Antiproton Flux, Antiproton-to-Proton Flux Ratio, and Properties of Elementary Particle Fluxes in Primary Cosmic Rays Measured with the Alpha Magnetic Spectrometer on the International Space Station

A precision measurement by AMS of the antiproton flux and the antiproton-to-proton flux ratio in primary cosmic rays in the absolute rigidity range from 1 to 450 GV is presented based on $3.49\times 10^5$ antiproton events and $2.42\times 10^9$ proton events. The fluxes and flux ratios of charged elementary particles in cosmic rays are also presented. In the absolute rigidity range $\sim 60$ to $\sim 500\mathrm{GV}$, the antiproton $\overline{p}$, proton $p$, and positron $e^{+}$ fluxes are found to have nearly identical rigidity dependence and the electron $e^{-}$ flux exhibits a different rigidity dependence. Below 60 GV, the ($\overline{p}/p$), ($\overline{p}/e^{+}$), and ($p/e^{+}$) flux ratios each reaches a maximum. From $\sim 60$ to $\sim 500\mathrm{GV}$, the ($\overline{p}/p$), ($\overline{p}/e^{+}$), and ($p/e^{+}$) flux ratios show no rigidity dependence. These are new observations of the properties of elementary particles in the cosmos.

Figure 1

(a) Negative rigidity and positive rigidity data samples in the [${\beta }_{\mathrm{RICH}}-\mathrm{sign}(R){\times \mathrm{\Lambda }}_{\mathrm{TRD}}$] plane for the absolute rigidity range 5.4–6.5 GV. The contributions of $\overline{p}$, $p$, $e^{+}$, $e^{-}$, ${\pi }^{+}$, and ${\pi }^{-}$ are clearly seen. The antiproton signal is well separated from the backgrounds. (b) For negative rigidity events, the distribution of data events in the (${\mathrm{\Lambda }}_{\mathrm{TRD}}-{\mathrm{\Lambda }}_{\mathrm{CC}}$) plane for the absolute rigidity bin 175–211 GV. (c) Fit with ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.=138/154$ of the antiproton signal template (magenta), the electron background template (blue), and the charge confusion proton background template (green) to the data in (b).

Figure 2

The measured antiproton flux (red, left axis) compared to the proton flux (blue, left axis) [16], the electron flux (purple, right axis), and the positron flux (green, right axis) [15]. All the fluxes are multiplied by ${\widehat{R}}^{2.7}$. The fluxes show different behavior at low rigidities while at $|R|$ above $\sim 60\mathrm{GV}$ the functional behavior of the antiproton, proton, and positron fluxes are nearly identical and distinctly different from the electron flux. The error bars correspond to the quadratic sum of the statistical and systematic errors.

Figure 3

(a) The measured ($\overline{p}/p$) flux ratio as a function of the absolute value of the rigidity from 1 to 450 GV. The PAMELA [6] measurement is also shown. (b) The measured ($\overline{p}/e^{+}$) (red, left axis) and ($p/e^{+}$) (blue, right axis) flux ratios. The solid lines show the best fit of Eq. (4) to the data above the lowest rigidity consistent with rigidity independence together with the 68% C.L. ranges of the fit parameters (shaded regions). For the AMS data, the error bars are the quadratic sum of statistical and systematic errors. Horizontally, the data points are placed at the center of each bin.

Figure 4

Sliding window fits of Eq. (4) to the ($\overline{p}/p$) flux ratio measured by AMS with parameter $C$ (green, left axis) and the slope $k$ (blue, right axis). The green and blue shaded regions indicate that the errors are correlated between adjacent points. The points are placed at $R_0$. The dashed blue line at $k=0$ is to guide the eye. The black arrow indicates the lowest rigidity above which the flux ratio is consistent with being rigidity independent and the black horizontal band shows the mean value and the 1-sigma error of the flux ratio above this rigidity.