Phys. Rev. Lett. 103, 132001 (2009)

A collaboration of physicists from the Tevatron Collider's D0 detector at the Fermi National Accelerator Laboratory in Batavia, Illinois, has successfully measured the mass of a top quark and its 'antitop' partner.

The fundamental symmetries of nature dictate that a particle and its antiparticle should have the same mass. But testing this prediction for quarks has been difficult because the tiny particles usually combine to make heavier things (such as protons and neutrons) before their mass can be measured. The top quark decays before combining with other quarks, making it easier to measure directly.

The team found a mass difference of 2.2% +/− 2.2. In other words, top quarks and antitops seem to have the same mass.