Making The Paper

摘要

Seventy years ago, scientists first calculated that galaxies must contain additional, undetectable sources of mass-up to five times the mass of the detectable gas and stars. Piergiorgio Picozza, a physicist at the University of Rome Tor Vergata in Italy, has spent his career searching for this invisible 'dark matter' which is proposed as the source of the added mass, and he might now have found evidence for it.rnPicozza has been investigating the formation of antimatter in space. Antimatter consists of particles that have the same mass as electrons and protons, but opposite properties such as charge. For example, the positively charged positron is the antimatter counterpart of the electron. Positrons can be produced by 'secondary processes', such as cosmic-ray nuclei smashing into interstellar dust, which occur at relatively low energies, but they might also arise directly from 'primary sources', such as dark-matter annihilations, that could generate positron-electron pairs at high energies. The latter process has not yet been confirmed. So a better understanding of positron formation could indicate the presence of dark matter. "A very important part of our job is to disentangle the sources of positrons," says Picozza.