Getting to grips with antihydrogen

In June 2011, the ALPHA Collaboration announced that they had successfully managed to trap and hold atoms of antimatter for 1000 seconds. Last week they announced that their success in changing the internal state of antihydrogen and made the first ever measurement of its spectrum. The Collaboration is now installing an all-new experimental set-up – ALPHA-2 – and shows no signs of slowing down its investigations into the anti-world.


The ALPHA experiment hall.

Newspapers and magazines around the world described the recent ALPHA announcement as the first step towards explaining why antimatter and matter did not cancel each other out in the first instances of creation, that is, why our universe of matter exists. Understanding the behaviour of matter and antimatter can help scientists solve this conundrum. With this in mind, the ALPHA collaboration has begun the study of the antihydrogen spectrum.

So far, the Collaboration has been focused on proving that they can alter the antihydrogen they’ve trapped by using microwave photons to drive a transition between the internal energy levels of the atoms. “A year ago, we couldn’t have imagined that we’d be in this position,” says ALPHA spokesperson Jeffrey Hangst. “We knew we’d trapped antimatter and we had an idea of how to go about an experiment using it, but we had no idea whether we could actually succeed.”

And succeed they did. While the Collaboration’s results are still at low precision (with an uncertainty at 3 decimal places), their new experimental set-up, ALPHA-2, could allow them to measure the antihydrogen spectrum to up to 15 decimal places. “In the ALPHA set-up, we used microwaves in order to flip the spin of the positron in the antihydrogen,” says Jeffrey. “ALPHA-2, on the other hand, will allow us to induce 2 different types of transition in the antihydrogen: using microwave and laser excitation.”

Instead of flipping the spin of the positron, the lasers in the ALPHA-2 set-up will be able to excite the orbit of the positron: taking it from the ground state to the first excited state. These lasers produce the much higher optical frequency required in order to induce this type of atomic transition.

“The question that guides our work is fundamental: does antihydrogen behave the same way as hydrogen?” says Jeffrey. “Perhaps there is no difference between the two, but perhaps there is variance at the 12th decimal place. The new ALPHA-2 set-up will allow us to improve our measurements in order to know for sure.”

by Katarina Anthony