Text 788, 95 rader
Skriven 2006-04-27 19:58:50 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 775
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PHYSICS NEWS UPDATE
                                        
The American Institute of Physics Bulletin of Physics News
Number 775  April 26, 2006  by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi
                
LOOKING FOR A CRACK IN THE UNIVERSE, in the form of very faint field
pervading the Cosmos, one that exerts a force on electron spin,
would be equivalent to the end of Lorentz invariance.  Lorentz
invariance is the proposition that says that the laws of physics are
the same for an observer at rest on the Earth or one who is rotated
through some angle or traveling at a constant speed relative to the
observer at rest.  An important ingredient in Einstein's theory of
special relativity, Lorentz invariance has been borne out in
numerous experiments.  A new experiment conducted at the University
of Washington has sought such an anomalous field and not found it
even at an energy scale no larger than 10^-21 eV.  This is the most
stringent search yet (by a factor of 100) for
Lorentz-invariance-violating effects involving electrons.  The
Washington work, described at this week's American Physical
Society's (APS) April Meeting in Dallas by Claire Cramer, is part of
an ongoing battery of tests carried out with a flexible and
sophisticated torsion-balance apparatus.  In this case, a pendulum
is made of blocks whose magnetism arises from both the orbital
motion of an electron around its nucleus and from the intrinsic spin
of the electron itself.  Carefully choosing and arranging the
blocks, one can create an assembly that has zero magnetization and
yet still have an overall nonzero electron spin.  Cramer refers to
this condition as a "spin dipole," analogous to the case of an
electric dipole, an object with zero net charge but which, because
of a displaced arrangement of positive and negative charge,
possesses a net electric field.  The existence of a
preferred-direction, Lorentz-violating spin-related force would have
shown up as a subtle mode in the rotation of the pendulum.  The
conclusion: any such quasi-magnetic field would have to be weaker
than about a femto-gauss.  At the APS meeting, Eric Adelberger,
leader of the Washington group, summarized some of the other efforts
underway in his lab such as the search for evidence of extra
dimensions in the form of departures from Newtonian gravity (for
instance, the inverse-square dependence) at a size scale of tens of
microns.  In fact, he said that something strange was happening at a
measurement scale of about 70 microns; the most likely explanation
of this, he conceded, was an experimental artifact.
        
QUARK-GLUON PLASMA---HAS IT BEEN OBSERVED?  Barbara Jacak of SUNY
Stony Brook is a member of the PHENIX team, a large detector
collaboration (one of four) studying the high-energy smashup of gold
nuclei at Brookhaven's Relativistic Heavy Ion Collider (RHIC).
Delivering a plenary talk at this week's APS meeting, Jacak argued
that new experimental data provide evidence that in collisions the
gold nucleus, including its complement of neutrons and protons, and
all their quark constituents, are being melted into a true plasma of
quarks and gluons.  This plasma possesses the highest energy density
of any substance made in a lab---up to 15 GeV/cubic-centimeter.  At
last year's APS April meeting all the RHIC teams unanimously agreed
that a peculiar liquid of quarks had been created in the
collisions.  Peculiar and unexpected: instead of a gas of weakly
interacting quarks, the collision fireball ensuing from a  head-on
interaction of the two nuclei resulted in a liquid of
strongly-interacting quarks
(http://www.aip.org/pnu/2005/split/728-1.html ).  But this wasn't
quite the same thing as claiming that this fluid was a true plasma.
To be a plasma, the quarks must reside outside their customary
groupings of two or three; two quarks (a quark-antiquark couplet)
together are called a meson while three-quark groupings are called
baryons.  Mesons and baryons in turn are collectively referred to as
hadrons. One of the observed properties of hadrons is that they are
color-neutral (just as ordinary atoms are charge neutral), "color"
being the fanciful name for the strong-nuclear-force equivalent of
electrical charge.  For example, a proton would normally consist of
a  red, blue, and green quark which (in a color sense) adds up to
zero.  And just as an electrical plasma is one in which the
particles are charged so a nuclear plasma would be one in which the
particles possess color.  At last year's April meeting the
observation that the matter is liquid was presented.  According to
Jacak, further  studies over the past year now provide, at least for
her and a growing number of RHIC scientists, the necessary proof for
a plasma state.
One notable fact supporting the plasma contention is the fact,
apparent from recent data analysis, that charm-quark jets are being
suppressed.  In the fireball, charm quarks are being produced,
albeit at much lower rates than the light quarks (up, down,
strange).  Because of their heft the charm quarks (or, to be
precise, the jets of hadrons they engender) ought to be able to
punch their way out of the plasma to be observed in outside
detectors, but they're not.  What seems to be happening is this: the
plasma of mostly light quarks are taking up or engulfing the heavy
quarks through frequent and intense interactions.  As Jacak says,
it's as if a strongly rushing river were picking up stones off the
riverbed and pulling them along with the stream.  A river of hadrons
(quarks bundled up into color-neutral clumps) wouldn't be able to do
this as readily as a river of mostly unattached quarks.

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