The ASACUSA experiment at CERN reports in Science a new precision measurement of the mass of the antiproton relative to that of the electron. Such measurements provide a unique tool for comparing with high precision the mass of an antimatter particle with its matter counterpart. 

This result is based on spectroscopic measurements with about 2 billion antiprotonic helium atoms cooled to extremely cold temperatures of 1.5 to 1.7 degrees above absolute zero. In antiprotonic helium atoms an antiproton takes the place of one of the electrons that would normally be orbiting the nucleus. 

The measurement of the antiproton’s mass is done by spectroscopy, by shining a laser beam onto the antiprotonic helium. ASACUSA has now managed to cool down the antiprotonic helium atoms to temperatures close to absolute zero by suspending them in a very cold helium buffer-gas. In this way, the microscopic motion of the atoms is reduced, enhancing the precision of the frequency measurement. The measurement of the transition frequency has been improved by a factor of 1.4 to 10 compared with previous experiments. Experiments were conducted from 2010 to 2014, with about 2 billion atoms, corresponding to roughly 17 femtograms of antiprotonic helium. 

According to standard theories, protons and antiprotons are expected to have exactly the same mass. The observation of even a minute breaking of CPT would call for a review of our assumptions about the nature and properties of space-time. 

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