Text 167, 72 rader
Skriven 2004-09-03 15:57:35 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 699
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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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 * Origin: Big Bang (1:106/2000.7)