A new look at the cosmic ray positron fraction

Abstract

Context. The positron fraction in cosmic rays has recently been measured with improved accuracy up to 500 GeV, and it was found to be a steadily increasing function of energy above ∼10 GeV. This behaviour contrasts with standard astrophysical mechanisms, in which positrons are secondary particles, produced in the interactions of primary cosmic rays during their propagation in the interstellar medium. The observed anomaly in the positron fraction triggered a lot of excitement, as it could be interpreted as an indirect signature of the presence of dark matter species in the Galaxy, the so-called weakly interacting massive particles (WIMPs). Alternatively, it could be produced by nearby sources, such as pulsars. Aims. These hypotheses are probed in light of the latest AMS-02 positron fraction measurements. As regards dark matter candidates, regions in the annihilation cross section to mass plane, which best fit the most recent data, are delineated and compared to previous measurements. The explanation of the anomaly in terms of a single nearby pulsar is also explored. Methods. The cosmic ray positron transport in the Galaxy is described using a semi-analytic two-zone model. Propagation is described with Green functions as well as with Bessel expansions. For consistency, the secondary and primary components of the positron flux are calculated together with the same propagation model. The above mentioned explanations of the positron anomaly are tested using χ 2 fits. The numerical package MicrOMEGAs is used to model the positron flux generated by dark matter species. The description of the positron fraction from conventional astrophysical sources is based on the pulsar observations included in the Australia Telescope National Facility (ATNF) catalogue. Results. The masses of the favoured dark matter candidates are always larger than 500 GeV, even though the results are very sensitive to the lepton flux. The Fermi measurements point systematically to much heavier candidates than the recently released AMS-02 observations. Since the latter are more precise, they are much more constraining. A scan through the various individual annihilation channels disfavours leptons as the final state. On the contrary, the agreement is excellent for quark, gauge boson, or Higgs boson pairs, with best-fit masses in the 10 to 40 TeV range. The combination of annihilation channels that best matches the positron fraction is then determined at fixed WIMP mass. A mixture of electron and tau lepton pairs is only acceptable around 500 GeV. Adding b-quark pairs significantly improves the fit up to a mass of 40 TeV. Alternatively, a combination of the four-lepton channels provides a good fit between 0.5 and 1 TeV, with no muons in the final state. Concerning the pulsar hypothesis, the region of the distance-to-age plane that best fits the positron fraction for a single source is determined. Conclusions. The only dark matter species that fulfils the stringent gamma ray and cosmic microwave background bounds is a particle annihilating into...

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“…At higher masses, the cross sections constrained are well above the thermal relic value, but are highly relevant for DM explanations of the positron excess observed by PAMELA [50], Fermi [51] and AMS-02 [52]. For example, the authors of [42] …”

“…The light and dark gray regions in the lower right panel correspond to the 5σ and 3σ regions in which the observed positron fraction can be explained by DM annihilation to μ þ μ − , for a cored DM density profile (necessary to evade γ-ray constraints), taken from [41]. The solid yellow line corresponds to the preferred cross section for the best fit 4-lepton final states identified by [42], who argued that models in this category can still explain the positron fraction without conflicts with nonobservation in other channels. The red and black circles correspond to models with 4e (red) and 4μ (black) final states, fitted to the positron fraction in [43]; as in that paper, filled and open circles correspond to different cosmic-ray propagation models.…”

“…Again, these preferred points are well above the bounds we derive for the corresponding channels. Figure 4 shows (1) the cross section band favored for the models of [42] as a solid yellow line, (2) the points favored by [43] as circles in red (4e final state) or black (4μ final state), and (3) a gray contour describing the region in which DM annihilation to μ þ μ − can explain the AMS-02 data [41]. These favored regions should be compared, respectively, to (1) an appropriately weighted combination of the green dot-dashed and red dot-dashed lines in Fig.…”

Recent results from the AMS-02 experiment

“…with 1/E S set to 0, is excluded at the 99.9% C.L. Several papers report dedicated analysis on the interpretation of AMS-02 electron and positron measurements in terms of astrophysical or exotic origin (see for example [20], [21] and [22]). It is possible to show that if the positron excess is due to the annihilation of DM in the vicinity of the Solar System, the mass of the DM particle has to be higher than 500 GeV.…”

AMS-02 antiprotons, at last! Secondary astrophysical component and immediate implications for Dark Matter

“…The sum of electrons and positrons [11] as well as their separate fluxes [12] have also been published, thus drawing a coherent and extremely precise picture of the lepton components of CR's. Despite the fact that DM interpretations of the positron and, more generally, leptonic 'excesses' have been attempted (for a review see [13]), even before the advent of Ams-02 it had been recognized that explanations involving astrophysical sources were both viable and favoured (for a review see [14]), a conclusion reinforced by updated analyses (see [15,16], and references therein, for recent assessments).…”