Antiproton successfully trapped for first time

The ALPHA Project, of which 15 Canadians are a part, is working at CERN’s antiproton decelerator facility in Switzerland. The team has, for the first time, been able to capture and hold atoms of antihydrogen, allowing them to be studied in detail.  This will allow future experiments to proceed, including examining properties like the absorption spectrum, of the antihydrogen atoms.

The results were published today online at the journal Nature.

Where hydrogen is the simplest atom, with a proton and electron, antihydrogen consists of an antiproton and an antielectron (positron). To create the antihydrogen, the team uses the antiproton beam and antielectrons from the decay of a radioactive source.

Our colleagues at the Science Media Centre of Canada have gone to Canadian experts for the following comment:

Prof. Scott Menary, of York University, who works on the ALPHA project,comments on the achievement’s significance:

“The point is that it’s a technical milestone that’s really a huge step towards realizing what is in principle an incredibly important measurement, and an extremely difficult measurement.

“The goal of everything is to compare matter and antimatter. At the Big Bang, matter and antimatter were created in equal amounts. It’s pure E=mc2. From pure energy, you produced mass, but if you produced an electron, you had to produce and anti-electron in equal amounts. We think the big bang is a pretty accurate picture of things – well, we know it is. But the problem is that if you look around the universe, there doesn’t seem to be any antimatter left, and the question is, why is that? It’s the central question.

“If you want to answer that question, what you need to do is compare matter and antimatter as precisely as you can. The most precisely known matter system is hydrogen. So if we can study antihydrogen at the same level, then we would have the most precise comparison possible. That’s the real power here. We have no idea whether we’ll see a difference or not.

“The reason for the article today is that matter and antimatter annihilate when they come in contact. And since the detector and everything is made of matter, we have to keep it away or we lose it.

“To do that, you use magnetic fields. That holding pattern is very weak, so we have to produce it with very, very low energy. If it has a lot of speed, it just flies away. Most of the last 15 number of years has been spent trying to produce this stuff at low enough energy that we could hold it in this magnetic trap. So that’s the big deal here. We finally managed to hold it. In principle, there’s not really a limit to how long we can hold it, but we’ve held it long enough to do the tests we want to do. ”

Rob McPherson, Professor at the University of Victoria and Principal Research Scientist at the Institute of Particle Physics, comments:

Is this a significant development in the study of matter and antimatter?

“We don’t know if studying antihydrogen is the right technique because we don’t know why there’s more matter than antimatter in the universe, and until we know that, we don’t really know what measurement is the right one to make. But this is an important breakthrough and it does have unique sensitivity to the possible theoretical reasons why there is more matter than antimatter.

“In the competing approaches, there are also unique measurements that are made at what are called the “B-factory” projects. One ran up until just a few years ago, from Stanford, called BaBar, and there was also an experiment in Japan called Belle. Both of them have stopped taking data and are performing analyses now. These study with unprecedented precision quark and antiquark properties, and those measurements are another way that we can look at difference between matter and antimatter. There is a proposal for the next generation of these, one in Japan and one in Italy.”

What are the benefits of one approach over the other?

“There are many things one approach would tell us but the other wouldn’t. There are many theories explaining why matter and antimatter are different. It depends on which of the theories are correct, and since we don’t know the right answer, we don’t really know the right experiment to build to discover the answer.

“One of the best studied bits of anything in science right now is the hydrogen atom. So now, if we do the same measurements of the antihydrogen atom, and compared the two, maybe that will lead us to a better understanding of the matter/antimatter difference.”

Next steps?

“Next step is to get more of the antihydrogen atoms and performing careful measurements of their spectroscopic energy levels. They said they have 38, but measurements with millions or billions might be needed to understand their differences from hydrogen atoms.

“Precise spectroscopic measurements will be first. You get the atomic electrons into excited states, (you can heat them up, use microwaves or you can shoot a laser at them,) and then you just watch them with a fine instrument that measures the photons they emit. And you very precisely measure the emitted photon energies as they de-excite. If we do these same measurements in the antimatter system, we may see some of the ways that antimatter behaves, versus matter. Initially, for the antihydrogen you just see if you hit an absorption resonance which kicks the antihydrogen out of the trap and then you see it annihilate. These absorption resonances can be compared with those in hydrogen.

“Something that’s a little bit of a holy grail in this field, and hydrogen atoms may be the best way to do this, is the effect of gravity on antimatter. It’s a hard force to study, and it’s thought that one of the differences between matter and antimatter could be how gravity attracts it.

“I would say that’s the next really important measurement to do with the antihydrogen.”

Dr. William Trischuk, Director of the Institute of Particle Physics and Professor of Physics at the University of Toronto, comments:

“The fact that they can trap them in significant numbers is incredible progress. Someday, the antihydrogen atom will be understood much better. At this stage, when we’re trying to understand why we’re all matter, and where did all the antimatter go from the big bang, they’re not quite there yet. But this is an important step along the way, no doubt.

“At this stage, the most precise measurements of things come from understanding the hydrogen atom, and I have no doubt that one day, the most precise measurements of matter and antimatter asymmetry will come from understanding the antihydrogen atom. But they’re not there yet.”

What about other methods of studying antimatter?

“Quarks have been chased down to parts-in-ten-thousand level. Through the 1990s until about a year or two ago, there were two big “B-factory” projects, and they’ve given us the most precise insight. But it’s not enough to explain the anti-matter asymmetry that apparently exists in the universe. We know it very precisely, but it’s too small to explain why so much matter was left over in the universe after the big bang.

“I think the very promising thing is that hydrogen is so well understood. [Quarks ] are only understood at the parts-in-ten-thousand level, but hydrogen is understood at a parts-in-a-billion level, from theory, from experiment. Once you get the study of antihydrogen down to the one-part-in-ten-thousand level, it offers a whole plethora of tests, and I think there will be very interesting measurements one can make.

“It’s an incredible piece of work, and it’s very gratifying to see the strong Canadian involvement in it.”