Narrowing the window for millicharged particles by CMB anisotropy

Abstract

We calculate the cosmic microwave background (CMB) anisotropy spectrum in models with millicharged particles of electric charge q\sim 10^{-6}-10^{-1} in units of electron charge. We find that a large region of the parameter space for the millicharged particles exists where their effect on the CMB spectrum is similar to the effect of baryons. Using WMAP data on the CMB anisotropy and assuming Big Bang nucleosynthesis value for the baryon abundance we find that only a small fraction of cold dark matter, Omega_{mcp}h_0^2 < 0.007 (at 95% CL), may consists of millicharged particles with the parameters (charge and mass) from this region. This bound significantly narrows the allowed range of the parameters of millicharged particles. In models without paraphoton millicharged particles are now excluded as a dark matter candidate. We also speculate that recent observation of 511 keV gamma-rays from the Galactic bulge may be an indication that a (small) fraction of CDM is comprised of the millicharged particles.

Figures (3)

• Figure 1: The exclusion plot in the parameter space for millicharged particles. Light grey area is excluded by accelerator experiments and BBN. Dashed region is excluded by the relic abundance of millicharged particles in models without paraphoton. Part of this region above the dash-dotted line is excluded by the relic abundance in models with paraphoton (assuming $\alpha'=0.1$). On the left of the dotted line millicharged particles cannot be thermally produced in the Early Universe. Dark grey and dashed dark grey areas are the previously allowed regions which are now excluded by Eq. (\ref{['result']}) in models without and with paraphoton, respectively. On the right of the dark solid line millicharged particles are coupled to baryons (see Eq. (\ref{['tightweak']}) in the text).
• Figure 2: Distribution of models in the ($\Omega_b h_0^2$, $\Omega_{mcp} h_0^2$) plane. Crosses and dots denote models agreeing with data at the 2$\sigma$ and 1$\sigma$ CL, respectively. The bold line illustrates the degeneracy of the CMB anisotropy spectrum. Two dotted lines show the range of $\Omega_b h_0^2$ allowed by BBN.
• Figure 3: Two different CMB anisotropy spectra compared with extended WMAP dataset. Solid line represents the best fit model without millicharged particles, $\Omega_bh_0^2 = 0.022$. Dashed line corresponds to model with $\Omega_b h_0^2= 0.014, \Omega_{mcp}h_0^2 = 0.007$.