Cosmic Ray Positrons: Constraints on Propagation Parameters and Dark Matter Searches in View of AMS-02 Data
Instituto de Física de São Carlos – Universidade de São Paulo (IFSC-USP)
Two years ago, the AMS-02 collaboration released the most precise measurement of the cosmic ray positron flux. In the conventional approach, in which positrons are considered as purely secondary particles, the theoretical predictions fall way below the data above 10 GeV. One suggested explanation for this anomaly is the annihilation of dark matter particles, the so-called WIMPs, into standard model particles. Most analyses have focused on the high-energy part of the positron spectrum, where the the anomaly lies, disregarding the complicated GeV low-energy region where Galactic cosmic ray transport is more difficult to model and solar modulation comes into play.
We derive the positron flux from sub-GeV to TeV energies for both gas spallation and dark matter annihilation. We carry out a scan over the cosmic ray propagation parameters which we strongly constrain by requiring that the secondary component does not overshoot the AMS-02 measurements. We find that only models with large diffusion coefficient are selected by this test. We then add to the secondary component the positron flux yielded by dark matter annihilation. We carry out a scan over WIMP mass to fit the annihilation cross section and branching ratios, successively exploring the cases of a typical WIMP standard annihilation through light mediators. We explicitly show that the cosmic ray positron flux is a powerful and independent probe of Galactic cosmic ray propagation. It should be used as a complementary observable to other tracers such as the boron-to-carbon ratio. This analysis shows also that the pure dark matter interpretation of the positron excess is strongly disfavored. This conclusion is based solely on the positron data, and no other observation, such as the antiproton flux or the CMB anisotropies, needs to be invoked.