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

A precision measurement by the Alpha Magnetic Spectrometer on the International Space Station of the positron fraction in primary cosmic rays in the energy range from 0.5 to 350 GeV based on $6.8\times {10}^6$ positron and electron events is presented. The very accurate data show that the positron fraction is steadily increasing from 10 to $\sim 250\mathrm{GeV}$, but, from 20 to 250 GeV, the slope decreases by an order of magnitude. The positron fraction spectrum shows no fine structure, and the positron to electron ratio shows no observable anisotropy. Together, these features show the existence of new physical phenomena.

Figure 1

A 1.03 TeV electron event as measured by the AMS detector on the ISS in the bending ($y\mathrm{\text{−}}z$) plane. Tracker planes 1–9 measure the particle charge and momentum. The TRD identifies the particle as an electron. The TOF measures the charge and ensures that the particle is downward-going. The RICH independently measures the charge and velocity. The ECAL measures the 3D shower profile, independently identifies the particle as an electron, and measures its energy. An electron is identified by (i) an electron signal in the TRD, (ii) an electron signal in the ECAL, and (iii) the matching of the ECAL shower energy and the momentum measured with the tracker and magnet.

Figure 2

Stability of the alignment of tracker planes over more than 18 months of operation. (a) The temperature of the AMS support structure varies by $25°\mathrm{C}$, whereas the temperature of the inner tracker planes remains within $1°\mathrm{C}$. As shown in (b), (c), and their insets, the position of the outer planes 1 and 9, respectively, aligned by using cosmic ray protons results in the precise tracing of their thermal movement over more than 18 months to a stability of $3\mu \mathrm{m}$.

Figure 3

Separation power of the TRD estimator in the energy range 83.2–100 GeV for the positively charged selected data sample. For each energy bin, the positron and proton reference spectra are fitted to the data to obtain the numbers of positrons and protons.

Figure 4

(a) Stability of the measurement in the energy range 83.2–100 GeV over wide variations of the cuts fitted with a Gaussian of width 1.1%. (b) The positron fraction shows no correlation with the number of selected positrons.

Figure 5

The positron fraction compared with the most recent measurements from PAMELA [22] and Fermi-LAT [23]. The comparatively small error bars for AMS are the quadratic sum of the statistical and systematic uncertainties (see Table 1 and [13]), and the horizontal positions are the centers of each bin.

Figure 6

The positron fraction measured by AMS fit with the minimal model. For the fit, both the data and the model are integrated over the bin width. Even with the high statistics and high accuracy of AMS, the spectrum shows no fine structure.