Antimatter is powerful stuff, but is it a genuine mirror image of matter? The coldest antiprotons ever made take us a step closer to finding out.
The simplest antiatom, antihydrogen, is made of an antiproton and a positron, the electron's antimatter counterpart. If it is a mirror image of hydrogen, it should emit an identical spectrum of light. The way to test this is to create ultra-cold antihydrogen, allowing it to be trapped and studied. The hard part is cooling the antiprotons.
The ALPHA team - one of two racing to trap antihydrogen using the Antiproton Decelerator at the CERN laboratory near Geneva, Switzerland - uses electrons to do this. The flaw in the method is that the electric fields needed to remove the electrons afterwards reheat some of the antiprotons. This meant ALPHA's antiprotons never got below an average temperature of about 100 kelvin - not cold enough to produce antihydrogen that can be trapped.
Now the team has come up with a potential solution: after removing the electrons, release the most energetic antiprotons. This yielded antiprotons with an average temperature of 9 K which, when combined with positrons, stand a good chance of producing antihydrogen that is cold enough (Physical Review Letters, in press). "We were a bit surprised how well this works," says ALPHA spokesman Jeffrey Hangst.
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