A new look at the cosmic ray positron fraction

International audienceThe positron fraction in cosmic rays was found to be a steadily increasing infunction of energy, above $\sim$ 10 GeV. This behaviour contradicts standardastrophysical mechanisms, in which positrons are secondary particles, producedin the interactions of primary cosmic rays during the propagation in theinterstellar medium. The observed anomaly in the positron fraction triggered alot of excitement, as it could be interpreted as an indirect signature of thepresence of dark matter species in the Galaxy. Alternatively, it could beproduced by nearby astrophysical sources, such as pulsars. Both hypotheses areprobed in this work in light of the latest AMS-02 positron fractionmeasurements. The transport of the primary and secondary positrons in theGalaxy is described using a semi-analytic two-zone model. MicrOMEGAs is used tomodel the positron flux generated by dark matter species. The description ofthe positron fraction from astrophysical sources is based on the pulsarobservations included in the ATNF catalogue. We find that the mass of thefavoured dark matter candidates is always larger than 500 GeV. The only darkmatter species that fulfils the numerous gamma ray and cosmic microwavebackground bounds is a particle annihilating into four leptons through a lightscalar or vector mediator, with a mixture of tau (75%) and electron (25%)channels, and a mass between 0.5 and 1 TeV. The positron anomaly can also beexplained by a single astrophysical source and a list of five pulsars from theATNF catalogue is given. Those results are obtained with the cosmic raytransport parameters that best fit the B/C ratio. Uncertainties in thepropagation parameters turn out to be very significant. In the WIMPannihilation cross section to mass plane for instance, they overshadow theerror contours derived from the positron data