We propose a dark matter (DM) scenario in an extension of a left-right symmetric model with a gauge-singlet scalar field. The gauge-singlet scalar can automatically become a DM candidate, provided that both P and CP symmetries are only broken spontan
eously. Thus no extra discrete symmetries are needed to make the DM candidate stable. After constraining the model parameters from the observed relic DM density we make predictions for direct detection experiments. We show that for some parameter range, the predicted WIMP-nucleon elastic scattering cross section can reach the current experimental upper bound, which can be tested by the experiments in the near future.
In the framework of Left-Right symmetric model, we investigate an interesting scenario, in which the so-called VEV seesaw problem can be naturally solved with Z_2 symmetry. In such a scenario, we find a pair of stable weakly interacting massive parti
cles (WIMPs), which may be the cold dark matter candidates. However, the WIMP-nucleon cross section is 3-5 orders of magnitude above the present upper bounds from the direct dark matter detection experiments for $m sim 10^2-10^4 $ GeV. As a result, the relic number density of two stable particles has to be strongly suppressed to a very small level. Nevertheless, our analysis shows that this scenario cant provide very large annihilation cross sections so as to give the desired relic abundance except for the resonance case. Only for the case if the rotation curves of disk galaxies are explained by the Modified Newtonian Dynamics (MOND), the stable WIMPs could be as the candidates of cold dark matter.
In the framework of type II seesaw mechanism we discuss the number of sterile right-handed Majorana neutrinos being the warm dark matter (WDM). When the type II seesaw mass term $M_ u ^{II}$ is far less than the type I seesaw mass term $M_ u ^{I}$, o
nly one of three sterile neutrinos may be the WDM particle. On the contrary, the WDM particles may contain all sterile neutrinos. If $M_ u ^{II} sim M_ u ^{I}$, the allowed number is not more than $N - 1$ for $N$ sterile neutrinos. It is worthwhile to stress that three different types of neutrino mass spectrum are permitted when $M_ u ^{II} gg M_ u ^{I}$ and $M_ u ^{II} sim M_ u ^{I}$.
