PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 611 October 29, 2002   by Phillip F. Schewe, Ben Stein, and James
Riordon

THE INTERNAL STATES OF ANTI-HYDROGEN have been studied, for the first time,
by the ATRAP collaboration, working at CERN where antiprotons are slowed and
then joined with positrons to form anti-hydrogen atoms within a detector-
and electrode-filled enclosure (a nested Penning trap) over the past year.
This work suggests that anti-hydrogen is preferentially formed in an excited
state via a three-body process when two positrons and an anti-proton
collide.  Only a month ago the ATHENA collaboration, working at the same
CERN facility, made the first report of cold anti-H atom detection (Update
605: http://www.aip.org/enews/physnews/2002/split/605-1.html) using
techniques largely pioneered by ATRAP (the antiproton accumulation
techniques, the nested Penning trap used, the positron cooling approach,
etc.)  So what has changed since then?
Three things. (1) First of all, the ATHENA detection of anti-atoms is
indirect.  The presumed presence of the anti-atom (positron plus antiproton)
is registered by a dual annihilation of the positron with an electron and
the antiproton with a nearby proton.  Complicating the detection scenario is
the fact that the proton-antiproton annihilation itself sometimes spawns
positrons which (when they annihilate in their turn) could falsely indicate
the prior presence of an anti-atom.  This class of events constitutes a
background which must be subtracted out in the analysis process, and it
precludes one from identifying any particular double-annihilation event as
having been a genuine anti-hydrogen (sometimes written as an H with a bar
over it).
By contrast, the ATRAP direct detection process unambiguously identifies
H-bar in a process called field ionization, which works as follows.  Having
formed in the center of the enclosure, neutral anti-atoms are free to drift
in any direction.  Some of them annihilate but others move into an
"ionization well," a region where strong electric fields tear the H-bar
apart.  Negatively charged antiprotons not in the company of a positively
charged positron cannot reach the well.  Once there, though, the field
sunders the atom, and the antiprotons are trapped in place, leaving the
positron to move off and annihilate elsewhere.  By counting the number of
antiprotons one knows how many anti-atoms had arrived at the well.  Every
event represents an anti-atom.  (Picture at
http://www.aip.org/mgr/png/2002/168.htm )
(2) Moreover, one can now make a statistical study of the electric field
needed to ionize the positron and deduce from this, in a rudimentary way,
some information about the internal energy states of the H-bar. Thus the
internal properties of an anti-atom have been studied for the first time.
The observed range in principal quantum number n (n=1 corresponding to the
ground state, or lowest level) goes from 43 up to 55.
(3) Finally, another thing that is different in this experiment is the much
higher rate of anti-H production.   The collaboration spokesperson, Gerald
Gabrielse of Harvard (617-495-4381, US cell 617-834-7929, CERN
41-22-767-9813, CERN cell 41-79-201-4281),  gabrielse{at}physics.harvard.edu)
says that more anti-H atoms can be recorded in a few hours than have been
reported in all previous experiments.
The ultimate goal of these experiments will be to trap neutral cold
anti-hydrogen atoms and to study their spectra with the same precision
(parts per 10^14 for an analysis of the transition from the n=2 to the n=1
state) as for plain hydrogen.  One could then tell whether the laws of
physics apply the same or differently to atoms and anti-atoms. (Gabrielse et
al., Physical Review Letters, probably to be published online Oct 30; other
ATRAP contacts are Walter Oelert at Forschungszentrum Julich,
49-2461-61-4156, CERN 41-22-767-1758, cell 49-1787-19-0524; Jochen Walz at
the Max Planck Institute for Quantum Physics, 49-89-32905-281, CERN
41-22-767-9813; Eric Hessels at York University, CERN 41-22-767-9813; ATRAP
website: http://hussle.harvard.edu/~ATRAP/).
In recent work ATRAP sees a further increase in the antihydrogen production
rate by using a small radio transmitter to heat antiprotons into making
repeated collisions with cold positrons.  With this higher production rate,
they are able to make the first measurements of a distribution (not just the
range) of excited states of antihydrogen.  (For an early background article,
by Gabrielse,  see Scientific American, Dec 1992.)

TESTING NEW PHYSICS WITH NOTHING.   To detect new forces, particles, and
dimensions in a sub-micron-sized force experiment, physicists must
inevitably confront the Casimir force, an exotic quantum phenomenon in which
empty space can push together a pair of metal plates.  Empty space, or the
"vacuum," is actually teeming with fleeting particles and electromagnetic
fields.  But in between a pair of narrowly spaced plates, the vacuum does
not pack energy as densely as it does outside the plates.  Just as an
underground tunnel blocks AM radio signals with wavelengths that are bigger
than the opening of the tunnel, the metal plates keep out electromagnetic
fluctuations with wavelengths greater than the distance between the plates.
And just as the invisible atmosphere pushed together Otto von Guericke's
pair of evacuated hemispheres so strongly in his 1600s demonstration that
even horses could not pull them apart.  The more energy-dense vacuum outside
the plates pushes together the metal plates, because they enclose a less
energy-dense vacuum, although this effect occurs much more subtly than the
1600s demonstration.  This vacuum pressure, which has been confirmed
experimentally (Update 300), can become large enough at short separations to
conceal the effects of new physics.
To overcome this problem, theorists at Purdue University and Wabash College
(contact Dennis Krause, kraused{at}wabash.edu) propose to exploit a key fact:
the metal material itself influences the strength of the Casimir force,
primarily through electronic interactions between the metal and the vacuum.
On the other hand, the plates' interaction with any new forces, particles,
or dimensions would likely depend on the metal's nuclear as well as
(Continued to next message)

---
 þ OLXWin 1.00b þ Each day a day goes by.