Text 303, 68 rader
Skriven 2005-01-03 17:43:00 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 714
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 714 January 3, 2005
by Phillip F. Schewe, Ben Stein

NEUTRINO SUPERFLUIDS aren't going to be observed any time soon, but the
mathematical proof that they could exist helps to augment the catalog of
possible physical reality.  Superfluids are closely related to superconductors.
 In both phenomena numerous particles---whether boson particles such as
helium-4 atoms or pairs of fermion particles such as electrons or helium-3
atoms---can coalesce into a single, all-encompassing quantum state; examples
include supercurrents, superfluids, and Bose-Einstein condensates (BEC).  Joe
Kapusta, a physicist at the University of Minnesota, has shown that neutrinos
too can become a superfluid.  First they must pair up, as electrons do in
superconductors.  Two electrons with opposite spins can  form pairs by the
exchange of slight disturbances in the underlying matrix of atoms in the solid
sample. Analogously, neutrinos with opposite helicity (for a "left-handed"
neutrino, its intrinsic spin is oriented opposite to its direction of motion;
for "right-handed" neutrinos it's the other way around) could pair up by
exchanging a disturbance in the all-pervasive sea of Higgs bosons in the
universe.  (The Higgs boson, in turn, is the much-sought cornerstone of the
current standard model of particle physics; it is the particle whose presence
confers mass on many of the other known particles.)  After pairing up, the nu
pairs could then form a superfluid condensate.  Kapusta admits that the chances
of observing his superfluid are slim since, first, right-handed neutrinos have
never been observed (and might be even more elusive or ghostly than their
left-handed partners) and, second, because the superfluid would only occur at
temperatures far colder than the 2.7-K average-temperature of the current
universe.  Kapusta points out that a superfluid of heavy neutrinos would make a
great medium for advanced civilizations to send messages over intergalactic
distances since the scattering length of pulses (the average distance they go
before scattering) moving through the neutrino fluid would be much greater than
for electromagnetic pulses. (Physical Review Letters, 17 December 2004;
kapusta@physics.umn.edu)
                                                                
ANTI-HYDROGEN PRODUCTION UNDER LASER CONTROL has been achieved in an
experiment conducted at the CERN lab in Geneva.   Cold anti-hydrogen (Hbar)
atoms are the antimatter counterparts of hydrogen atoms. Previously
antihydrogen was formed when positrons cooled antiprotons within the carefully
designed electric and magnetic fields of a nested Penning trap. That the
anti-atoms had  formed at all was verified, but they're not yet cold enough to
be held in place.  The ultimate goal is to make a goodly supply of anti-atoms,
store them, and then probe their internal structure with laser light to
determine whether they have the same quantum behavior as ordinary hydrogen. An
incremental step would be not just to make the anti-atoms but to see to it that
they are in specific internal energy states, and this is what the ATRAP
(http://hussle.harvard.edu/~atrap/ ) collaboration has now done.  To gain some
extra control over anti-H production, they have to make the production process
a bit more complicated. Where the lasers come into the picture is to initiate a
three-step process.  First, laser light selectively excites cesium atoms into
special "Rydberg" states.  Second, positrons collide with the Cs atoms, an
encounter which cedes one of the atom's electrons to the positron; the
positron-electron pair, which constitutes a sort of atom-like entity of its
own, known as positronium (abbreviated Ps), inherits the cesium atom's
excitation.  (By the way, this excited Ps is a thousand times bigger than plain
Ps).  Third, the positron part of the Ps can occasionally be captured by an
antiproton moving in the same direction.  In the process the anti-hydrogen
atoms assumes the same binding energy as the former Ps.  The rate for producing
anti-H this way is still lower than with the older methods, but the use of the
intermediate cesium process and laser excitation offers an extra measure of
control over atomic conditions within the trap (useful in experiments yet to
come) and, furthermore, may have resulted, in this case, in the coldest
anti-atoms ever created in a lab.  (Storry et al., Physical Review Letters, 31
December 2004; contact Gerald Gabrielse, 617-495-4381,
gabrielse@physics.harvard.edu)

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