Narrowing the window for millicharged particles by CMB anisotropy

We calculate the cosmic microwave background (CMB) anisotropy spectrum in models with millicharged particles of electric charge q ∼ 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, Ωmcph 2 0 < 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 γ-rays from the Galactic bulge may be an indication that a (small) fraction of CDM is comprised of the millicharged particles. Search for particles carrying small but non-vanishing electric charge (millicharged particles) has long history. If observed, millicharged particles would either cause serious doubts on the concept of Grand Unification or imply the existence of a new massless gauge boson — paraphoton [1, 2]. Furthermore, the existence of millicharged particles would hint towards processes with apparent electric charge non-conservation, like electron or proton decay to “nothing” [3]. There are various constraints on the parameters (charge and mass) of millicharged particles, coming from collider and laboratory experiments and from cosmology and astrophysics (see, e.g., Refs. [4, 5, 6, 7, 8, 9] for the latest results and Refs. [10, 11] for reviews), see Fig. 1. Interestingly, reported bounds did not exclude a possibility [12] that a significant part (or even all) of the cold dark matter (CDM) is comprised of millicharged particles. 1 e-mail: [email protected] 2 e-mail: [email protected] 3 e-mail: [email protected]