New results from the Alpha Magnetic Spectrometer on the International Space Station
The new results on energetic cosmic ray electrons and positrons are announced today. They are based on the first 41 billion events measured with the Alpha Magnetic Spectrometer (AMS) on the International Space Station (ISS). These results provide a deeper understanding of the nature of high energy cosmic rays and shed more light on the dark matter existence.
AMS has analyzed 41 billion primary cosmic ray events. Of these, 10 million have been identified as electrons and positrons. AMS has measured the positron fraction (ratio of the number of positrons to the combined number of positrons and electrons) in the energy range 0.5 to 500 GeV. We have observed that the energy at which the fraction starts to quickly increase is 8 GeV (see Figure 1) indicating the existence of a new source of positrons. Figure 2 shows that the exact rate at which the positron fraction increases with energy has now been accurately determined and the fraction shows no observable sharp structures. The energy at which the positron fraction ceases to increase (corresponding to the turning point energy at which the positron fraction reaches its maximum) has been measured to be 275+32 GeV as shown in Figure 2 (upper plot). This is the first experimental observation of the positron fraction maximum after half a century of cosmic rays experiments. The excess of the positron fraction is isotropic within 3% strongly suggesting the energetic positrons may not be coming from a preferred direction in space.
Precise measurement of the positron fraction is important for understanding of the origin of dark matter. Dark matter collisions will produce an excess of positrons and this excess can be most easily studied by measuring the positron fraction. Ordinary cosmic ray collisions result in the positron fraction decreasing steadily with energy. Different models on the nature of dark matter predict different behavior of the positron fraction excess above the positron fraction expected from ordinary cosmic ray collisions. Depending on the nature of dark matter, the excess of the positron fraction has a unique signature. The characteristic features are highlighted in the following illustration:
The new results from AMS (published today in Physical Review Letters) show that items (1)-(4) have been unambiguously resolved and are observations of a new phenomena. They are consistent with a dark matter particle (neutralino) of mass on the order of 1 TeV. To determine if the observed new phenomena is from dark matter or from astrophysical sources such as pulsars, measurements are underway by AMS to determine the rate of decrease at which the positron fraction falls beyond the turning point, (item 5), as well as the measurement of the anti-proton fraction (anti-proton to proton plus anti-proton ratio). These will be reported in future publications.
Secondly, AMS reports the precise measurements of the electron flux and the positron flux,i.e. intensities of cosmic ray electrons and positrons. These measurements show that the behavior of electrons and positrons are significantly different from each other both in their magnitude and energy dependence. Figure 3 (upper plot) shows the electron and positron fluxes multiplied by the energy cubed (E3, for the purpose of presentation). The positron flux first increases (0.5 to 10 GeV), then levels out (10 to 30 GeV), and then increases again (30 to 200 GeV). Above 200 GeV, it has a tendency to decrease. This is totally different from the scaled electron flux.
The behavior of the flux as a function of energy is described by the spectral index and the flux was expected to be proportional to energy E to the power of the spectral index. The result shows that neither flux can be described with a constant spectral index, see Figure 3 (lower plot). In particular, between 20 and 200 GeV, the rate of change of the positron flux is surprisingly higher than the rate for electrons. This is important proof that the excess seen in the positron fraction is due to a relative excess of high energy positrons, as expected from dark matter collisions, and not the loss of high energy electrons. These results are published today in Physical Review Letters in a separate article.
This new observation of the electron and positron fluxes also demonstrates, as pointed out by Dr. Michael S. Turner, that there is a fundamental difference between matter (electrons) and antimatter (positrons).
In 1932, Carl Anderson discovered the positron in cosmic rays. Non-magnetic detectors in space and on the ground can measure the flux of the sum of electrons plus positrons. Over the last 50 years, there have been many experiments that measured the combined flux of electrons plus positrons in cosmic rays. These measurements have yielded interesting results and few of them indicated the possible existence of a structure at 300-800 GeV.
AMS, being a particle physics detector, provides many independent measurements of electrons, positrons, and electrons plus positrons. After collecting 41 billon cosmic ray events, AMS has been able to provide a measurement of the flux of electrons plus positrons, shown in Figure 4 (upper plot).The combined flux is smooth and reveals new and distinct information. Most interesting is the observation that, at high energies and over a wide energy range, the combined flux can be described by a single, constant spectral index (see Figure 4, lower plot).
The precision measurements of the positron fraction, the individual fluxes and the combined flux are complementary to one to another. Together they will provide a deeper understanding of the origin of high energy cosmic rays and shed more light on the existence of dark matter.
Figure 1. The positron fraction measured by AMS (red circles) compared with the expectation from the collision of ordinary cosmic rays showing that above 8 billion electron volts (8 GeV) the positron fraction begins to quickly increase. This increase indicates the existence new sources of positrons.
Figure 2. Upper plot shows the slope of positron fraction measured by AMS (red circles) and a straight line fit at the highest energies (blue line). The data show that at 275±32 GeV the slope crosses zero. Lower plot shows the measured positron fraction as function of energy as well as the location of the maximum. No sharp structures are observed.
Figure 3. The upper plot highlights the difference between the electron flux (blue dots, left scale) and the positron flux (red dots, right scale). The lower plot shows the spectral indices of the electron flux and of the positron flux as functions of energy.
Figure 4. (Upper plot) The combined flux of electrons plus positrons measured by AMS multiplied by E3 together with the results from earlier experiments [1-7]. (Lower plot) The combined flux of e± multiplied by E3 versus energy and the result of a single power law fit.
 S. Torii et al., Astrophys. J. 559, 973 (2001);  M. A. DuVernois et al., Astrophys. J. 559, 296 (2001);  J. Chang et al., Nature (London) 456, 362 (2008);  K. Yoshida et al., Adv. in Space Res. 42, 1670 (2008);  F. Aharonian et al., Phys. Rev. Lett. 101, 261104 (2008);  F. Aharonian et al., Astron. Astrophys. 508, 561 (2009);  M. Ackermann et al., Phys. Rev. D 82, 092004 (2010).
Background of AMS
AMS was assembled and tested at the European Organization for Nuclear Research, CERN, Geneva, Switzerland. Detector components were constructed at universities and research institutes around the world. Fifteen countries from Europe, Asia, and America participated in the construction of AMS (Finland, France, Germany, Netherlands, Italy, Portugal, Spain, Switzerland, Turkey, China, Korea, Taiwan, Russia, Mexico and the United States). The Principal Investigator of AMS is Prof. Samuel Ting of MIT and CERN. AMS is a U.S. Department of Energy sponsored particle physics experiment on the ISS under a DOE-NASA Implementing Arrangement. The Collaboration works closely with the NASA AMS Project Management team from Johnson Space Center as it has throughout the entire process. AMS was launched by NASA to the ISS as the primary payload onboard the final mission of space shuttle Endeavour (STS-134) on May 16, 2011. Once installed on the ISS, AMS was powered up and immediately began collecting data from primary sources in space and these were transmitted to the AMS Payload Operations Control Center (POCC). The POCC is located at CERN, Geneva, Switzerland.
After 40 months of operations in space, AMS has collected 54 billion cosmic ray events. To date 41 billion have been analyzed. The data is analyzed at the AMS Science Operations Center (SOC) located at CERN as well as AMS universities around the world. Over the lifetime of the Space Station, AMS is expected to measure hundreds of billions of primary cosmic rays. Among the physics objectives of AMS is the search for antimatter, dark matter, and the origin of cosmic rays. The Collaboration will also conduct precision measurements on topics such as the boron to carbon ratio, nuclei and antimatter nuclei, and antiprotons, precision measurements of helium flux, proton flux and photons as well as the search for new physics and astrophysics phenomena such as strangelets.
It is important to note that, in the search for an understanding of dark matter, there are three distinct approaches:
Production experiments, such as those being carried at the LHC with the ATLAS and CMS experiments, use particle collisions to produce dark matter particles and detect their decay products. This is similar to experiments at the Brookhaven, Fermilab, CERN-SPS and CERN-LHC which led to the discovery of CP violation, the J particle, Z and W bosons, the b and t quarks, and the Higgs boson.
Scattering experiments utilize the fact that dark matter can penetrate deep underground and that it can be detected by recoil nuclei from the scattering of dark matter with pure liquid or solid targets. This is similar to electron-proton scattering experiments performed at SLAC leading to the discovery of partons and the electro-weak effects.
Annihilation experiments for dark matter are done in space and are based on the fact that dark matter collisions can produce excesses of positrons and anti-protons. These are the main goals of AMS. On the ground, annihilation experiments are done in electron-positron colliders (SPEAR, PETRA, LEP, BaBar, TRISTAN) leading to the discovery of the psi particle, the heavy electron (tau) and gluons, precision measurements of CP violation effects and the properties of Z and W bosons.
The scattering experiments, the production experiments, and the annihilation experiments each produce unique physics discoveries. The absence of a dark matter signal from one of these three ways does not exclude its discovery by the other two.
The U.S. participation in AMS involves MIT, Yale (Professor Jack Sandweiss), the University of Hawaii (Professors Veronica Bindi and Philip von Doetinchem), the University of Maryland (Professor Roald Sagdeev and Professor Eun Suk SEO) and NASA’s Johnson Space Center (Mr. Trent Martin and Mr. Ken Bollweg). The AMS project is coordinated by the Laboratory for Nuclear Science at MIT under the leadership of Professor Richard Milner. The major responsibility for space operations and data analysis is carried by Drs. U. J. Becker, J. Burger, X.D. Cai, M. Capell, V. Choutko, F.J. Eppling, P. Fisher, A. Kounine,V. Koutsenko, A. Lebedev, Z.Weng, and P. Zuccon of MIT.
Germany made a major contribution to the detector construction and data analysis under the leadership of Professors Dr. Stefan Schael, Henning Gast, and Iris Gebauer. Germany’s participation is supported by DLR and RWTH Aachen.
Italy made a major contribution to the detector construction and presently to the data analysis, under the leadership of Professors Roberto Battiston, Deputy PI and currently President of ASI, Bruna Bertucci, Italian Coordinator, Franco Cervelli, Andrea Contin, Giovanni Ambrosi, Marco Incagli, Giuliano Laurenti, Federico Palmonari, and Pier-Giorgio Rancoita. Italy’s participation is supported by ASI and INFN.
Spain made a major contribution to the detector construction and presently to the data analysis under the leadership of Manuel Aguilar, Javier Berdugo, Jorge Casaus, Carlos Delgado and Carlos Mana. Spain’s participation is supported by CIEMAT and CDTI.
France has made major contributions to the detector construction and to the data analysis both from LPSC, Grenoble and LAPP, Annecy under the leadership of Professors Laurent Derome, Sylvie Rosier-Lees, and Jean-Pierre Vialle. France’s participation is supported by IN2P3 and CNES.
Taiwan made a major contribution to the detector construction and presently to the data analysis, under the leadership of Academician Shih-Chang Lee and Profs. Y.H. Chang and S. Haino. Taiwan’s participation is supported by Academia Sinica, National Science Council and CSIST. Taiwan also maintains the AMS Asia POCC.
From China, Shandong University made a major contribution to the detector construction and to the data analysis under the leadership of Professor Cheng Lin. The Institute of High Energy Physics in Beijing has made major contributions to the detector construction and data analysis under the leadership of Academician Hesheng Chen. Southeast University in Nanjing has made major contributions to the detector construction and data analysis under the leadership of Professor Hong Yi and J. Z. Luo. Beihang University under the leadership of Academician Wei Li, Professor Zhi-Ming Zheng and Dr. Baosong Shan made important contributions to the data analysis. Sun Yat-Sen University in Guangzhou has made major contributions to the detector construction and data analysis under the leadership of Professor N.S. Xu. Shanghai Jiaotong University in Shanghai has made important contributions to the detector construction. The Institute of Electrical Engineering under Q. L. Wang and the Chinese Academy of Launch Vehicle Technology were responsible for the AMS permanent magnet.
Switzerland has made a major contribution to the detector construction and the data analysis, both from ETH/Zurich and the University of Geneva under the leadership of Professors Maurice Bourquin, Catherine Leluc, and Martin Pohl of the University of Geneva.
Collaborating Insititutes on the two Physical Review Letters:
I. Physics Institute and JARA-FAME, RWTH Aachen University, D-52056 Aachen, Germany
Department of Physics, Middle East Technical University, METU, 06800 Ankara, Turkey
Laboratoire d’Annecy-Le-Vieux de Physique des Particules, LAPP, IN2P3/CNRS and Universite de Savoie, F-74941 Annecy-le-Vieux, France
Beihang University, BUAA, Beijing, 100191, China
Institute of Electrical Engineering, IEE, Chinese Academy of Sciences, Beijing, 100080, China
Institute of High Energy Physics, IHEP, Chinese Academy of Sciences, Beijing, 100039, China
INFN-Sezione di Bologna, I-40126 Bologna, Italy
Universita di Bologna, I-40126 Bologna, Italy
Massachusetts Institute of Technology, MIT, Cambridge, Massachusetts 02139, USA
National Central University, NCU, Chung-Li, Tao Yuan 32054, Taiwan
East-West Center for Space Science, University of Maryland, College Park, Maryland 20742, USA
IPST, University of Maryland, College Park, Maryland 20742, USA
CHEP, Kyungpook National University, 702-701 Daegu, Korea
CNR-IROE, I-50125 Firenze, Italy
European Organization for Nuclear Research, CERN, CH-1211 Geneva 23, Switzerland
DPNC, Universite de Geneve, CH-1211 Geneve 4, Switzerland
Laboratoire de Physique subatomique et de cosmologie, LPSC, Universite Grenoble-Alpes, CNRS/IN2P3, F-38026 Grenoble, France
Sun Yat-Sen University, SYSU, Guangzhou, 510275, China
University of Hawaii, Physics and Astronomy Department, 2505 Correa Road, WAT 432; Honolulu, Hawaii 96822, USA
Julich Supercomputing Centre and JARA-FAME, Research Centre Julich, D-52425 Julich, Germany
NASA, National Aeronautics and Space Administration, Johnson Space Center, JSC, and Jacobs-Sverdrup, Houston, TX 77058, USA
Institut fur Experimentelle Kernphysik, Karlsruhe Institute of Technology, KIT, D-76128 Karlsruhe, Germany
Instituto de Astrofisica de Canarias, IAC, E-38205, La Laguna, Tenerife, Spain
Laboratorio de Instrumentacao e Fisica Experimental de Particulas, LIP, P-1000 Lisboa, Portugal
National Chung-Shan Institute of Science and Technology, NCSIST, Longtan, Tao Yuan 325, Taiwan
Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas, CIEMAT, E-28040 Madrid, Spain
Instituto de Fisica, Universidad Nacional Autonoma de Mexico, UNAM, Mexico, D. F., 01000 Mexico
INFN-Sezione di Milano and Universita di Milano, I-20090 Milano, Italy
INFN-Sezione di Milano-Bicocca, I-20126 Milano, Italy
Universita di Milano-Bicocca, I-20126 Milano, Italy
Laboratoire Univers et Particules de Montpellier, LUPM, IN2P3/CNRS and Universite de Montpellier II, F-34095 Montpellier, France
Southeast University, SEU, Nanjing, 210096, China
Physics Department, Yale University, New Haven, Connecticut 06520, USA
INFN-Sezione di Perugia, I-06100 Perugia, Italy
Universita di Perugia, I-06100 Perugia, Italy
INFN-Sezione di Pisa, I-56100 Pisa, Italy
Universita di Pisa, I-56100 Pisa, Italy
INFN-TIFPA and Universita di Trento, I-38123 Povo, Trento, Italy
INFN-Sezione di Roma 1, I-00185 Roma, Italy
Universita di Roma La Sapienza, I-00185 Roma, Italy
Department of Physics, Ewha Womans University, Seoul, 120-750, Korea
Shandong University, SDU, Jinan, Shandong, 250100, China
Shanghai Jiaotong University, SJTU, Shanghai, 200030, China
Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan
Space Research Laboratory, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland