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@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@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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