PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 699 September 3, 2004
by Phillip F. Schewe and Ben Stein
                                
THE QUARK-MESON COUPLING (QMC) model, a theory which takes the radical step
of incorporating self-consistent changes in the quark structure of a
nucleon when it is bound in matter, has been transformed into  a theory of
quasi-nucleons interacting through many-body forces.  Thanks to this, the
QMC model can now challenge the time-honored descriptions of the nucleus
where nucleon structure was supposed to play no role.   The conventional
hierarchy of nuclear matter at the smallest scale goes like this: quarks
are the most elemental.  Nucleons, the next bigger things, are clumps of
three quarks held together by a force carried from place to place by
gluons.  Then the nucleus is made from nucleons held together by mesons,
which are themselves clumps of two quarks.  Next up in size are atoms,
which consist of electrons (members of a separate category of particle
called leptons) hovering around the nucleus. At all these levels different
models would apply.  In other words, no one theory would apply everywhere;
one would need instead several "effective theories" with limited
validity outside their own realm. For instance, in experiments conducted at
very high energies (many GeV)---equivalent to using a microscope able to
see individual quarks inside the nucleons---it is customary to see nuclear
physics as being a bunch of quarks interacting via the exchange of gluons.
At lower energies, where the spatial resolution is lower (i.e.,
experimental studies are less able to resolve details inside the nucleon),
one is apt to see nuclear physics as being a bunch of nucleons interacting
via the exchange of  mesons. Actually, even in the lower energy range, one
should keep the quarks in mind because their motion  inside a nucleon may
change when the latter resides in a nucleus.  That is, a nucleon is one
thing when on its own and another thing when inside a nucleus, in which
case it becomes a "quasi-nucleon." This is what the QMC model
takes into account by describing the interactions between a quark in one
nucleon with a quark in another nucleon by meson exchange (see illustration
at www.aip.org/png). The quarks in that nucleon are in turn interacting
with the quarks in another and so on. The resulting picture of the nucleus
is then that of quasi-nucleons interacting through forces which involve 2,
3, or even 4 bodies. The necessity of  such  many-body forces was
empirically known from traditional nuclear physics and the merit of the QMC
model is that it explains their origin and predicts their intensity. This
makes for a more realistic description, particularly for the border area
between higher energy  (a province sometimes called particle physics) and
lower energy (to which the generic term "nuclear physics"
applies).  The QMC theory has stood up to experimental tests for some years
now. For example, it has been helpful in explaining changes in hadron
masses in dense matter and there are even hints from extremely precise
measurements of the ratio of electric to magnetic form factors of a proton
bound in helium (at Mainz and Jefferson Lab) supporting the subtle changes
predicted  there.  Now, the authors of the QMC model, Pierre Guichon
(Saclay, France) and Tony Thomas (Adelaide, Australia --- now Chief
Scientist at Jefferson Lab), believe the newer version of their model will
really help in interpreting data coming from heavy-ion collision
experiments aiming to create a quark-gluon plasma state. (Physical Review
Letters, upcoming article; pguichon{at}cea.fr, 33-1690-87207)

NEW EVIDENCE FOR A SUPERFLUID SOLID.  In January 2004, two physicists at
Penn State presented the results of an experiment in which at very low
temperatures one solid (solidified helium-4) passed through another solid
(a glasslike material called vycor) without any friction
(www.aip.org/pnu/2004/split/669-1.html).  Now, the same researchers, Moses
Chan (chan{at}phys.psu.edu) and Eun-Seong Kim, have modified their approach to
demonstrate "superflow," the superfluid-like behavior of a solid,
in a new way.  This time, the solidified helium is not ensconced in any
glass matrix.  The He atoms are admitted to an open ring-shaped channel in
a simple chamber which is free to swivel. Next the He are chilled and
submitted to high pressure, causing solidification.  One can tell that the
helium at this point is solid and not liquid because of the characteristic
oscillation (swiveling) properties.  At an even lower temperature, 230 mK,
the swiveling changes again, suggesting to Chan and Kim that a portion of
the solid (about 1.5% of the sample) has metamorphosed into a freely
flowing---but still solid---state of matter, or a frictionless
"supersolid." (Science Express, 3
September.)

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