From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu Dec 18 2003 - 18:25:05 PST
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From: physnews_at_aip.org
Sent: Thursday, December 18, 2003 1:55 PM
To: ljk4_at_MSN.COM
Subject: Physics News Update 666
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 666 December 18, 2003 by Phillip F. Schewe, Ben Stein, and
James Riordon
BRINGING THE NUCLEON INTO SHARPER FOCUS. Working at the Thomas
Jefferson National Accelerator Facility in Virginia, a multinational
research team has determined how quarks in a proton orient their
"spins," which, roughly speaking, can be visualized as tiny bar
magnets that point in a certain direction and have a certain
strength. Information about a quark's spin can provide new details
of how the tiny particles arrange themselves inside a nucleon
(proton or neutron). In high-school physics classes, students are
taught that a proton or neutron simply consists of three quarks,
which specialists call "valence quarks." A more complete picture
includes these three valence quarks, plus a sea of quark-antiquark
pairs that pop in and out of empty space (the vacuum), as well as
particles called gluons which hold the quarks together.
Now, for the first time, researchers have precisely measured the
distribution of spin for a neutron's valence quarks. Strikingly,
their results reveal the importance of once-neglected orbital
motions of quarks around the nucleon. Aiming an electron beam at a
helium-3 target in JLab's Hall A, researchers (led by Jian-Ping
Chen, jpchen_at_jlab.org and Zein-Eddine Meziani, meziani_at_temple.edu)
selected a 5.7 GeV beam energy so that the electrons interacted
mainly with the neutron's valence quarks and not its sea quarks and
gluons. Interestingly, the researchers applied their new neutron
data, along with existing proton data, to find out more about the
proton. Their conclusions: the spins of the proton's two valence up
quarks are aligned parallel to the overall proton spin, but the same
is not true for the proton's valence down quark (see image at
http://www.aip.org/mgr/png/2003/207.htm).
This result disagrees with predictions from an approximation of
perturbative quantum chromodynamics (pQCD), a widely accepted theory
of the strong force (which holds the nucleon together). This
approximation does not account for the quarks' orbital angular
momenta, which describes the orbital paths of quarks inside the
nucleon. However, the results agree well with predictions from
relativistic valence quark model, which does consider quarks'
orbital angular momenta as they move inside the nucleon. (Zheng et
al., Physical Review Letters, upcoming article; for more
information, contact Xiaochao Zheng, Argonne, 630-252-3431,
xiaochao_at_jlab.org) Once omitted in simpler pictures of the nucleon,
quark orbital angular momentum is also proving important for
exploring questions about the shape of the proton (see for example
New Scientist, May 3, 2003.)
A TRUE ONE-DIMENSIONAL ATOMIC SYSTEM, consisting of a Bose Einstein
condensate (BEC) of rubidium atoms pulled out into a thin tubelike
shape, has been experimentally demonstrated for the first time, in
the ETH lab in Zurich. The ETH researchers begin by loading their
condensate into an optical lattice, an artificial configuration in
which atoms are held and moved about in 3D space by criss-crossing
beams of laser light. In contrast to previous efforts to make
one-dimensional BECs, this experiment succeeded in extruding a
condensate into 1000 small needle-like condensates---one dimensional
strings of 100 atoms or so and not merely cigar shaped
lozenges---because they used a far more intense laser trapping field
and higher quality laser beams (more truly Gaussian in their
profile), the better to keep atoms from tunneling from one needle
into a neighboring needle (see figure at
http://www.aip.org/mgr/png/2003/208.htm ). Once the ETH physicists
had established their one-dimensional atomic gas what did they do
with it? They set their lean stack of atoms into motion by slightly
moving the magnetic center of their apparatus. This caused the
atoms to move up and down in a "breathing mode"at a characteristic
frequency. Studying this oscillation was analogous to listening a
one dimensional bell ringing.
How unusual is the ETH 1D condensate? Well, even two dimensional
atomic systems are rare in physics: helium films and hydrogen atoms
sitting atop helium are the prominent examples. The only other 1D
gas studied in physics consists of electrons moving in "quantum
wires." One-dimensional systems are interesting because they are
more intrinsically dominated by quantum effects than 2- or
3-dimensional systems. According to Tilman Esslinger
(tilman.esslinger_at_iqe.phys.ethz.ch,
http://www.quantumoptics.ethz.ch/ ) 1D ensembles of atoms should
play an important role wherever precision handling of atoms is
needed: in atom optics, atom interferometry, or sending signals from
atom lasers down an atom waveguide. (Moritz et al., Physical Review
Letters, 19 December 2003)
***********
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