arXiv:1001.1341v5 [hep-ph] 15 Apr 2010 A Left-Right Symmetric Model for Neutrino Masses, Baryon Asymmetry and Dark Matter Pei-Hong Gu ∗ Max-Plank-Institut f¨ ur Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany In the left-right symmetric models without bi-doublet Higgs scalars, the standard model fermions can obtain masses by integrating out heavy charged singlet fermions. We find the decays of heavy neutral singlet fermions, responsible for generating small neutrino masses, can simultaneously pro- duce a left-handed lepton asymmetry for baryon asymmetry and a relic density of right-handed neutrinos for dark matter. Benefited from the left-right symmetry, the properties of the dark mat- ter can be related to the generation of the neutrino masses and the baryon asymmetry. We also indicate that the decays of the non-thermally produced right-handed neutrinos can explain the observed fluxes of 511 keV photons from the Galactic bulge. PACS numbers: 95.35.+d, 14.60.Pq, 98.80.Cq, 12.60.Cn, 12.60.Fr which is well consistent with the cosmological limits on the sum of the neutrino masses [1], The SU (3)$_{c}$ × SU (2)$_{L}$ × U (1)$_{Y}$ standard model (SM) has been tested to a very high accuracy, but it has been suffering big challenges from particle physics and cos- mology. For example, it can’t explain the existence of non-baryonic dark matter, which has been confirmed by precisely cosmological observations [1], The smallness of the neutrino masses inspires a seesaw [4–8] extension of the SM. The seesaw models [4–9] also allow the attractive leptogenesis [10–15] mechanism for generating the observed baryon asymmetry. Here h = 0 . 719 +0 . 026 − 0 . 027 [1]. So far we know little on the true identity of the dark matter although there have been many interesting candidates. The cosmological obser- vations also indicates that the present universe doesn’t contain significant amount of baryonic antimatter. This baryon asymmetry again requires supplementing new in- gredients to the existing theory. The density of the bary- onic matter is measured by [1] On the other hand, new physics beyond the SM is also motivated by some theoretical considerations. For exam- ple, the SM accommodates the left- and right-handed fermions in different ways: the left-handed fermions transform as doublets but the right-handed ones trans- form as singlets. The unequal treatment on the left- and right-handed fermions means parity violation. If we start with a theory with left-right symmetry [16] at high en- ergy, where the left- and right-handed fermions are both placed in doublets and the parity is conserved, the spon- taneously left-right symmetry breaking can induce the parity violation as observed at low energy. The left- right symmetric models are based on the gauge group SU (3)$_{c}$ × SU (2)$_{L}$ × SU (2)$_{R}$ × U (1)$_{B}$$_{−}$$_{L}$. Within this con- text, the electric charge ( Q ) can be well defined in terms of lepton ( L ) and baryon ( B ) numbers, which is intriguingly comparable to that of the dark matter. The coincidence between the dark and bary- onic matter implies that they may have a specifical rela- tion [2] although their creation and evolution are usually understood by unrelated mechanisms. Furthermore, ob- servations on solar, atmospheric, reactor and accelerator neutrino oscillations have established the phenomenon of massive and mixing neutrinos [3], where − → T $_{L}$, − → T R and B − L are the generators of SU (2)$_{L}$, SU (2)$_{R}$ and U (1)$_{B}$$_{−}$$_{L}$, respectively. This provides us a natural explanation for the choice of hypercharge charge in the SM. An important prediction of the left-right sym- metry is the existence of right-handed neutrinos, which are trivial and hence absent in the SM. In the most popular left-right symmetric models with bi-doublet and triplet Higgs scalars [8], the right-handed neutrinos can naturally obtain heavy Majorana masses to realize the seesaw and the leptogenesis after the left-right symmetry breaking. In this paper we consider the left-right symmetric mod- els without bi-doublet Higgs scalars [17]. If the Higgs I. INTRODUCTION Σ m$_{i}$ < 1 . 3 eV (95% CL) . (4) Ω$_{χ}$ h 2 = 0 . 1099 ± 0 . 0062 . (1) Ω$_{b}$ h 2 = 0 . 02273 ± 0 . 00062 , (2) Q = T$_{L}$$_{3}$ + T$_{R}$$_{3}$ + B − L 2 , (5) Δ m 2 31 = ± 2 . 4 +0 . 12 − 0 . 11 10 − $^{3}$eV $^{2}$, (3b) sin $^{2}$θ$_{23}$ = 0 . 50 − 0 . $_{06}$, (3c) sin $^{2}$θ$_{12}$ = 0 . 304 +0 . 022 − 0 . $_{016}$, (3d) sin $^{2}$θ$_{13}$ = 0 . 01 +0 . 016 − 0 . $_{011}$, (3e) Δ m 2 21 = 7 . 65 +0 . 23 − 0 . 20 × 10 − $^{5}$eV $^{2}$, (3a) ∗ Electronic address: peihong.gu@mpi-hd.mpg.de