After repeated pounding, antihydrogen reveals its charge: Zero

The charge is identical to that of the hydrogen atom, once again demonstrating that the properties of antimatter and matter are mirror images of one another.

A non-zero charge would have meant that the antiproton in the nucleus and the positron buzzing around it have slightly different charges, which would violate the rules of the Standard Model of particle physics and possibly provide an explanation for the dominance of matter over antimatter in the universe.

"The asymmetry of matter and antimatter in the universe is one of the most important outstanding problems with the Big Bang theory, which is otherwise very successful," said Joel Fajans, a professor of physics at the University of California, Berkeley, and a leader of the experiment. "Our experiment was a long shot to see if there are differences between matter and antimatter, in this case hydrogen atoms and antihydrogen atoms. Both should be neutral."

Theoretically, matter and antimatter should have been created in equal quantities at the birth of the cosmos in the Big Bang, 13.8 billion years ago. Yet today, antimatter is rare in the universe, leading physicists to search for minute violations of the known laws of physics that could explain the asymmetry.

"In a sense, this is the first precision measurement done on antihydrogen, because the measurement exceeds anything that could be inferred from previous measurements," Fajans said. "People had separately set bounds on the charge of the antiproton and the positron, which are opposite and, experimentally, approximately equal. But with this paper, we have improved on the bound obtained by adding the measured charge of the antiproton and positron."

The charge is zero to within 0.7 parts per billion, a limit 20 times smaller than previous measurements.

The experiment also allowed the researchers to calculate the charge of the positron, which is the same -- except for the sign of the charge -- as that of the electron, within 1 part in a billion. This limit is 25 times better than previous measurements.

The results were published in the Jan. 21, 2016 issue of the journal Nature. The ALPHA collaboration at the European Organization for Nuclear Research in Geneva, Switzerland, is led by Jeffrey Hangst of Aarhus University in Denmark.

Searching for differences between matter and antimatter

Fajans, UC Berkeley physics professor Jonathan Wurtele and their ALPHA colleagues have probed antihydrogen in previous experiments to search for violations of the Standard Model, so far to no avail. One such attempt, to discover a difference between the gravitational attraction of matter and antimatter, will be tested with more precision thanks to a new $15 million grant to ALPHA from Canada and Denmark to look for gravitational anomalies in antihydrogen atoms.

In the most recent experiment, conducted at the end of 2014, Fajans and Wurtele employed a novel technique called stochastic acceleration, which is more sensitive than more direct methods. They trapped antihydrogen atoms as in previous experiments, but this time pulsed them repeatedly with an electric field to try to bump them out of the trap. If the antiatoms are really neutral, these fields would have no effect.

"We continually pounded on the antihydrogen with an electric field, randomly, about 80,000 times. If they were charged, knocking them back and forth, back and forth would eventually give them enough energy to escape out of the trap," he said. "The antihydrogen remained in the trap, allowing us to set a bound on what the charge could have been."

He compared this technique to bumping a balloon around a football stadium, hit repeatedly by hundreds or thousands of fans. Without air friction to slow it down, the balloon would eventually zoom out of the stadium.

Thanks to years of work by the 50 or so ALPHA scientists and students, the ALPHA experiment is now at a critical juncture, Fajans said.

"We've gotten to the point where we can confidently and reliably do experiments on trapped antihydrogen, but it has taken us thousands and thousands of hours to get to this point," he said. "It opens a new era of precision measurement on antihydrogen."

UC Berkeley graduate students Marcello Baquero-Ruiz and Len Evans and lecturer Andrew Charman worked with Fajans, Wurtele and the ALPHA team to obtain the experimental data and analyze it over the past year. The UC Berkeley team was supported by the U.S. National Science Foundation and Department of Energy.