A new particle, the cosmion, has been proposed to be the dark matter of the Universe and to explain the solar ν deficit by cooling the solar core to reduce ${}_{}{}^8{}_{}{}^{}\mathrm{B}$ ν production. Such cosmions in the galactic halo would scatter from nuclei in terrestrial detectors. Measurements were made in Si ionization detectors in a very-low-background environment down to energies of 1.1 keV. These results exclude nearly all of the mass range possible for cosmions with coherent nuclear interactions.

• Received 5 June 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.1305