Leptophilic-portal dark matter in the light of AMS-02 positron excess

We revisit dark matter annihilation as an explanation of the positron excess reported recently by the AMS-02 satellite-borne experiment. To this end, we propose a particle dark matter model by considering a two Higgs doublet model (2HDM) extended with an additional singlet boson and a singlet fermion. The additional (light) boson mixes with the pseudoscalar inherent in the 2HDM, and the singlet fermion, which is the dark matter candidate, annihilates via this bosonic portal. The dark matter candidate is made leptophilic by choosing the lepton-specific 2HDM and a suitable high value of $\tan \beta$. We identify the model parameter space which explains the muon g-2 anomaly while evading the experimental constraints. After establishing the viability of the singlet fermion to be a dark matter candidate, we calculate the positron excess produced from its annihilation to the light bosons which primarily decay to muons. Incorporating the Sommerfeld effect caused by the light mediator and an appropriate boost factor, we find that our proposed model can satisfactorily explain the positron fraction excess as well as the positron spectrum data reported by the AMS-02 experiment.

Figure 1

The combined $2\sigma$ allowed parameter space in the $\tan \beta$ vs $m_A$ plane with $m_a=1.5\mathrm{GeV}$ (left panel) and $m_a=3.5\mathrm{GeV}$ (right panel) from the $a_{\mu }$ results (blue) and upper limits from the lepton-flavor universality bounds in the $Z$ (red) and $\tau /\mu$ (black) decays. We have assumed $m_H=m_{H^{\pm }}=300\mathrm{GeV}$.

Figure 2

The shaded regions are excluded by BABAR (light brown) and ORSAY and E137 (light blue), and the solid contours are future sensitivities at NA64-muon (purple), SEAQUEST (blue), and lepton colliders (pink on the right upper corner). The shaded region inside the dotted line is excluded by LHCb (light brown), and the other dotted contours are the future sensitivity at BELLE II (blue) and SHIP/MATHUSLA (red).

Figure 3

The diagrams for DM annihilation into $aa$.

Figure 4

The allowed parameter region in the $g_{\chi }$ vs $y_{\chi }$ plane that satisfies the PLANCK observed DM relic density for dark matter masses of 1.3 and 1.6 TeV.

Figure 5

The dominant tree and loop (box and triangle) diagrams for DM direct detection. The coupling are given by $g_{a\chi \chi }=g_{\chi }\cos \theta$, $y_{a\chi \chi }=y_{\chi }\cos \theta$, and $g_{aqq}=(m_q/v)\sin \theta /\tan \beta$.

Figure 6

Left panel: the positron fraction obtained from the DM annihilation for the different benchmark points tabulated in Table 2. The experimental data as reported by AMS-02, Fermi-LAT, and PAMELA are also shown. Right panel: the comparison of the positron flux using the best fit parameters obtained from fitting the AMS-02 observed positron fraction, with the experimental data published by the three experiments.

Figure 7

Comparison of the total $e^{+}+e^{-}$ flux as observed by the CALET [89], DAMPE [90], and AMS-02 [7] experiments.