RESULTS
AMS-02 was installed on the
International Space Station (ISS) on May
19, 2011. In the first 18 months of
operations, AMS-02 recorded 6.8 million
positron (an antimatter particle with the
mass of an electron but a positive charge)
and electron events produced from
cosmic ray collisions with the interstellar
medium in the energy range between 0.5
giga-electron volt (GeV) and 350 GeV.
These events were used to determine the
positron fraction (ratio of positrons to the
total number of electrons and positrons).
Below 10 GeV, the positron fraction
decreased with increasing energy, as
expected. However, the positron fraction
increased steadily from 10 GeV to 250
GeV. This increase, seen previously
though less precisely by instruments such as the Payload for Matter/antimatter Exploration and
Light-nuclei Astrophysics (PAMELA) and the Fermi Gamma-ray Space Telescope, conflicts with
the predicted decrease of the positron fraction and indicates the existence of a currently
unidentified source of positrons, such as pulsars or the annihilation of dark matter particles.
Furthermore, researchers observed an unexpected decrease in slope from 20 GeV to 250 GeV.
The measured positron to electron ratio is isotropic, the same in all directions. Above 250 GeV,
more data is required to determine the positron fraction behavior.
PUBLICATION(S)
Aguilar-Benitez M, Aisa D, Alpat B, Alvino A, Ambrosi G, Andeen K. Precision measurement of
the (e++e-) flux in primary cosmic rays from 0.5 GeV to 1 TeV with the Alpha Magnetic
Spectrometer on the International Space Station. Physical Review Letters. November 28,
2014;113:221102. doi: 10.1103/PhysRevLett.113.221102.
Accardo L, Aguilar-Benitez M, Aisa D, et al. High statistics measurement of the positron fraction
in primary cosmic rays of 0.5–500 GeV with the Alpha Magnetic Spectrometer on the
International Space Station. Physical Review Letters. September 18, 2014;113:121101. doi:
10.1103/PhysRevLett.113.121101.
Aguilar-Benitez M, Aisa D, Alvino A, Ambrosi G, Andeen K, Arruda MF. Electron and positron
fluxes in primary cosmic rays measured with the Alpha Magnetic Spectrometer on the
International Space Station. Physical Review Letters. September 18, 2014;113:121102. doi:
10.1103/PhysRevLett.113.121102.
Alpha Magnetic Spectrometer- 02 is an international
collaboration of 600 physicists from 56 institutes in 16
countries led by Nobel Laureate Dr. Samuel Ting of MIT.