Text 361, 75 rader
Skriven 2005-03-16 21:07:52 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 723
===============
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 723 March 15, 2005
by Phillip F. Schewe, Ben Stein

DEGENERATE GAS STUCK IN OPTICAL LATTICE.  The forces that govern the motions of
macroscopic objects like planets and tennis balls are complicated enough. 
Forces among atoms at ultracold temperatures are even more complicated.  In
this regime atoms (pictured as being waves) spread out so much that they
overlap with neighboring atoms.
If the atoms are bosons (that is, if the total spin of each atom is an integer)
then they all fall into a single quantum state, namely a Bose Einstein
condensate (BEC).  If, however, the atoms are fermions (the total spin is
half-integral-valued), then quantum reality, in the form of the Pauli exclusion
principle, also decrees a special status: not a single ensemble BEC state (all
atoms having the same energy), but a state in which none of the atoms has the
same energy.  In this "Fermi degenerate" state the atoms fill up all possible
quantum energy levels, one by one (or two by two, providing that the two atoms
sharing a level have opposite spins), until the last atom is accounted for. 
(For the first demonstration of a Fermi degenerate state in atoms, see
www.aip.org/pnu/1999/split/pnu447-1.htm.)  Now, physicists at the ETH lab in
Zurich have, for the first time, not only made a quantum degenerate Fermi gas
but have been able to load the atoms into the criss-cross interstices of an
optical lattice, an artificial 3D crystal in which atoms are held in place by
the electric fields of well-aimed laser beams.  Then, by adjusting an external
magnetic field, the pairs of atoms lodged in their specified sites can be made
to interact (courtesy of the "Feshbach resonance") with a
varying strength.   According to Tilman Esslinger (41-1-633-2340,
esslinger@phys.ethz.ch), it is this ability to put atoms where you want them in
a crystal-like scaffolding, and then to make them interact with a strength that
you can control, that makes this setup so useful.  It might be possible to test
various condensed matter theories, such as those that strive to explain
high-temperature superconductivity, on a real physical system.  (Kohl et al.,
Physical Review Letters, March 4; lab site, www.quantumoptics.ethz.ch)
                                                
A PUZZLING SIGNAL IN RHIC EXPERIMENTS has now been explained by two researchers
as evidence for a primordial state of nuclear matter believed to have
accompanied a quark-gluon plasma or similarly exotic matter in the early
universe.  Colliding two beams of gold nuclei at Brookhaven's Relativistic
Heavy Ion Collider (RHIC) in New York, physicists have been striving to make
the quark-gluon plasma, a primordial soup of matter in which quarks and gluons
circulate freely.  However, the collision fireball has been smaller and
shorter-lived than expected, according to two RHIC collaborations (STAR and
PHENIX) of pions (the lightest form of quark-antiquark pairs) coming out of the
fireball.  The collaborations employ the Hanbury-Brown-Twiss method, originally
used  in astronomy to measure the size of stars.  In the subatomic equivalent,
spatially separated detectors record pairs of pions emerging from the collision
to estimate the size of the fireball.  Now an experimentalist and a theorist,
both from the University of Washington, John G. Cramer (206-543-9194,
cramer@phys.washington.edu) and Gerald A. Miller (206-543-2995,
miller@phys.washington.edu), have teamed up for the first time to propose a
solution to this puzzle. Reporting independently of the RHIC collaborations,
they take into account the fact that the low-energy pions produced inside the
fireball act more like waves than classical, billiard-ball-like particles; the
pions' relatively long wavelengths tend to overlap with other particles in the
crowded fireball environment.  This new quantum-mechanical analysis leads the
researchers to conclude that a primordial phenomenon has taken place inside the
hot, dense RHIC fireballs.  According to Miller and Cramer, the strong force is
so powerful that the pions are overcome by the attractive forces exerted by
neighboring quarks and anti-quarks. As a result, the pions act as nearly
massless particles inside the medium.  Such a situation is believed to have
existed shortly after the big bang, when the universe was extremely hot and
dense. As the pions work against the attraction to escape RHIC's primordial
fireball, they must convert some of their kinetic energy into mass, restoring
their lost weight.  But the pions' experience in the hot, dense environment
leaves its mark: the strong attractive force (and the absorption of some of the
pions in the collision) would make the fireball appear reduced in size to the
detectors that record the pions.  According to Miller, looking at the fireball
using pions is like looking through a distorted  lens: the pions see the radius
as about 7 fermi (fm), about the radius of an ordinary gold nucleus, while the
researchers deduce the true radius of the fireball to be about 11.5 fm 
(Cramer, Miller, Wu and Yoon, Phys Rev Lett, tent. 18 March 2005).

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