Dark Matter Dilution Mechanism through the Lens of Large-Scale Structure

Entropy production is a necessary ingredient for addressing the overpopulation of thermal relics. It is widely employed in particle physics models for explaining the origin of dark matter. A long-lived particle that decays to the known particles, while dominating the universe, plays the role of the dilutor. We point out the impact of its partial decay to dark matter on the primordial matter power spectrum. For the first time, we derive a stringent limit on the branching ratio of the dilutor to dark matter from large scale structure observation using the sloan digital sky survey data. This offers a novel tool for testing models with a dark matter dilution mechanism. We apply it to the left-right symmetric model and show that it firmly excludes a large portion of parameter space for right-handed neutrino warm dark matter.

Figure 1

Schematic illustration of the DM dilution mechanism: relativistic freeze-out, dilutor matter domination, entropy injection (dashed red line), and DM repopulation (solid green line).

Figure 2

Phase space distribution of ultrarelativistic $X$. The blue and orange curves correspond to the two models listed in Table 1. Solid (dashed) curve corresponds to $m_X=10\mathrm{keV}$, $m_Y=100\mathrm{GeV}$ ($m_Y=1\mathrm{PeV}$), while ${\text{Br}}_X=0.1$ for all cases.

Figure 3

Primordial matter power spectrum in standard $\mathrm{\Lambda }\mathrm{CDM}$ (black solid curve) and diluted DM models listed in Table 1 (colorful curves). Like Fig. 2, we set ${\text{Br}}_X=0.1$. Data points from SDSS DR7 LRG and Lyman-$\alpha$ observations are shown in blue and orange, respectively.

Figure 4

Upper bound on ${\mathrm{Br}}_X$, the branching ratio of dilutor $Y$ decaying into $X$ from the fit to LSS data (SDSS DR7 LRG), for the LRSM with $Y=N_2$, $X=N_1$, and a long-lived scalar $Y=\mathrm{\Phi }$. For each point in the $m_X-m_Y$ parameter space, the $Y$ lifetime is solved by requiring $X$ to comprise all of the DM in the universe.