Résumé

AMS-02 mesure les rayons cosmiques avec précision

Déployée le 19 mai 2011 sur la Station spatiale internationale (ISS), l'expérience AMS-02 a pour objet une étude de longue durée et de haute précision des noyaux cosmiques, des particules chargées et des rayons gamma. Le détecteur, qui jusqu'à présent a fonctionné parfaitement, a fourni des données sur 36,5 milliards de rayons cosmiques en 28 mois de service. Lors de la 33e Conférence internationale sur les rayons cosmiques (ICRC 2013), la collaboration AMS a présenté les premiers résultats relatifs à deux sujets importants : le premier concerne les flux, les taux et les anisotropies des leptons ; le second les noyaux cosmiques chargés (protons, hélium, bore, carbone).

More than 100 years have passed since the discovery of cosmic rays by Victor Hess in 1912 and there are still no signs of decreasing interest in the study of the properties of charged leptons, nuclei and photons from outer space. On the contrary, the search for a better understanding and clarification of the long-standing questions – the origin of ultrahigh energy cosmic rays, the composition as a function of energy, the existence of a maximum energy, the acceleration mechanisms, the propagation and confinement in the Galaxy, the extra-galactic origin, etc. – are more pertinent than ever. In addition, new ambitious experimental initiatives are starting to produce results that could cast light on more recent challenging questions, such as the nature of dark matter, the apparent absence of antimatter in the explored universe and the search for new forms of matter.

The 33rd International Conference on Cosmic Rays (ICRC 2013) – The Astroparticle Physics Conference – took place in Rio de Janeiro on 2–9 July and provided a high-profile platform for the presentation of a wealth of results from solar and heliospheric physics, through cosmic-ray physics and gamma-ray astronomy to neutrino astronomy and dark-matter physics. A full session was devoted to the presentation of new results from the Alpha Magnetic Spectrometer, AMS-02. Sponsored by the US Department of Energy and supported financially by the relevant funding and space agencies in Europe and Asia, this experiment was deployed on the International Space Station (ISS) on 19 May 2011 (figure 1). The results, which were presented for the first time at a large international conference, are based on the data collected by AMS-02 during its first two years of operation on the ISS.

AMS-02 is a large particle detector by space standards and built using the concepts and technologies developed for experiments at particle accelerators but adapted to the extremely hostile environment of space. Measuring 5 × 4 × 3 m3, it weighs 7.5 tonnes. Reliability, performance and redundancy are the key features for the safe and successful operation of this instrument in space (CERN Courier July/August 2011 p18 and p23).

The main scientific goal is to perform a high-precision, large-statistics and long-duration study of cosmic nuclei (from hydrogen to iron and beyond), elementary charged particles (protons, antiprotons, electrons and positrons) and γ rays. In particular, AMS-02 is designed to measure the energy- and time-dependent fluxes of cosmic nuclei to an unprecedented degree of precision, to understand better the propagation models, the confinement mechanisms of cosmic rays in the Galaxy and the strength of the interactions with interstellar media. A second high-priority research topic is an indirect search for dark-matter signals based on looking at the fluxes of particles such as electrons, positrons, protons, antiprotons and photons.

Another important item on the list of priorities – which will be addressed in future – is the search for cosmic antimatter nuclei. This variety of matter is apparently absent in the region of the universe currently explored but – according to the Big Bang theory – it should have been highly abundant in the early phases of the universe. Last but not least, AMS-02 will explore the possible existence of new phenomena or new forms of matter, such as strangelets, which this state-of-the-art instrument will be in a unique position to unravel.

The AMS-02 detector was designed, built and is now operated by a large international collaboration led by Nobel laureate Samuel C C Ting, involving researchers from institutions in America, Europe and Asia. The detector components were constructed and tested in research centres around the world, with large facilities being built or refurbished for this purpose in China, France, Germany, Italy, Spain, Switzerland and Taiwan. The final assembly took place at CERN, benefiting from the laboratory's significant expertise and experience in the technologies of detector construction. The instrument was then tested extensively with cosmic rays and particle beams at CERN, in the Maxwell electromagnetic compatibility chamber and the large-space thermal simulator at ESA-ESTEC in Noordwijk, as well as in the large facilities at the NASA Kennedy Space Center in the US.

The construction of AMS-02 has stimulated the development of important and novel technologies in advanced instrumentation. These include the first operation in space of a large two-phase CO2 cooling system for the silicon tracker (CERN Courier June 2012 p29) and the two-gas (Xe-CO2) system for the operation of the transition-radiation detector, as well as the overall thermal system. The latter must protect the experiment from the continual changes of temperature that the detector undergoes at every position on its orbit, which affect various parts of the detector subsystems in a manner that is not easy to reproduce. The use of radiation-tolerant fast electronics, a sophisticated trigger, redundant systems for data acquisition, associated protocols for communications with the NASA on-board hardware and a high-rate downlink system for the real-time transmission of data from AMS-02 to the NASA ground facilities, are a few examples that illustrate the complexity and the kind of challenges that the project has had to meet.

The operation of the Payload Operation and Control Center (POCC) at CERN, 24 hours a day and 365 days a year, in permanent connection with the ISS and the NASA Johnson Space Center, has also been a major endeavour (CERN Courier September 2011 p49). Fast processing of data on reception at the Science Operation Center at CERN has been a formidable tour de force, resulting in the timely reconstruction of 36.5 × 109 cosmic rays during the period 19 May 2011 – August 2013.

After almost 28 months of operation, AMS-02 – with its 300,000 electronics channels, 650 computers, 1100 thermal sensors and 400 thermostats – has worked flawlessly. To maintain performance and reliability, three space-flight simulators operate continuously at CERN, at the NASA Johnson Space Center and at the NASA Marshall Space Flight Center, where they test and certify the numerous upgrades of the software packages for the on-board computers and the communication interfaces and protocols.

First results

At ICRC 2013, the AMS collaboration presented data on two important areas of cosmic-ray physics. One addresses the fluxes, ratios and anisotropies of leptons, while the other concerns charged cosmic nuclei (protons, helium, boron, carbon). The following presents a brief summary of the results and of some of the most critical experimental challenges.

In the case of electrons and positrons, efficient instrumental handles for the suppression of the dominant backgrounds are: the minimal amount of material in the transition-radiation and time-of-flight detectors; the magnet location, separating the transition-radiation detector and the electromagnetic calorimeter; and the capability to match the value of the particle momentum reconstructed in the nine tracker layers of the silicon spectrometer with the value of the energy of the particle showering in the electromagnetic calorimeter.

The performance of the transition-radiation detector results in a high proton-rejection efficiency (larger than 103) at 90% positron efficiency in the rigidity range of interest. The performance of the calorimeter with its 17 radiation lengths provides a rejection factor better than 103 for protons with momenta up to 103 GeV/c. The combination of the two efficiencies leads to an overall proton-rejection factor of 106 for most of the energy range under study.

A precision measurement of the positron fraction in primary cosmic rays, based on the sample of 6.8 million positron and electron events in the energy range of 0.5–350 GeV – collected during the initial 18 months of operation on the ISS – was recently published and presented at the conference (Aguilar et al. 2013 and Kounine ICRC 2013). The positron-fraction spectrum (figure 2), does not exhibit fine structure and the highly precise determination shows that the positron fraction steadily increases from 10–250 GeV, while from 20–250 GeV, the slope decreases by an order of magnitude. The AMS-02 measurements have extended the energy ranges covered by recent experiments to higher values and reveal a different behaviour in the high-energy region of the spectrum.

AMS-02 has also extended the measurements of the positron spectrum to 350 GeV – that is, above the energy range of determinations by other experiments. The individual electron and positron spectra, with the E3 multiplication factor and the combined spectrum, were presented at the conference (Schael, Bertucci ICRC 2013). Figure 3 shows the electron spectrum, which appears to follow a smooth, slowly falling curve up to 500 GeV. The positron spectrum, by contrast, rises to 10 GeV, flattens from 10–30 GeV, before rising again above 30 GeV (figure 4). For the time being, it is not obvious that the models or simple parametric estimations that are currently used to describe the rate spectrum can also describe the behaviour of the individual electron and positron spectra.

Using a larger data sample, comprising of the order of 9 million of electrons and positrons, the collaboration has performed a preliminary measurement of the combined fluxes of electrons and positrons in the energy range 0.5–700 GeV (Bertucci ICRC 2013). The data do not show significant structures, although a change in the spectral index with increasing lepton energies is clearly observed. However, the positron flux increases with energy and a promising approach to identifying the physics origin of this behaviour lies in the determination of the size of a possible anisotropy, arising in primary sources, in the arrival directions of positrons and electrons measured in galactic co-ordinates. AMS-02 has obtained a limit on the dipole anisotropy parameter d <0.030 at the 95% confidence level for energies above 16 GeV (Casaus ICRC 2013).

Turning to cosmic nuclei, the first AMS-02 measurements of the proton and helium fluxes were presented at the conference (Haino, Choutko ICRC 2013). The rigidity ranges were 1 GV – 1.8 TV for protons and 2 GV – 3.2 TV for helium (figures 5 and 6). In both cases, the experiment observed gradual changes of the fluxes owing to solar modulation, as well as drastic changes after large solar flares. Otherwise, the spectra are fairly smooth and do not exhibit breaks or fine structures of the kind reported for other recent experiments.

The ratio of the boron to carbon fluxes is particularly interesting because it carries important information about the production and propagation of cosmic rays in the Galaxy. Boron nuclei are produced mainly by spallation of heavier primary elements present in the interstellar medium, whereas primary cosmic rays – such as carbon and oxygen – are predominantly produced at the source. Precision measurements of the boron-to-carbon ratio therefore provide important input for determining the characteristics of the cosmic-ray sources by deconvoluting the propagation effects from the measured data. The capability of AMS-02 to do multiple independent determinations of the electric charges of the cosmic rays allows a separation of carbon from boron with a contamination of less than 10–4. Figure 7 presents a preliminary measurement of the boron-to-carbon ratio in the kinetic-energy interval 0.5 – 670 GeV/n (Oliva ICRC 2013).

For the future

After nearly 28 months of successful operation, the results presented at ICRC 2013 already give a taste of the scientific potential of the AMS-02 experiment. In the near future, the measurements sketched in this article will extend the energy or rigidity coverage and the study of systematic uncertainties will be finalized. The experiment will measure the fluxes of more cosmic nuclei with unprecedented precision to constrain further the size and energy dependence of the underlying background processes.

High on the priority list for AMS-02 is the measurement of the antiproton flux and the antiproton/proton rate – a relevant and most sensitive quantity for disentangling, among the possible sources, those that induce the observed increase of the positron flux with energy. With the growing data sample and a deeper assessment of the systematic uncertainties, the searches for cosmic antinuclei will become extremely important, as will the search for unexpected new signatures.

By the end of the decade AMS-02 will have collected more than 150 × 109 cosmic-ray events. In view of what has been achieved so far, it is reasonable to be fairly confident that this massive amount of new and precise data will contribute significantly to a better understanding of the ever exciting and lively field of cosmic rays.