Cooling Technique Improves Antiproton Mass Measurement

Scientists from CERN’s ASACUSA experiment have announced a new precision measurement of the mass of the antiproton relative to that of the electron, a result based on spectroscopic measurements of approximately two billion antiprotonic helium atoms cooled to temperatures near absolute zero.

“A pretty large number of atoms containing antiprotons were cooled below -271°C,” says Masaki Hori, spokesperson for the ASACUSA collaboration. “It’s kind of surprising that a half-antimatter atom can be made so cold by simply placing it in a refrigerated gas of normal helium.”

In antiprotonic helium atoms, an antiproton takes the place of one of the electrons that would normally be orbiting the nucleus. Such measurements, according to scientists, provide a unique tool for comparing with high precision the mass of an antimatter particle with its matter counterpart. The two, researchers say, should be strictly identical.

Matter and antimatter particles are always produced as a pair in particle collisions. Particles and antiparticles have the same mass and opposite electric charge. The positively charged positron, for example, is an anti-electron, the antiparticle of the negatively charged electron. Positrons have been observed since the 1930s, both in natural collisions from cosmic rays and in particle accelerators. However, studying antimatter particles with high precision remains a challenge, however, because matter and antimatter annihilate when they come into contact, disappearing in a flash of energy.

In order to make measurements with these antiprotons, many experiments trap them for long periods using magnetic devices. But the ASACUSA approach is different. Its researchers create very special hybrid atoms made of a mix of matter and antimatter—antiprotonic helium atoms composed of an antiproton and an electron orbiting a helium nucleus. Researchers make these by mixing antiprotons with helium gas. In this mixture, about 3 percent of the antiprotons replace one of the two electrons of the helium atom. In antiprotonic helium, the antiproton is in orbit around the helium nucleus and protected by the electron cloud that surrounds the whole atom, making antiprotonic helium stable enough for precision measurements.

The measurement of the antiproton’s mass is done by spectroscopy, by shining a laser beam onto the antiprotonic helium. Tuning the laser to the right frequency causes the antiprotons to make a quantum jump within the atoms. From this frequency, the antiproton mass relative to the electron mass can be calculated. This method has been successfully used before by the ASACUSA collaboration to measure with high accuracy the antiproton’s mass. However, the microscopic motion of the antiprotonic helium atoms introduced a significant source of uncertainty in previous measurements.

The major new achievement of the collaboration, as reported in Science, is that researchers have 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. In fact, researchers improved the measurement of the transition frequency by a factor of 1.4 to 10 compared with previous experiments. Scientists conducted experiments from 2010 to 2014, with about two 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. To date, no difference has been found between their masses, but pushing the precision limits of this comparison is a very important test of key theoretical principles such as the CPT symmetry. CPT is a consequence of basic symmetries of space-time, such as its isotropy in all directions. 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.

ASACUSA scientists say the collaboration is confident that it will be able to further improve the precision of antiproton’s mass by using two laser beams. In the near future, the start of the ELENA facility at CERN will also allow the precision of such measurements to be improved.