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Impearls: Antihydrogen makes the big time

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Earthdate 2002-11-22

Antihydrogen makes the big time

Interesting news item (link requires subscription or pay per view) by Charles Seife in the journal Science (organ of the American Association for the Advancement of Science) on the race to be first to make significant quantities of antihydrogen — enough to study and hopefully find the spectrum of that most volatile of substances.  (Antihydrogen is the negative subatomic mirror of normal hydrogen, consisting of duos of antiprotons and antielectrons [positrons] electronically united as antiatoms, though apparently not, at least for the moment, as antihydrogen molecules.)

Achieving the holy grail of the spectrum of antihydrogen, “could rattle the foundations of physics and will likely net a Nobel Prize for whichever team gets there first,” Seife writes.  “Scientists think that antihydrogen's spectrum should be identical to hydrogen's.  If it's not, a key principle in physics known as CPT symmetry will have to be discarded, forcing a drastic revision of physicists' understanding of subatomic particles.”

Neither of the two teams now closing in on the prize of the spectrum has yet achieved this goal, but that “shouldn't detract from the real story: the production of significant amounts of cold, slow antihydrogen.  ‘The fact that both groups have gotten antihydrogen is a major accomplishment,’” Seife quotes MIT physicist Daniel Kleppner.

The two teams, known as ATRAP and ATHENA, which are in the running are both using “similar techniques, nearly identical equipment, and the same particle beam, an antiproton factory at CERN, the European particle physics laboratory near Geneva.”  Both groups, Seife reports:

… had access to the same beamline, the Antiproton Decelerator (AD) at CERN, which takes protons created at near the speed of light down to about 10% of that speed.  Drawing on years of experiments with AD's predecessor, LEAR, both groups settled on electromagnetic bottles known as Penning traps to cool antiprotons down to about 4 kelvin, confine them with antielectrons, and induce the two to combine.

ATHENA and ATRAP follow the same basic recipe for antimatter.  Each gets its antiprotons from AD and its antielectrons from a radioactive sodium isotope that emits the particles as it decays.  Each captures the antiprotons, cools them to a few kelvin, and shoots them and the antielectrons into opposite ends of a trap where they can mix.

The traps — meter-long cylinders that corral the particles with electromagnetic fields — face a daunting challenge:  Because antiprotons and antielectrons have opposite charges, a potential that captures antielectrons repels antiprotons and vice versa.  That makes it difficult to build a single trap that can hold both of them, because a trap that appears like a valley to an antielectron looks like a hill to the antiprotons.  To bring the particles together, both teams use a trap within a trap — in effect, two hills framing a valley (from the antielectrons' point of view) or two valleys flanking a hill (from the antiprotons' point of view)….  When an antielectron binds to an antiproton (something that occurs with only a handful of the thousands of cold antiprotons in each shot), the resulting neutral antiatom can no longer be easily controlled by electric or magnetic fields.  It escapes the trap and floats away.

That's where the big differences start.  To tell that they've created antihydrogen, the ATHENA physicists look for gamma rays that are produced when an antihydrogen atom is annihilated by collisions with ordinary matter.  By subtracting the background gamma rays from their total count, they can estimate the number of antiatoms.  Gabrielse's ATRAP team, by contrast, lets the untrapped neutral antihydrogens float into an additional trap, which tears apart the antihydrogen atom.  The team then counts the liberated antiprotons as they annihilate on contact with ordinary matter.  As a bonus, the ionization trap also yields information about how tightly the antielectron is bound to the antiproton.

Unfortunately, the two teams are now “frozen neck and neck,” as Seife put it, in their race.  He writes:  “… the teams' source of antiprotons has just dried up.  The current run of CERN's beamline just ended, so both teams will have to wait until next year to resume the race.  And the budget problems at CERN … will interfere with their scramble to collect antihydrogen.  ‘[AD] will be shut down for 1 year, which is a huge disappointment,’” Seife quotes ATRAP leader Harvard physicist Gerald Gabrielse.  “‘Without antiprotons, it's hard to make progress.’”

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