Gravitino, Axino, Kaluza-Klein Graviton Warm and Mixed Dark Matter and Reionisation

Abstract

Stable particle dark matter may well originate during the decay of long-lived relic particles, as recently extensively examined in the cases of the axino, gravitino, and higher-dimensional Kaluza-Klein (KK) graviton. It is shown that in much of the viable parameter space such dark matter emerges naturally warm/hot or mixed. In particular, decay produced gravitinos (KK-gravitons) may only be considered cold for the mass of the decaying particle in the several TeV range, unless the decaying particle and the dark matter particle are almost degenerate. Such dark matter candidates are thus subject to a host of cosmological constraints on warm and mixed dark matter, such as limits from a proper reionisation of the Universe, the Lyman-alpha forest, and the abundance of clusters of galaxies.. It is shown that constraints from an early reionsation epoch, such as indicated by recent observations, may potentially limit such warm/hot components to contribute only a very small fraction to the dark matter.

Figures (4)

• Figure 1: Contour plots of constant present-day free-streaming velocities in a variety of scenarios where the dark matter $\tilde{G}$ is generated by the late decay of a non-relativistic primary $\chi\to\tilde{G} + \gamma$ (cf.Eq.\ref{['eq1']}). Results are shown in the plane of $m_{\tilde{G}}$ and $\Delta m\equiv m_{\chi}-m_{\tilde{G}}$ (all in GeV). The scenarios are bino-decay into gravitinos (red-solid), slepton decay into gravitinos (blue-dotted), and $B^1$ decays into KK-gravitons (green-dashed). Shown are the contours of velocities (from top to bottom) $v = 0.002,0.1,$ and $1\,$ km s$^{-1}$, respectively. Also shown, by the thin dotted lines, are the contours where the effects of small-scale suppression due to the coupling of a charged slepton to the CMBR decaying later into a gravitino are similar to warm dark matter with $v = 0.002,0.1,$ and $1\,$ km s$^{-1}$, respectively (see text for details).
• Figure 2: Contour plot of the redshift $z$ when a fraction $10^{-2}$ of all baryons is collapsed within halos exceeding a virial temperature of $10^4$K. Results are shown depending on the warm/hot dark matter fractional contribution to the present critical density $\Omega_x$ and the present free-streaming velocity $v_x$ in km s$^{-1}$ of the warm/hot component. The calculation assumes the remainder of the dark matter $\Omega_c = 0.26 - \Omega_x$ to be cold. The shown redshifts may be identified with the approximate reionsation redshift when a "standard" reionsation efficiency is assumed. For comparison, the WMAPIII results of optical depth $\tau = 0.09\pm 0.03$ imply a reionization redshift within the $\Lambda$CDM concordence model between $z\approx 8.5-15$ at 95% confidence level. The star indicates a recent limit Viel:2005qj on a small contribution of $m_{\tilde{G}}\,{}^>_{\sim}\, 16\,$ eV thermal gravitinos to the dark matter ruled out by a combination of CMBR and Lyman-$\alpha$ forest data. Regions above and right of the dashed line should be ruled out by these considerations (see text).
• Figure 3: As Fig.2, but for a collapse fraction $10^{-4}$, corresponding to a factor $\sim 100$ more efficient reionization than inferred from present-day properties of star clusters and star formation.
• Figure 4: As Fig.2, but for a spectral index for the adiabatic primordial perturbations of $n_s=1$.