Abstract
We show that the existence of a sub-dominant form of dark matter, made of dark “antiatoms” of mass m∼ 1 TeV and size ȧ0∼ 3 fm, can explain the results of direct detection experiments, with a positive signal in DAMA/NaI and DAMA/LIBRA and no signal in other experiments. The signal comes from the binding of the dark antiatoms to thallium, a dopant in DAMA, and is not present for the constituent atoms of other experiments. The dark antiatoms are made of two particles oppositely charged under a dark U(1) symmetry and can bind to terrestrial atoms because of a kinetic mixing between the photon and the massless dark photon, such that the dark particles acquire an electric millicharge ∼ ± 5.10−4e. This millicharge enables them to bind to high-Z atoms via radiative capture, after they thermalize in terrestrial matter through elastic collisions.
Highlights
“antiatoms” of mass m ∼ 1 TeV and size a 0 ∼ 30 fm, can explain the results of direct detection experiments, with a positive signal in DAMA/NaI and DAMA/LIBRA and no signal in other experiments
We checked that there are no bound states with xenon, and with the elements of atomic numbers Z ≤ 54, so that the negative results of CDMS-II/Ge and superCDMS and XENON100 and LUX can be naturally explained
The model assumes that a subdominant form of dark matter is made of dark antiatoms
Summary
We take a very simple dark sector, in which two fermions p− and e+, of masses mp− and me+ and of respective charges −eand +eunder a dark U(1), bind to each other and form dark antihydrogen atoms. In view of the Planck data, it sets an upper limit on the cosmological density of millicharges Ωmch2 < 0.001 (95%), but is assumes that the millicharged dark matter is fully ionized This should be weakened here as the oppositely charged particles form neutral atomic structures while only an ionized fraction remains. Constraints on self-interacting dark matter from halo shapes [22] and colliding clusters can be completely avoided [26], if no more than 10% of the whole dark matter in halos has self-interactions while the rest is collisionless This is an additional weakening factor of the CMB constraint that leaves us with a subdominant sector to account for the results of the direct searches. We assume that the dark atoms are at rest in the frame of the dark disk, which itself is at rest with respect to the halo of collisionless particles
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