Earlier this year, a research collaboration at CERN announced that it had created a few dozen atoms of antihydrogen, the antimatter equivalent of the more familiar hydrogen atoms. These anti-atoms were kept in existence for just under 200 milliseconds before they annihilated in collisions with the container walls. Now, the same team is back with the announcement that it has created hundreds of atoms of antihydrogen, some of which were kept around for over 15 minutes—long enough to start contemplating doing some serious science with them.
The trap used to catch the antihydrogen is the same one used in the last experiment. It uses superconducting magnets to keep the antiatoms away from the container walls, taking advantage of the tiny magnetic moment created by the spatial distance between the antiproton nucleus and the positron (antielectron) orbiting it. The differences are so tiny that the trap will only work if the antihydrogen has an energy of 50μeV (micro electron Volt), which makes for quite a challenge, since the antiprotons start the process at 3keV. The big increase in the number of trapped atoms has largely come about through better ways of slowing down and cooling the starting materials.
The antiprotons are generated by bombarding a static target with a proton beam. The output is gathered and fed into a bit of hardware that is a bit unusual at CERN: a particle decelerator. These are slowed and cooled further by sending them through a cloud of cold electrons, after which some antiprotons are allowed to evaporate off, carrying even more energy with them; further evaporative cooling takes place after mixing with an excess of positrons. The team behind the work even figured out a way to push the positrons into a cooling chamber while imparting as little excess energy as possible.
Even so, most of the 6,000 antihydrogen atoms that are generated by a typical mixing are too energetic and bump into the trap's walls, ending their short lifetimes. Just a bit over half of the typical experiments result in an atom getting caught in the trap and being available for further studies.
It's not just the kinetic energy of the atoms that limits our ability to study them. The positrons are initially bound very weakly in the outer orbitals of the antiatom, and take a bit of time to reach the ground state by emitting photons. So keeping them around for longer is essential if we're going to study antihydrogen in its native state. Fortunately, the authors calculate that over 99 percent of the antiatoms will be in the ground state in a half a second, and there's so little gas contaminating the trap that the antiatoms should survive for hundreds of seconds on average.
In fact, they show that seven antiatoms survived all the way out to 1,000 seconds, or a bit over 15 minutes. In three tries, they also saw a single antihydrogen atom make it to 2,000 seconds, or over a half-hour. There's little doubt that these atoms spent most of their time in the ground state.
Most of the rest of the paper is spent comparing the predicted behavior of antihydrogen with where it eventually ran into the trap walls, creating a signal that the authors could locate. This data confirms that the antimatter is being created exactly as planned, and provides some information on how the trap is operating.
There's a lot of room to slow down the antiparticles a bit further before putting them into the trap, which would boost the efficiency considerably. Even so, efficiencies are already at the point where the authors suggest we can start doing detailed studies of antihydrogen, looking for ways in which it might differ from its regular matter counterpart. These include spectroscopic measurements to determine if the orbitals occupied by the positron are at identical energy levels to those in hydrogen.
The antihydrogen also survives long enough that we might start to consider performing some laser cooling to slow its kinetic energy down even further. This could slow the antiatoms enough that they'll become subject to measurable gravitational impact. In both these cases, theory predicts that antimatter atoms will behave precisely as regular matter does, with regularly spaced atomic orbitals and a weak, but measurable, pull from gravity. Any deviations from the expectations would have profound implications for physics, so you can bet that the team at CERN is eager to get started on this work.
Listing image by Image courtesy of CERN/ALPHA