The Alpha Magnetic Spectrometer (AMS) was flown in June 1998

on the space shuttle Discovery during flight STS-91 in a 51.7◦ orbit at altitudes between 320 and 390 km. The major detector elements were a permanent magnet with an analyzing power B∗L2 of 0.14 Tm2, a six-layer, double-sided silicon tracker, time-of-flight hodoscopes, a ˇCerenkov counter and anti-coincidence counters. A total of 2.86 × 106 He nuclei were observed in the rigidity range 1 to 140 GeV. No He

nuclei were detected at any rigidity. The upper limit on the flux ratio of He to He is 1.1×10−6. The proton spectrum in the kinetic energy range 0.1 to 200 GeV was measured, and is parameterized by a power law above the geomagnetic cutoff. Below the geomagnetic cutoff, a substantial second spectrum was observed concentrated at equatorial latitudes with a flux of ∼70m−2s−1sr−1. The lepton spectra in the

kinetic energy ranges 0.2 to 40 GeV for electrons and 0.2 to 3 GeV for positrons were measured. Two distinct spectra were observed, a higher energy spectrum and a substantial second spectrum with positrons much more abundant than electrons. Tracing leptons from the second spectra shows that most of these leptons travel for an extended period of time in the geomagnetic field and that the positrons and electrons originate from two complementary geographic regions. Long-lived secondary spectra protons (antiprotons) originate from the same regions as positrons (electrons)

Steuer, M.

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IL NUOVO CIMENTO Vol. 24 C, N. 4-5 Luglio-Ottobre 2001Results of the AMS experiment(∗)M. SteuerMassachusetts Institute of Technology - Cambridge, Massachusetts 02139, USAand the AMS Collaboration(ricevuto l’1 Dicembre 2000; approvato il 12 Febbraio 2001)Summary. — The Alpha Magnetic Spectrometer (AMS) was flown in June 1998on the space shuttle Discovery during flight STS-91 in a 51.7◦ orbit at altitudesbetween 320 and 390 km. The major detector elements were a permanent magnetwith an analyzing power B∗L2 of 0.14 Tm2, a six-layer, double-sided silicon tracker,time-of-flight hodoscopes, a Cˇerenkov counter and anti-coincidence counters. A totalof 2.86 × 106 He nuclei were observed in the rigidity range 1 to 140 GeV. No Henuclei were detected at any rigidity. The upper limit on the flux ratio of He to Heis 1.1× 10−6. The proton spectrum in the kinetic energy range 0.1 to 200 GeV wasmeasured, and is parameterized by a power law above the geomagnetic cutoff. Belowthe geomagnetic cutoff, a substantial second spectrum was observed concentratedat equatorial latitudes with a flux of ∼70m−2s−1sr−1. The lepton spectra in thekinetic energy ranges 0.2 to 40 GeV for electrons and 0.2 to 3 GeV for positronswere measured. Two distinct spectra were observed, a higher energy spectrum anda substantial second spectrum with positrons much more abundant than electrons.Tracing leptons from the second spectra shows that most of these leptons travelfor an extended period of time in the geomagnetic field and that the positronsand electrons originate from two complementary geographic regions. Long-livedsecondary spectra protons (antiprotons) originate from the same regions as positrons(electrons).PACS 96.40 – Cosmic rays.PACS 01.30.Cc – Conference proceedings.1. – IntroductionAMS is an international collaboration spanning 3 Continents, 13 Countries, 33 Insti-tutions. It is based on an agreement between NASA and DOE. It is a CERN Recognized(∗) Paper presented at the Chacaltaya Meeting on Cosmic Ray Physics, La Paz, Bolivia,July 23-27, 2000.c© Societa` Italiana di Fisica 661662 M. STEUER^z^y^xPermanent MagnetNd-Fe-B: 46 MGOe1mCerenkovAerogelTrackerPlanesT1T2T3T4T5T6S3S4S2S1TOF LayersParticle TrajectoryLow Energy Particle ShieldsACCB=0.15TFig. 1. – Schematic view of AMS as flown on STS-91.Experiment. First results are published in [1-3], which can be consulted for details. Theapparatus, as flown on STS-91, is described in detail in [4] (see fig. 1).The purpose of the AMS experiment is to study the antimatter composition of the pri-mary cosmic rays. The existence (or absence) of antimatter nuclei in space is closely con-nected to the theories of elementary particle physics, like CP -violation, baryon numbernonconservation, Grand Unified Theory (GUT), etc. Balloon-based cosmic ray searchesfor antinuclei at altitudes up to 40 km have been carried out for more than 20 years; allsuch searches have been negative.The Alpha Magnetic Spectrometer (AMS) is scheduled for a particle physics programon the International Space Station. Using a high-precision, large acceptance magneticspectrometer, searches for dark matter, the origin of cosmic rays, and for antinuclei areforeseen.The AMS flight on the space shuttle Discovery (STS-91 in June 1998, see fig. 2)was primarily a test flight to verify the detector’s performance under actual space flightconditions.2. – Data takingAfter the shuttle had attained orbit, data collection commenced on 3 June 1998 andcontinued over the next nine days for a total of 184 hours, resulting in 100 million triggersrecorded, half during the MIR docking period. Around 12% of physics data were sentvia slow links to various NASA/USAF ground stations and were delivered in almost realtime.During data taking the shuttle altitude varied from 320 to 390 km and the latituderanged between ± 51.7 ◦. Before the rendezvous with the MIR space station the attitudeof the shuttle was maintained as to keep the z-axis of AMS pointed within 45 ◦ of theRESULTS OF THE AMS EXPERIMENT 663Fig. 2. – AMS (near the tail) as seen from MIR during STS-91 docking on June 4, 1998.zenith. While docked, the attitude was constrained by MIR requirements and variedsubstantially. After undocking the pointing was maintained with a precision of 1◦ at 0◦,20◦ and then 40 ◦ of the zenith. Shortly before descent the shuttle turned over and thepointing was towards the nadir.Events were triggered by the coincidence of signals in all four TOF planes consistentwith the passage of a charged particle through the active tracker volume. Triggers witha coincident signal from the anti-counters (ACC) were vetoed.The detector performance as well as temperature and magnetic field were monitoredcontinuously.3. – Event reconstruction– The sign of the particle charge Z was derived from the deflection in the rigidity fitand the particle direction.– The particle mass was derived from |Z|R and β.– The particle rigidity, R = pc/|Z|e (GV), was obtained from the measurement of thedeflection of the trajectory measured by the tracker in the magnetic field. Hits inat least four tracker planes were required and the fitting was performed with twodifferent algorithms, the results of which were required to agree.– The particle velocity, β, and direction, zˆ = ±1, was obtained from the TOF, wherezˆ = −1 signifies a downward-going particle in the AMS coordinate system.– The magnitude of the particle charge Z was obtained from the measurements ofenergy losses in the TOF counters and tracker planes (corrected for β).4. – Search for antihelium in cosmic raysThe results of our search are summarized in fig. 3. As seen, we obtain a total of2.86× 106 He events up to a rigidity of 140 GV. We found no He event at any rigidity.664 M. STEUEREvents110102103104105-150 -100 -50 0 50 100 150HeHe2.86×106 events0 eventsSign ×Rigidity GVAMSSTS - 91Fig. 3. – Measured rigidity times the charge sign for selected |Z| = 2 events.Since no He nuclei were observed, we can only establish an upper limit on their flux.If the incident He spectrum is assumed to have the same shape as the He spectrum overthe range 1 < R < 140GV, then one obtains at the 95% C.L. a limit ofNHeNHe< 1.1× 10−6.5. – Protons in near earth orbitData were collected in the cosmic ray proton spectrum region from kinetic energiesof 0.1 to 200 GeV, taking advantage of the large acceptance, the accurate momentumresolution, the precise trajectory reconstruction and the good particle identification ca-pabilities of AMS.The high statistics (∼ 107) available allow to determine the variation of the spectrumwith position both above and below the geomagnetic cutoff. Because the incident particledirection and momentum were accurately measured in AMS, it is possible to investigatethe origin of protons below cutoff by tracking them in the earth’s magnetic field.The spectrum above cutoff is referred to as the “primary” spectrum and below cutoffas the “second” spectrum.5.1. Properties of the primary spectrum. – The primary proton spectrum can beparameterized by a power law in rigidity, Φ0×R−γ . Fitting the measured spectrum overthe rigidity range 10 < R < 200 GV, i.e. well above cutoff, yieldsγ = 2.79± 0.012 (fit)± 0.019 (sys),Φ0 = 16.9± 0.2 (fit)± 1.3 (sys)± 1.5 (γ) GV2.79m2s srMV.RESULTS OF THE AMS EXPERIMENT 665 Downward Upward0.9 < θM <1.010-1 1 10 102 Downward0 < θM < 0.3 UpwardFlux (m2  sec sr MeV)-110-510-410-310-210-1110-1 1 10 102 Downward Upward0.6 < θM< 0.7Kinetic Energy (GeV)10-1 1 10 102a) b) c)Fig. 4. – Comparison of upward and downward second proton spectrum at different geomagneticlatitudes. As seen, below cutoff, the upward and downward fluxes agree in the range 0 ≤ ΘM <0.8.The systematic uncertainty in γ was estimated from the uncertainty in the acceptance(0.006), the dependence of the resolution function on the particle direction and tracklength within one sigma (0.015), variation of the tracker bending coordinate resolution by±4microns (0.005) and variation of the selection criteria (0.010). The third uncertaintyquoted for Φ0 reflects the systematic uncertainty in γ.5.2. Properties of the second spectrum. – A substantial second spectrum of downward-going protons is observed for all but the highest geomagnetic latitudes.The upward- and downward-going protons of the second spectrum have the followingunique properties:i) At geomagnetic equatorial latitudes, ΘM < 0.2, this spectrum extends from thelowest measured energy, 0.1 GeV, to ∼6 GeV with a flux ∼70m−2s−1sr−1.ii) The second spectrum has a distinct structure near the geomagnetic equator: achange in geomagnetic latitude from 0 to 0.3 causes the proton flux to drop by afactor of 2 to 3 depending on the energy.iii) Over the much wider interval 0.3 < ΘM < 0.8, the flux is nearly constant.iv) In the range 0 ≤ ΘM < 0.8, detailed comparison in different latitude bands (fig. 4)indicates that the upward and downward fluxes are nearly identical, agreeing within1 %.To understand the origin of the second spectrum, we traced back 105 protons from theirmeasured incident angle, location and momentum, through the geomagnetic field for 10 sflight time or until they impinged on the top of the atmosphere at an altitude of 40 km,which was taken to be the point of origin. All second spectrum protons were found tooriginate in the atmosphere, except for few percent of the total detected near the SouthAtlantic Anomaly (SAA).The trajectory tracing shows that about 30% of the detected protons flew for less than0.3 s before detection. The origin of these “short-lived” protons is distributed uniformlyaround the globe.666 M. STEUEROrigin Latitude (degrees)-80-60-40-20020406080-150 -100 -50 0 50 100 150Origin Longitude (degrees)Long Lived p Detected at ΘM < 0.3 radFig. 5. – Origin of long-lived protons (ΘM < 0.3, p < 3 GeV/c) in geomagnetic coordinates.In contrast, the remaining 70% of protons with flight times greater than 0.3 s, classi-fied as “long-lived”, originate from a geographically restricted zone. Figure 5 shows thestrongly localized distribution of the point of origin of these long-lived protons in geo-magnetic coordinates. Though data is presented only for protons detected at ΘM < 0.3,these general features hold true up to ΘM ∼ 0.7.6. – Leptons in near earth orbitData were collected to study the spectra of electrons and positrons in cosmic raysover the respective kinetic energy ranges of 0.2 to 40 GeV and 0.2 to 3 GeV, the latterrange being limited by the proton background. The large acceptance of AMS and highstatistics (∼ 105) enable us to study the variation of the spectra with position and angleboth above and below the geomagnetic cutoff. The origin of particles below cutoff canbe obtained by tracking them in the geomagnetic field.For this study the acceptance was restricted to events with an incident angle within25◦ of the positive z-axis of AMS and data from four periods are included. In the firstperiod the z-axis was pointing within 1◦ of the zenith. Events from this period arereferred to as “downward” going. In the second period the z-axis pointing was within1◦ of the nadir. Data from this period are referred to as “upward” going. The effect ofthe geomagnetic cutoff and the decrease in this cutoff with increasing ΘM is particularlyvisible in the downward electron spectra. The spectra above and below cutoff differ. Tounderstand this difference the trajectory of electrons and positrons were traced back fromtheir measured incident angle, location and momentum, through the geomagnetic field.This was continued until the trajectory was traced to outside the Earth’s magnetosphereor until it crossed the top of the atmosphere at an altitude of 40 km. The spectra fromparticles which were traced to originate far away from Earth are classified as “primary”and those from particles which originate in the atmosphere as “second” spectra. Inpractice, particles below the geomagnetic cutoff are from the second spectra, howeverthis classification provides a cleaner separation in the transition region.Similar to the procedure for second spectra protons, leptons with flight times < 0.2 sare defined as “short-lived”, the remaining as “long-lived”. For ΘM < 0.3, most (75% ofe+, 65% of e−) leptons are long-lived.RESULTS OF THE AMS EXPERIMENT 667-80-60-40-20020406080-80-60-40-20020406080-150 -100 -50 0 50 100 150a) e-b) e+LongitudeLatitudeB AABFig. 6. – The geographical origin of long-lived second spectra leptons (< 3 GeV).6.1. Origin of long-lived leptons. – Figure 6 shows for long-lived second spectra leptons(< 3 GeV) the geographical origin of (a) electrons and (b) positrons. The lines indicatethe geomagnetic field contours at 380 km. One can see the strongly localized distributionsof the point of origin for long-lived leptons. Tracking shows that regions of origin forpositrons coincide with regions of sink for electrons and vice versa. b) a) Short Lived  Long LivedEk (GeV)e+/e-110654321Fig. 7. – The e+/e− ratio of second spectra leptons (< 3 GeV, ΘM < 0.3) as a function of energy.668 M. STEUERLong-lived second spectra positrons have the same points of origin as long-lived secondspectra protons.6.2. Lepton charge ratio. – An interesting feature of the observed second lepton spectrais the predominance of positrons over electrons. The energy dependence of the e+/e−ratio for 0◦ attitude and ΘM < 0.3 is shown in fig. 7. As seen, short-lived and long-lived leptons behave differently. For short-lived leptons the ratio does not depend on theparticle energy in the range 0.2 to 3 GeV but for long-lived leptons the ratio does dependon the lepton energy, reaching a maximum value of ∼ 5.7. – Conclusion– The short test flight of the AMS detector has shown its viability for an extendedperiod of several years of data taking at the International Space Station (ISS).– The detector is undergoing several upgrades, the most prominent one being a su-perconducting magnet.– It will be ready for its next flight and installation on the ISS, foreseen for October2003.∗ ∗ ∗Many thanks to my colleagues in the AMS data acquisition and analysis groups.REFERENCES[1] The AMS Collaboration (J. Alcaraz et al.), Phys. Lett. B, 461 (1999) 387.[2] The AMS Collaboration (J. Alcaraz et al.), Phys. Lett. B, 472 (2000) 215.[3] The AMS Collaboration (J. Alcaraz et al.), Phys. Lett. B, 484 (2000) 10.[4] Viertel G. M. and Capell M., Nucl. Instrum. Methods A, 419 (1998) 295.

Results of the AMS experiment

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Results of the AMS experiment

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