keV sterile neutrino dark matter in gauge extensions of the standard model

It is known, that a keV scale sterile neutrino is a good warm dark matter candidate. We study how this possibility could be realized in the context of gauge extensions of the standard model. The naïve expectation leads to large thermal overproduction of sterile neutrinos in this setup. However, we find that it is possible to use out-of-equilibrium decay of the other right-handed neutrinos of the model to dilute the present density of the keV sterile neutrinos and achieve the observed dark matter density. We present the universal requirements that should be satisfied by the gauge extensions of the standard model, containing right-handed neutrinos, to be viable models of warm dark matter, and provide a simple example in the context of the left-right symmetric model.

Figure 1

Schematic evolution of the light relic abundance in the Universe. The dashed line is a thermal relic decoupled while being relativistic (hot thermal relic), leading to the overclosure of the Universe. The blue decreasing line is the same hot thermal relic, but with the abundance diluted by rapid expansion of the Universe (entropy production), leading to correct DM abundance. The lowest (magenta) line depicts the evolution of the nonthermally produced particle with zero primordial abundance.

Figure 2

Unitary-gauge diagrams contributing to the radiative neutrino decay with charged leptons propagating in the loop.

Figure 3

${\theta }_2^2$ as a function of $M_2$. $M_1=1.6\mathrm{keV}$ (continuous); $M_1=5\mathrm{keV}$ (dotted); $M_1=20\mathrm{keV}$ (dashed) and $M_1=40\mathrm{keV}$ (dashed-dotted). It is accounted for the lower bound, Eq. (22). The long-dashed (red) line shows the ratio $(M_W/M{)}^4$ and illustrates that processes mediated by $W_R$ bosons can be neglected in the decay rate of $N_2$.

Figure 4

$v_R$ as a function of $M_2$. The dependence on $M_1$ is very weak.

Figure 5

The ratio ${\Gamma }_{N_2}/2{\Gamma }_1$ calculated in the specific LR model of Sec. 4. We used Eq. (a1) together with the formulas for the two-body decays into mesons [39] for the left (red) curve and Eq. (a1) together with Eq. (a2) for the right (blue) curve. The dotted curves correspond to the region, where both approximations are not entirely reliable.