Dark matter from late decays and the small-scale structure problems

TL;DR

This work evaluates whether dark matter produced in late decays of quasi-stable particles can alleviate small-scale problems of CDM. By mapping the decay parameters to observables $Q_p$ and $\lambda_{\rm FS}$, the authors derive stringent constraints on viable models and lifetimes $\tau$ and mass splittings $\delta$. They find that gravitino and KK graviton DM generally cannot solve both small-scale issues within thermally produced WIMP scenarios, while right-handed sneutrino and right-handed KK neutrino DM can do so only under subdominant Dirac neutrino masses and with fine-tuned mass degeneracies. In universal extra dimensions, KK graviton scenarios are excluded and RH KK neutrino cases face similar hurdles. Overall, late-decay DM remains a plausible mechanism, but requires highly constrained particle-physics realizations to be consistent with BBN, CMB, and Lyman-\alpha bounds.

Abstract

The generation of dark matter in late decays of quasi-stable massive particles has been proposed as a viable framework to address the excess of power found in numerical N-body simulations for cold dark matter cosmologies. We identify a convenient set of variables to illustrate which requirements need to be satisfied in any generic particle physics model to address the small scale problems and to fulfill other astrophysical constraints. As a result of this model-independent analysis, we point out that meeting these requirements in a completely natural way is inherently difficult. In particular, we re-examine the role of gravitinos and Kaluza-Klein gravitons in this context and find them disfavoured as a solution to the small-scale problems in case they are DM candidates generated in the decay of thermally produced WIMPs. We propose right-handed sneutrinos and right-handed Kaluza-Klein neutrinos as alternatives. We find that they are viable dark matter candidates, but that they can contribute to a solution of the small scale problems only in case the associated Dirac neutrino mass term appears as a subdominant contribution in the neutrino mass matrix.

Figures (10)

• Figure 1: From bottom to top, the solid (dashed) lines correspond to $\lambda_\mathrm{FS}/\mathrm{Mpc}=0.3,0.4,0.5$ ($Q_p/Q_0=1,0.1,0.01$). In the dark shaded region both small scale structure problems could be resolved, while in the lighter shaded areas this is true for only one of them, respectively. The upper-right part is excluded from Lyman-$\alpha$ forest measurements of the power spectrum, while in the lower-left part of the plot, the mechanism of dark matter generation through the decay of a meta-stable species does not leave an observable imprint in the sky.
• Figure 2: This plot shows the tightest available constraints on a scenario with DM from late decays. The yellow dashed-dotted (red dotted) lines give the limits from CMB (BBN), from bottom to top, when a fraction $f_\gamma=0.05,0.01,0.005\,$ of the total energy in light decay products is released into electromagnetically active species. The contribution to the ISW excludes the area above the green solid line. Finally, the blue dashed lines show, from bottom to top, the Super-Kamionkande limit for masses $M_Y=1,0.5,0.1$ TeV. The shaded regions are the same as in Fig. \ref{['fig_Qlambda']}. Not included in the figure are constraints from hadronic decay products, which become important for $\tau\lesssim10^8\,$s; see text for further details.
• Figure 3: The required coupling strength as a function of the mass splitting, for the case that both $X$ and $Y$ have spin 0, 1/2 or 1. From top to bottom, the solid (dashed) lines correspond to $\lambda_\mathrm{FS}/\mathrm{Mpc}=0.3,0.4,0.5$ ($Q/Q_0=1,0.1,0.01$). The lower part of this plot is thus excluded and the dark shaded area shows the region where both small scale structure problems can be resolved. Also included is the combined bound from BBN and CMB, shown as a red dotted line for $f_\gamma=0.05,0.01,0.005$.
• Figure 4: Same as Fig. \ref{['fig_particle']}, now for the case that either $X$ or $Y$ is a spin 3/2 or 2 particle. See text for further details.
• Figure 5: Correlation between sneutrino mass $M_{\tilde{\nu}}$ and sneutrino-gravitino mass splitting $\delta$ for models matching the dark matter abundance measured by WMAP. Predictions within the MSSM span from the case of 3 families of mass-degenerate left-handed sneutrinos (dashed curve) to the case of coannihilations of 1 light sneutrino with gluinos (dotted curve). Also shown are the cases of 1 light sneutrino coannihilating with Winos and that of 1 light sneutrino family without coannihilations (other than with the light charged sleptons).