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By the late 1930s it was well established that matter in our everyday world essentially consists of electrons, protons and neutrons. However, the existence of antimatter, including positrons, antiprotons, antineutrons, antideuterons, antitritium and antihelium 3, had also been established in the laboratory. Scientists today are still puzzled about the apparent imbalance of matter and antimatter in the Universe. Why shouldn't the Universe contain equal amounts of matter and antimatter? Big Bang theory for the origin of the Universe makes hypothesis that in the first instants of its existence matter and antimatter had to be present in equal amount.

 

Pre-PAMELA results
Pre-PAMELA results 
Therefore, detection of antimatter of primary origin in cosmic rays would be a discovery of fundamental significance. If there is primordial antimatter, antihelium is the most likely form to be detected in cosmic rays, likewise in matter primordial nucleosynthesis in which helium is the next most abundant element to hydrogen. The detection of antinuclei with Z>2 in cosmic rays would provide, instead, direct evidence of the existence of antistellar nucleosyntesis. However, on the basis of gamma-ray observations and other considerations, the coexistence of condensed matter and antimatter on scales up to clusters of galaxies has been virtually ruled out. On the other side, no observation presently excludes the possibility that a small fraction of high energy cosmic rays from nearby superclusters, escaped from a distant antigalaxy and traversed the intergalactic space filled by turbulent magnetic field, enters the Milky Way against the galactic wind and finds its way to the Earth. Nevertheless, no quantitative assessment of the probabilities is available. PAMELA is searching for antinuclei in a wide energy range with a sensitivity on the antihelium/helium  ratio better than some units of 10-7.

Antiprotons and positrons are not direct indicators for the existence of antimatter domains, because they are primarily produced by collisions of the cosmic rays with the ISM. Evaporation by the Hawking process of primordial black holes, produced very early in the hot Big Bang, in the quantum gravity era, and exotic particles annihilation might give other possible contributions. The signature of antiprotons and positrons coming from distant antigalaxies would appear as a distortion on the detected secondary production fluxes.

 

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