First results of the AMS-02 experiment

The Alpha Magnetic Spectrometer (AMS) collaboration announced its first physics result in Physical Review Letters on 3 April 2013 [AMS Collaboration, Phys. Rev. Lett. 110 (2013)]. This was a long awaited event for the astroparticle physics community. Indeed, this large particle detector was first proposed by Nobel laureate Samuel Ting in 1994, to study primordial cosmic-ray particles in the energy range from 0.5 to 2 TeV. A proof-of-principle spectrometer (AMS-01) flew successfully for 10 days on the space shuttle Discovery during flight STS-91 in June 1998.

AMS-02 is visible at centre of the International Space Station's starboard truss.
The encouraging results have strengthen the undertaking of building the first high-precision astroparticle detector (AMS-02) to be installed on the International Space Station (ISS). Following the loss of the space shuttle Columbia and its crew on 1 February 2003, the space shuttle programme was suspended by NASA, cancelling a number of flights, including the AMS-02 one. On 15 October 2009, hence over six years later, President George W. Bush signs the bill authorising NASA to add another space shuttle to launch AMS-02 on the ISS. On 16 May 2011 the space shuttle Endavour finally takes the AMS detector to space. The crew of the STS-134 mission successfully installed AMS three days later on the ISS S3 truss (see picture), from where AMS is taking continually data ever since.

The first AMS-02 results reported two weeks ago concern the positron fraction, i.e. the ratio of the positron flux to the combined flux of positrons and electrons, in the energy range from 0.5 to 350 GeV. The positron fraction received a lot of attention since the PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) experiment measured an increasing fraction from 10 to 100 GeV in 2009. This increase cannot be explained by positrons produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium, so-called secondary source. Hence other primary sources of positrons are under investigation, in particular astrophysical sources such as pulsars or exotic sources such as dark matter annihilation. This rise was later confirmed and extended to 200 GeV by the Fermi LAT (Large Area Telescope), which uses in the absence of an onboard magnet the Earth’s magnetic shadow to distinguish electrons and positrons. To ensure the precise measurement of cosmic rays over the whole energy range, AMS-02 has a set of redundant and complementary detectors to identify the nature of the incoming particles and their energy. In contrast to PAMELA, AMS-02 additionally uses a transition radiation detector (TRD) to separate electrons and positrons from protons, allowing a background rejection of 10^6. This is achieved in conjunction with the 3D sampling electromagnetic calorimeter (ECAL) up to the highest energies, hence eliminating the hypothesis of a detector effect or experimentally induced rise of the positron fraction. The AMS-02 results are spectacular in many ways. First, the positron fraction was measured with an unprecedented precision. The ECAL energy resolution facilitates a fine binning which allows to search for a structure in the positron fraction. Such a structure has not been found in the data. Secondly, the positron fraction was measured up to 350 GeV, an energy not reached before by any dedicated cosmic-ray experiment. And finally, the AMS-02 positron fraction confirms a steady rise which is not a detector effect and hence must be induced by a primary source not identified so far.

The AMS-02 positron fraction compared to the most recent measurements from PAMELA (Picozza, private communication; Adriani, Astropart. Phys. 34 (2010)) and Fermi-LAT (Ackermann et al., Phys. Rev. Lett. 108 (2012)). The comparatively small error bars for AMS-02 are the quadratic sum of the statistical and systematic uncertainties.
The question of the nature of the additional positron source is still unanswered. The more likely origin, and preferred by most experts, is of astrophysical nature, i.e. pulsars. Even measurements of the positron fraction at higher energies maybe showing a cutoff in energy would not resolve the issue. To clearly see a dark matter signal in cosmic-ray spectra one needs to look into different channels such as photons, and anti-deuterons. The first one is a very promising one, since a dark matter signal would result in a line-like feature, clearly distinguishable from the continuous background, and in reach for the AMS-02 detector for small dark matter masses thanks to a long exposure time of around 20 years. At low energies below a few GeV, anti-deuterons resulting from dark-matter annihilations would significantly contribute to the spectrum. The amount of produced anti-deuterons is inverse proportional to the mass of the dark matter particle, rendering AMS sensitive to small dark matter masses. In any case, to get a complete picture of the dark matter particle, one needs complementary information from indirect, direct, and collider experiments. Together, experiments like AMS-02, XENON100/1T, and at the LHC can find the truth of dark matter. Exciting times are ahead!

– Antje Putze, former OKC fellow is now at I. Physikalisches Institut B, RWTH Aachen, Germany

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