Search for Anti-matter in Primary Cosmic Rays

Abstract

COSMIC rays provide the only sample of the material outside the solar system which is available for examination and it even seems likely that some part of the cosmic ray flux originates outside our own galaxy. In the circumstances, it is understandable that there have been several searches for anti-nuclei in primary cosmic rays, partly so as to throw light on the question of whether the universe is symmetrical in matter and antimatter. An upper limit of p⁻/p⁺ ⩽ 3 × 10⁻⁴ has been established¹ for energy E⩽200 MeV while an upper limit on the fraction of anti-nuclei to nuclei with charge Z⩾2 and energies less than 700 MeV/nucleon has been put^2,3 at 0.6%. All these measurements have been restricted in their energy range by the need to observe particles stopping in nuclear emulsions. Even if matter and anti-matter co-exist on the scale of the universe, it is nevertheless likely that the two forms are segregated on a galactic scale, so that the fraction of particles coming from outside the galaxy is likely to be greater at greater energy. It follows that the higher the energy at which the search is conducted, the more significant will be any upper limit that can be established for the amount of anti-matter. We report a new method which we have used to search for anti-nuclei of charge Z⩾6 in the energy range 5.0≲E≲9.0 GeV/nucleon.