One of the bearing walls of modern physics is that particles of antimatter and matter are perfect counterparts, down to their mass. That wall is standing strong, according to new results presented this week at a meeting of the American Physical Society's Division of Atomic, Molecular, and Optical Physics in Santa Fe, New Mexico. The international team has caged a proton and antiproton in a trap and deduced they have the same mass to within a part in 10 billion.
Some speculative theories predict that matter and antimatter could have slightly different masses, so Harvard University physicists Gerald Gabrielse and Anton Khabbaz, with collaborators from the University of Bonn in Germany and elsewhere, decided to check. The team trapped a single antiproton from the LEAR accelerator at Europe's CERN laboratory near Geneva, Switzerland, in a web of electric and magnetic fields, where it spun in circles like a firefly in a jar. The researchers also introduced a negative hydrogen ion (a proton with two electrons circling around it) to the same trap. Protons and antiprotons have opposite charges, but the hydrogen ion has the same charge as an antiproton, which makes the two easy to compare.
To see if their masses differed, the team watched how fast the particles raced around inside the trap. If one particle were heavier, it would take a little while longer to make an orbit. They used tiny electrodes to check. "Each time the particle passes one electrode ... it induces a current to pass through a resistor, and that current we amplify and measure," Khabbaz explains.
The group found that the two raced around in almost identical circles, about 100 micrometers across, 90 million times per second, and concluded--after correcting for the tiny mass of the two electrons--that the proton and antiproton had the same mass to about 10 decimal places, a factor of 10 times better than previous measurements. "[The precision of] this result is extraordinary," says Jook Walraven, a physicist at the Institute for Atomic and Molecular Physics in Amsterdam.