content_image/431156.jpg content_image/431157.jpg
Here α and G$_{F}$ , respectively, are the fine-structure con- stant and the Fermi constant, θ 2 i is the mixing angle de- fined by 

With the previous parameter choice, we can determine the mixing angle to be θ 2 i ≃ 10 − $^{22}$. Therefore the decay into the electron-positron pairs (42b) can [26] provide a natural explanation for the flux of 511 keV photons from the galactic bulge observed by INTEGRAL [27] satellite, 

It is easy to check that our scenario is consistent with other astrophysical and cosmological constraints [28]. Alternatively, we may explain the observed cosmic positron/electron excess [29–33], which is probably from continuum distribution of pulsars [34, 35], by fine tuning the parameters. 

In summary we have shown the dark matter can be well determined by the neutrino masses and the baryon 

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## V. CONCLUSION 

asymmetry in the left-right symmetric model with dou- blet and singlet fields. In this model, the SM fermions ob- tain masses by integrating out charged singlet fermions. In the neutrino sector, the right(left)-handed neutral fermions, associated with the left(right)-handed Higgs doublet, can generate the left(right)-handed neutrino masses through the seesaw. The mass matrices of the left- and right-handed neutrinos have a same structure as a re- sult of the left-right symmetry. The neutral singlets are also responsible for the baryon asymmetry and the dark matter. Specifically their 2-body decays can produce a desired lepton asymmetry in the left-handed leptons and then the observed baryon asymmetry can be realized by the sphaleron induced lepton-to-baryon conversion. At the same time, the right-handed neutrinos can serve as the dark matter as they have a right relic density from the 3-body decays of the neutral singlets. The decays of these non-thermally produced right-handed neutrinos can easily induce the observed fluxes of 511 keV photons from the Galactic bulge. The attractive feature of our scenario is that the left-right symmetry can connect the properties of the dark matter to the neutrino masses and the baryon asymmetry. 

Acknowledgement : I thank Manfred Lindner for hospitality at Max-Plank-Institut f¨ ur Kernphysik. This work is supported by the Alexander von Humboldt Foun- dation. 

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