Semiannihilation of fermionic dark matter

The continued nonobservation of events emanating from dark matter annihilations in various direct and indirect detection experiments calls into question the mechanism for determining the relic density of a weakly interacting massive particle. However, if the relic density is determined primarily by a semiannihilation process, as opposed to the usual annihilation, this tension can be ameliorated. Here, we investigate a $Z_3$ symmetric effective field theory incorporating a fermionic dark matter that semiannihilates to right-handed neutrinos. The dynamics of the right-handed neutrinos and the impact of its late decays are also scrutinized while obtaining the correct dark matter relic. Finally, indirect detection bounds on the semiannihilation cross sections are drawn from the gamma-ray observations in the direction of dwarf spheroidal galaxies (Fermi-LAT), and including the projections obtained for the H.E.S.S. and the CTA detectors.

Figure 1

Feynman diagram of the semiannihilation dark matter to RHN and the decays of RHN.

Figure 2

Different terms in expansion of $⟨\sigma v⟩\times {\mathrm{\Lambda }}^4$ plotted as a function of $x=m_{\chi }/T$ for SS interaction with (a) $m_{\chi }=300,m_N=100\mathrm{GeV}$, (b) $m_{\chi }=1000,m_N=100\mathrm{GeV}$.

Figure 3

Yield of $\chi$ and right-handed neutrino for $m_{\chi }=120,\mathrm{\Lambda }=500,m_N=100$ as a function of $x=\frac{m_{\chi }}{T}$. The yellow and blue curves show the extrapolated equilibrium abundances of the DM and the RHN, respectively. The green and red (purple) curves show the actual evolution of abundance of DM and RHN for different cases of Yukawa coupling: (a) $y_{N_i}=({10}^{-7},{10}^{-7},{10}^{-7})$, (b) $y_{N_i}=({10}^{-7},{10}^{-7},{10}^{-10})$, (c) $y_{N_i}=({10}^{-7},{10}^{-10},{10}^{-10})$, and (d) $y_{N_i}=({10}^{-10},{10}^{-10},{10}^{-10})$. The red and purple curves in (b) and (c) correspond to RHN with largest and smallest Yukawa couplings.

Figure 4

Contour on $m_{\chi }-\mathrm{\Lambda }$ plane satisfying $\mathrm{\Omega }{h}^2=0.1200\pm 0.0012$ for different interactions for different values of $m_N$ and $y_N$.

Figure 5

Photon energy spectrum for different values of $m_{\chi }$ at a fixed value of $m_N=100\mathrm{GeV}$.

Figure 6

Photon energy spectrum for $m_{\chi }=3\mathrm{TeV}$ for different values of RHN mass and different final state leptons (a) electron and (b) tau lepton.

Figure 7

Upper limits on $⟨\sigma v⟩$ as a function of $m_{\chi }$ for (a) electron-RHN final state and (b) tau-RHN final state. We obtained these limits from different experiments Fermi-LAT, H.E.S.S., and CTA for different values of RHN mass, $m_N=100\mathrm{GeV}$ (solid), $m_N=750\mathrm{GeV}$ (dashed), and $m_N=1500\mathrm{GeV}$ (dotted).