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. In the Galaxy, destruction of OHe and acceleration of free X⁻⁻ can result in anomalous component of cosmic rays. Collisions of OHe atoms in the central part of Galaxy results in their excitation with successive emission of electron-positron pairs, what can explain excessive radiation of positron annihilation line, observed by INTEGRAL. Slowed down in the terrestrial matter, OHe is elusive for direct methods of underground dark matter detection based on the search for effects of nuclear recoil in WIMP-nucleus collisions. 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. The concentration of OHe in the matter of underground detectors is rapidly adjusted to the incoming flux of cosmic O-helium. Therefore the rate of energy release in radiative capture of Na by OHe should experience annual modulations. It explains the results of DAMA/NaI and DAMA/LIBRA experiments. The existence of low energy bound state in OHe-Na system follows from the solution of Schrodinger equation for relative motion of nucleus and OHe in a spherically symmetrical potential, formed by the Yukawa tail of nuclear scalar isoscalar attraction potential, acting on He beyond the nucleus, and its Coulomb repulsion at distances from nuclear surface, smaller than the size of OHe. Within the uncertainties of nuclear physics parameters the values of coupling strength and mass of sigma meson, mediating scalar isoscalar nuclear potential, are found, at which the sodium nuclei have a few keV binding energy with OHe. Transitions to more energetic levels of Na₊OHe system imply tunneling through Coulomb barrier that leads to suppression of annual modulation of events with MeV-tens MeV energy release in the correspondence with the results of DAMA experiments. The puzzles of direct dark matter searches appear in this solution as a reflection of nontrivial nuclear physics of OHe.