Text 722, 70 rader
Skriven 2006-01-31 06:46:24 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 763
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PHYSICS NEWS UPDATE                                                            

The American Institute of Physics Bulletin of Physics News
Number 763 January 30, 2006  by Phillip F. Schewe, Ben Stein

RARE e-/e+/e- STATE.  The best study of the rare "atom" consisting
of two electrons and one positron is being reported.  Positronium
(abbreviated Ps) is a very "clean" two-body object: it consists of
an electron and a positron which after about 150 nanoseconds
annihilate each other.  For studying the theory of quantum
electrodynamics (QED), Ps is in some ways better even than the
hydrogen atom: with pointlike constituents and with no complicating
nuclear forces (the size of the proton and its own internal
structure interject uncertainties into QED estimates of H behavior),
Ps is a simpler, albeit fragile, quantum system.  An even more
fragile "atom" is the tripartite object consisting of two electrons
and one positron.  Ps-, as it is known, is less suitable for QED
studies than Ps, but has the great virtue of being the simplest
three-body system in physics.  Again, it is simpler than H-, H2+,
and He because of  its pointlike constituents and the absence of
nuclear forces.  Ps- is, like Ps, a bound state with discrete
quantum energy states, although only the ground state is calculated
to be stable against dissociation into Ps and a free electron. Very
little is known about Ps- beyond  its lifetime.  Now, a new
experiment carried out at the Max Planck Institute for Nuclear
Physics in Heidelberg has measured the lifetime of Ps- with a
sixfold increase in precision (the new value is half a nanosecond).
Ps- is formed by  shooting a positron beam into a thin carbon foil,
and its size is actually a bit bigger than a hydrogen atom.
(Fleischer et al., Physical Review Letters, upcoming article;
contact Frank Fleischer, f.fleischer@mpi-hd.mpg.de, 49-6221-516-516)
                        
RELATIVISTIC ELECTRON COOLING of an antiproton beam has been
demonstrated at Fermilab.  Increasing the density of antiprotons by
reducing the spread in longitudinal speeds leads to a larger
collision rate in particle colliders, producing more sought-after
scattering events that contain rare particles and decays.
Antiprotons, made artificially by smashing protons into a metal
target, must be collected on the fly and focused before they can be
accelerated and collided with opposite-moving batches of protons;
such proton-antiproton smashups are the premier activity at
Fermilab's Tevatron facility.  The more compact and tightly focused
the two beams are, the more desirable high-energy collision there
will be.  The degree of focus and beam density is expressed in a
parameter called luminosity.  To achieve interesting results it is
desirable to have both high collision energy and high luminosity.
Taming swarming antiprotons, however, is difficult.   One would like
all the antiprotons to be co-moving at the same velocity, but
because of the way they're made in the first place, they will be
flying at high speeds through a beam pipe with a variety of motions,
both longitudinal and lateral.  The lateral motions can be largely
suppressed by a process called stochastic cooling, in which electric
signals are dispatched to
various electrodes stationed around the Fermilab's three antiproton
storage rings; the electrodes offer minor kicks which serve to lower
the lateral "temperature" of the swarm.  Reducing the spread in
longitudinal speeds has been harder to accomplish, until now.  In
the new Fermilab process a continuous beam of electrons at an energy
of 4.8 MeV is made to overlap with a beam of 8.9 GeV antiprotons
which, because of their higher mass, move at the same speed as the
electrons.  The electron beam---in effect an electrical current of
0.5 ampere and 2 megawatts---removes some of the unwanted
longitudinal velocity spread, increasing thereby the luminosity by
a factor of 30%.  E-cooling of this kind has been used before but
only with much lower-energy particle beams.  (Nagaitsev et al.,
Physical Review Letters, 3 February 2006; contact Sergei Nagaitsev,
nsergei@fnal.gov; http://www-ecool.fnal.gov )

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 * Origin: Big Bang (1:106/2000.7)