Martin Pohl, Center for Astroparticle Physics, CAP Genève
Beginning of April 2013, the Alpha Magnetic Spectrometer (AMS) Collaboration published its first physics result in Physical Review Letters 1. The AMS experiment is a powerful and sensitive particle physics spectrometer. As seen in Figure 1, AMS is located on the exterior of the International Space Station (ISS). Since its installation on 19 May 2011 it has measured over 30 billion cosmic rays in the GeV to TeV energy range. Its permanent magnet and array of precision particle detectors collect and identify charged cosmic rays passing through. Over its long duration mission on the ISS, AMS will record signals from 16 billion cosmic rays every year and transmit them to Earth for analysis by the AMS Collaboration. This is the first of many physics results to be reported.
The first publication from the AMS Experiment is a major milestone for the AMS international collaboration. Hundreds of scientists, engineers, technicians and students from all over the world have worked together for over 18 years to make AMS a reality. The collaboration represents 16 countries from Europe, Asia and North America (Finland, France, Germany, Italy, the Netherlands, Portugal, Spain, Switzerland, Romania, Russia, Turkey, China, Korea, Taiwan, Mexico and the United States) under the leadership of Nobel Laureate Samuel Ting of M.I.T. The collaboration continues to work closely with the NASA AMS Project Management team from Johnson Space Center as it has throughout the entire process. Many countries have made important contributions to the AMS detector construction and presently to the data analysis. These include two groups from Switzerland, University of Geneva and ETHZ, supported by federal and cantonal authorities as well as the SNF.
AMS was constructed at universities and research institutes around the world and assembled at the European Organization for Nuclear Research, CERN, Geneva, Switzerland. It was launched by NASA to the ISS as the primary payload onboard the final mission of space shuttle Endeavour (STS-134) on 16 May 2011. Once installed on 19 May 2011, 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 located at CERN, Geneva, Switzerland.
Once AMS became operational, the first task for the AMS Collaboration was to ensure that all instruments and systems performed as designed and as tested on the ground. The AMS detector, with its multiple redundancies, has proven to perform flawlessly in space. Over the last 22 months in flight, AMS collaborators have gained invaluable operational experience in running a precision spectrometer in space and mitigating the hazardous conditions to which AMS is exposed as it orbits the Earth every 90 minutes. Conditions like this are not encountered by ground-based accelerator experiments or satellite-based experiments and require constant vigilance in order to avoid irreparable damage. They include the extreme thermal variations caused by solar effects and the re-positioning of ISS onboard radiators and solar arrays. In addition, the AMS operators regularly transmit software updates from the AMS POCC at CERN to the AMS computers in space in order to match the regular upgrades of the ISS software and hardware.
Positron fraction measurement
In the initial 18 months period of space operations, from 19 May 2011 to 10 December 2012, AMS analyzed 25 billion primary cosmic ray events. Of these, an unprecedented number, 6.8 million, were unambiguously identified as electrons and their antimatter counterpart, positrons. The 6.8 million particles observed in the energy range 0.5 to 350 GeV are the subject of the precision study reported in this first paper.
Electrons and positrons are identified by the accurate and redundant measurements provided by the various AMS instruments against a large background of protons. Positrons are clearly distinguished from this background through the robust rejection power of AMS of more than one in one million.
Currently, the total number of positrons identified by AMS, in excess of 400,000, is the largest number of energetic antimatter particles directly measured and analyzed from space. The first paper can be summarized as follows:
AMS has measured the positron fraction (ratio of the positron flux to the combined flux of positrons and electrons) in the energy range 0.5 to 350 GeV. We have observed that from 0.5 to 10 GeV, the fraction decreases with increasing energy. The fraction then increases steadily between 10 GeV to ~250 GeV. Yet the slope (rate of growth) of the positron fraction decreases by an order of magnitude from 20 to 250 GeV. At energies above 250 GeV, the spectrum appears to flatten but to study the behavior above 250 GeV requires more statistics – the data reported represents ~10% of the total expected. The positron fraction spectrum exhibits neither structure nor time dependence. The positron to electron ratio shows no anisotropy indicating the energetic positrons are not coming from a preferred direction in space. Together, these features show evidence of a new physics phenomena. Figure 2 illustrates the AMS data presented in the first publication.
The exact shape of the spectrum, as shown in Figure 2, extended to higher energies, will ultimately determine whether this spectrum originates from the collision of dark matter particles or from pulsars in the galaxy. The high level of accuracy of this data indicates that AMS may soon resolve this issue.
Over the last few decades there has been much interest on the positron fraction from primary cosmic rays by both particle physicists and astrophysicists. The underlying reason is that by measuring the ratio between positrons and electrons and by studying the behavior of any excess across the energy spectrum, a better understanding of the origin of dark matter and other physics phenomena may be obtained.
The first AMS result has been analyzed using several phenomenological models, one of which is described in the paper and included in Figure 2. This generic model, with diffuse electron and positron components and a common source component, fits the AMS data surprisingly well. This agreement indicates that the positron fraction spectrum is consistent with electron positron fluxes each of which is the sum of its diffuse spectrum and a single energetic common source. In other words, a significant portion of the high-energy electrons and positrons originate from a common source. More specific models 2 based on dark matter self annihilation and/or pulsar sources in the Milky Way have been published immediately after the release of the AMS data.
As shown in Figure 3, the accuracy of AMS and the high statistics available distinguish the reported positron fraction spectrum from earlier experiments 3, by extending the energy range and improving the precision by an order of magnitude.
AMS is a magnetic spectrometer with the ability to explore new physics because of its precision, statistics, energy range, capability to identify different particles and nuclei and its long duration in space. It is expected that hundreds of billions of cosmic rays will be measured by AMS throughout the lifetime of the Space Station. The volume of raw data requires a massive analysis effort. The parameters of each signal collected are meticulously reconstructed, characterized and archived before they undergo analysis by multiple independent groups of AMS physicists thus ensuring the accuracy of the physics results.
With the wealth of data emitted by primary cosmic rays passing through AMS, the Collaboration will also explore other topics such as the precision measurements of the boron to carbon ratio, nuclei and antimatter nuclei, and antiprotons, precision measurements of the helium flux, proton flux and photons, as well as the search for new physics and astrophysics phenomena such as strangelets.
The AMS Collaboration will provide new, accurate information over the lifetime of the Space Station as the AMS detector continues its mission to explore new physics phenomena in the cosmos.
(This article is based on http://press.web.cern.ch/tags/ams.)
¹ AMS Collaboration, M. Aguilar et al., First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV, Phys. Rev. Lett. 110, 141102 (2013)
² See e.g.: Andrea De Simone, Antonio Riotto, Wei Xuec, CERN-PH-TH/2013-054 (April 3, 2013). Tim Linden and Stefano Profumo, arXiv:1304.1791v1 [astro-ph.HE], (April 5, 2013). Peng-Fei Yin, Zhao-Huan Yu, Qiang Yuan and Xiao-Jun Bi, arXiv:1304.4128v1 [astro-ph.HE] (April 15, 2013)
³ TS93: R. Golden et al., Astrophys. J. 457 (1996) L103. Wizard/CAPRICE: M. Boezio et al., Adv. Sp. Res. 27-4 (2001) 669. HEAT: J. J. Beatty et al., Phys. Rev. Lett. 93 (2004) 241102; M. A. DuVernois et al., Astrophys. J. 559 (2001) 296. AMS-01: M. Aguilar et al., Phys. Lett. B 646 (2007) 145. PAMELA: P. Picozza, Proc. of the 4th International Conference on Particle and Fundamental Physics in Space, Geneva, 5-7 Nov. 2012, to be published. O. Adriani et al., Astropart. Phys. 34 (2010) 1; O. Adriani et al., Nature 458 (2009) 607. Fermi-LAT: M. Ackermann et al., Phys. Rev. Lett. 108 (2012) 011103.
[Released: July 2013]