PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 287 September 20, 1996 by Phillip F. Schewe and Ben
Stein
THE QUEST FOR ANTIHYDROGEN continues. Last year in a sort of
shotgun wedding the first antihydrogen atoms made in a lab (at CERN)
came about when a few antiprotons were hitched to positrons at the collision
point between a jet of xenon gas and an antiproton beam. However, there
was no chance to study the private lives of the newlyweds because they
quickly annihilated against a nearby wall. To create, trap, and examine anti-
H at leisure will take patience and finesse. The anti-ingredients for making
anti-H are of course anti-protons and positrons. These species have
separately been cooled and trapped. To herd both species simultaneously is
more difficult, however, since traps designed for both positively and
negatively charged particles are usually leaky. As a prelude to making anti-
H atoms, Harvard physicists Gerald Gabrielse
(gabrielse@hussle.harvard.edu) and David Hall have spent some time
working with protons and electrons, seeing if they can get some of these
humdrum particles to combine into ordinary H atoms (which they refer to as
"anti-anti-hydrogen"). Recently they succeeded in trapping both electrons
and protons in a single device which consists of one Penning trap (which
uses a combination of electric and magnetic fields) for the negative particles
nested inside another for the positive particles. Outnumbering the protons
by a factor of thousands to one, the electrons help to cool the protons to a
common temperature of only 4 K. Why so cold? Estimates have shown that
the likelihood of one important reaction---in which two positrons and an
antiproton form an anti-H plus an extra positron to carry off surplus energy--
-improves by 8 orders of magnitude if the temperature is reduced from 300
to 4 K. Besides cooling protons, did Gabrielse make any hydrogen atoms
as a dry run for making anti-H atoms later on? Detecting the neutral H
atoms, Gabrielse says, is quite difficult. Knowing you have anti-H atoms in
the trap will actually be much easier since some of them can be allowed to
escape, whereupon they spectacularly annihilate with any ordinary matter in
their path. The next step is to try his new trap with positrons, which he
obtains from radioactive decays, and antiprotons from the Low Energy
Antiproton Ring at CERN. The Harvard trap may be the final resting place
of the very last antiprotons from LEAR, which will permanently shut down
at the end of the year. (D.S Hall and G. Gabrielse, Physical Review Letters,
2 September 1996.)