The dark atoms of dark matter

Abstract

The nonbaryonic dark matter of the Universe is assumed to consist of new stable particles. A specific case is possible, when new stable particles bear ordinary electric charge and bind in heavy "atoms" by ordinary Coulomb interaction. Such possibility is severely restricted by the constraints on anomalous isotopes of light elements that form positively charged heavy species with ordinary electrons. The trouble is avoided, if stable particles $X^{--}$ with charge -2 are in excess over their antiparticles (with charge +2) and there are no stable particles with charges +1 and -1. Then primordial helium, formed in Big Bang Nucleosynthesis, captures all $X^{--}$ in neutral "atoms" of O-helium (OHe), thus creating a specific Warmer than Cold nuclear-interacting composite dark matter. Slowed down in the terrestrial matter, OHe is elusive for direct methods of underground dark. However OHe-nucleus interaction leads to their binding and in OHe-Na system the energy of such level can be in the interval of energy 2-4 keV. It explains the results of DAMA/NaI and DAMA/LIBRA experiments. The puzzles of direct dark matter searches appear in this solution as a reflection of nontrivial nuclear physics of OHe. (abridged)