost of putting a superconducting magnet in 16. Fiandrini, E.: Proc. 6th Topical Seminar orbit. AMS will be the first large acceptance mag- on ”Experimental Apparatus for Particle netic detector in space. It will allow a measure- Physics and Astrophysics”, San Miniato al ments of the flux of all kind of cosmic rays with Todesco, Italy, 20-24 May 1996, Nucl.Phys. an accuracy orders of magnitude better than be- B. Proc. Suppl. in press. fore. The large improvement in sensitivity given 17. Battiston, R., Salamon, M., Fiandrini, E., to by this new instrument, will allow us to enter into be published. a totally new domain to explore the unknown. 18. E.D. Bloom, these Proceedings. REFERENCES 1. Steigmann, G.: 1976, Ann. Rev. Astron. As- troph., 14 p. 339. 2. Kolb, E.W., Turner, M.S.: 1983, Ann. Rev. Nucl. Part. Sci. 33 p. 645. 3. Peebles, P.J.E.:1993, Principles of Physi- cal , Princeton University Press, Princeton N.J. 4. Ellis, J. et al.: 1988, Phys. Lett. B214,p. 403. 5. Turner, M.S., Wilzek, F.: 1990, Phys. Rev. D42, p. 1001. 6. S.P. Ahlen et al.: 1994, N.I.M. A350 351. 7. Sakharov, A.D.: 1967, JETP Lett 5,p.24.