Researchers at CERN have created and trapped antihydrogen in an attempt to study the underpinnings of the standard model of physics. Antihydrogen is made of antiparticles, specifically an antiproton and a positron, instead of the proton and an electron that are present in natural hydrogen. It has the same mass but opposite charge of its normal matter counterparts.

Antimatter has a bad reputation for being dangerous because it annihilates on contact with regular matter, releasing prodigious amounts of energy. However, the clever Ars reader will note that they have not been annihilated by the antimatter produced at CERN. The reality is that if you gathered all of the antimatter CERN has ever created, you wouldn't garner enough energy to power your laptop through reading this article.

The Universe seems to be made of mostly regular matter, so any antimatter encounters matter and is annihilated immediately after it has been created. Production and detection of cold antihydrogen atoms also happened at CERN in 2002, but those were short-lived. The new Nature letter describes how to overcome the difficulty of containing antihydrogen so that it isn't immediately destroyed.

Antiparticles behave predictably in the presence of electric or magnetic fields and so can be contained in a special magnetic container called a Penning trap. Antihydrogen, which has no net electric charge, is much harder to contain.

Production begins by creating antiprotons, which are slowed down by the Antiproton Decelerator at CERN, then stored in the magnetic Penning trap and further cooled. Positrons are supplied by a radioactive sodium-22 source and then cooled as well. These two antiparticles are then allowed to mix and, if this is done at low enough energies, they combine to form antihydrogen.

At this point, there is a problem: the antihydrogen is chargeless and can no longer be contained in the magnetic Penning trap. The researchers constructed what they called an ALPHA apparatus, which features a novel superconducting magnetic trap that interacts with the antihydrogen's magnetic moment. The magnetic moment of an individual atom comes from the structure or interaction of the orbiting particle (the positron) and the nucleus (the antiproton) and allows the particle to interact with electric fields in a weak manner. The ALPHA trap can confine antihydrogen in the ground state if it's kept at temperatures of less than half a Kelvin.

One challenge of this experiment is mixing the antiprotons and positrons at relatively low velocities such that antihydrogen can form efficiently. The efficiency is relative; The authors had to mix 10 million positrons with 700 million antiprotons in order to get get 38 certifiable antihydrogen events.

Those 38 certifiable events then had to be distinguished from spurious background events originating from cosmic rays or residual antiprotons left in the trap by running several control versions of the experiment. In order to demonstrate that the antihydrogen has actually been trapped, it is quickly released (172 milliseconds) and the subsequent anti-atom annihilations on the detector's trap walls are detected.

A lot of effort went into simulating and verifying that the annihilations seen come from antihydrogen. Despite these meager returns, this is a valuable physics experiment as a proof-of-principle demonstration of antimatter confinement.

It remains one of the largest unsolved problems in physics today as to why the Universe contains more regular matter than antimatter. Symmetry would suggest the Universe should have produced equal parts matter and antimatter, which would have annihilated—because we are here, we know this was not the case. Now that they have a bit of antihydrogen on hand, the researchers will test fundamental symmetries in nature (charge conjugation/parity/time reversal) by examining the excited states of antihydrogen.

Nature, 2010. DOI: 10.1038/nature09610 (About DOIs).

Listing image by Niels Madsen ALPHA/Swansea