IT IS a mystery of cosmic proportions: why is the universe filled with matter and not antimatter? Physicists hoping to find the answer have been left scratching their heads this week by an analysis which claims that some antihydrogen atoms created last year may not be normal antiatoms after all. Instead they may sit on the blurry line between atoms and plasma.
Antihydrogen atoms consist of a positron (a positively charged 鈥渁nti-electron鈥�) orbiting a negatively charged antiproton. Physicists hope that by creating and studying such antiatoms they will discover why the universe apparently contains so much more matter than antimatter.
A major step towards this goal was achieved in 2002, when researchers running the ATRAP and ATHENA experiments at the CERN particle physics laboratory near Geneva, Switzerland, independently reported that they had created antihydrogen atoms for the first time. The researchers believed neutral antihydrogen formed when separate charged clouds of antiprotons and positrons were brought together. The ATRAP experiment went further and probed the internal energy states of these novel atoms (Physical Review Letters, vol 89, p 233401).
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Now Fred Driscoll of the University of California at San Diego (UCSD) claims these energy states match those that would be expected if the atoms were in the form of weakly bound particles known as 鈥渄rift atoms鈥�. 鈥淭hese are on the borderline between plasma and atoms,鈥� Driscoll says.
According to his calculations, positrons in drifting antihydrogen orbit around antiprotons some 25 million times more slowly than they would in a normal antihydrogen atom. What鈥檚 more, the force holding the drift antihydrogen atoms together is around 5000 times weaker than the energy that holds together a typical hydrogen atom. However, Gerald Gabrielse, a member of the ATRAP team, says more work is needed to confirm the match Driscoll sees between the theory that describes drift atom formation and the measured energy states of antihydrogen.
Even if Driscoll鈥檚 analysis turns out to be right, it is not all bad news for the experimenters. A key aim for ATRAP is to make drift atoms relax into a strongly bound state that will make them antimatter mirrors of typical hydrogen atoms. Without a theory to describe the way the antihydrogen atoms behave, it has proved difficult to develop techniques to manipulate them or change their state. The drift atom theory could change that. It even suggests a way to distinguish the desired antiatoms from the drift atoms, says UCSD physicist Tom O鈥橬eil, who with Michael Glinsky came up with the idea of drift atoms.
Drift atoms should be more susceptible than strongly bound atoms to an electric field that is stronger at one end of the atom than at the other. This is because the antiproton and positron are further apart in a drift atom, so one end of the atom reacts more to an uneven field than the other.
Driscoll believes his analysis also applies to antihydrogen produced by the ATHENA experiment. But without data on the energy states of those particles it is hard to tell. Rolf Landua, who heads the ATHENA team, insists the neutral objects seen in that experiment were not drift atoms because they moved at right angles to an applied magnetic field, and drift atoms tend to move along magnetic field lines. But Driscoll believes that drift atoms could travel at right angles to a magnetic field if they happened to form moving in that direction.
