Composite dark matter from the fourth generation

The hypothesis of a heavy stable quark of the fourth family can provide a nontrivial solution for cosmological dark matter if baryon asymmetry in the fourth family has a negative sign and an excess of (U) over bar antiquarks with charge (-2/3) is generated in the early Universe. Excessive (U) over bar antiquarks form ((U) over bar(U) over bar(U) over bar) antibaryons with electric charge -2, which are all captured by He-4 and trapped in a [He-4(++)((U) over bar(U) over bar(U) over bar)(--)] O-helium OHe atom as soon as 4 He is formed in Big Bang nucleosynthesis. Interaction of O-helium with nuclei opens a new path to the creation of heavy nuclides in Big Bang nucleosynthesis. Due to the large mass of the U quark, OHe atomic gas decouples from baryonic matter and plays the role of dark matter in large-scale structure formation with structures on small scales being suppressed. Owing to nuclear interaction with matter, cosmic O-helium. from the galactic dark matter halo is slowed in the Earth below the thresholds of underground dark matter detectors. However, an experimental test of this hypothesis is possible in the search for OHe in balloon-borne experiments and for U hadrons in cosmic rays and accelerators. OHe atoms might form anomalous isotopes and could cause cold nuclear transformations in matter, offering a possible way to exclude (or prove) their existence.