Number 714, January 3, 2005
by Phil Schewe and Ben Stein
Neutrino Superfluids
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. (Kapustaet al.; Physical Review Letters, 17 December 2004; kapusta@physics.umn.edu,
612-624-0506x)
Anti-Hydrogen Production Under Laser Control
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. (Storryet al., Physical Review Letters, 31 December 2004; contact Gerald
Gabrielse, 617-495-4381, gabrielse@physics.harvard.edu)
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