The proton is slightly lighter than previously believed, according to the most accurate measurement so far of this particle’s mass.
Protons weigh slightly more than one atomic mass unit (defined as one-twelfth the mass of a carbon-12 atom) and scientists have been steadily refining the precision of this measurement. Florian Köhler-Langes of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, and his colleagues measured the mass of a proton relative to that of a carbon-12 nucleus, by comparing how the particles moved inside an electromagnetic trap.
They achieved a precision of 32 parts per trillion in their result, making it about three times better than previous attempts. They found the proton to be about 296 parts per trillion lighter than the current accepted value. Researchers want to compare this measurement with the mass of antiprotons. Any resulting discrepancy between the proton and its antiparticle could help to explain why the Universe contains vastly more matter than antimatter.