Text 407, 102 rader
Skriven 2005-04-20 17:22:37 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 728
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 728 April 20, 2005
by Phillip F. Schewe, Ben Stein
        
AN OCEAN OF QUARKS.  Nuclear physicists have now demonstrated that the material
essence of the universe at a time mere microseconds after the big bang consists
of a ubiquitous quark-gluon liquid.
This huge insight comes from an experiment carried out over the past five years
at the Relativistic Heavy Ion Collider (RHIC), the giant crusher of nuclei
located at Brookhaven National Lab, where scientists have created a toy version
of the cosmos amid high-energy collisions.  RHIC is of course not a telescope
pointed at the sky but an underground accelerator on Long Island; it is,
nevertheless, in effect, a precision cosmology instrument for viewing a very
early portion of the universe, a wild era long before the time of the first
atoms (which formed about 400,000 years after the big bang), before the first
compound nuclei such as helium (about a minute after the big bang), before even
the time when protons are thought to have formed into stable entities (ten
microseconds).

In our later, cooler epoch quarks conventionally occur in groups of two or
three.  These groupings, called mesons and baryons, respectively, are held
together by particles called gluons---which act as agents for the strong
nuclear force.  Baryons (such as protons and neutrons), collectively called
hadrons, are the normal building blocks of any nucleus.  Could hadrons be
melted or smashed into their component quarks through violent means?  Could a
nucleus be made to rupture and spill its innards into a common swarm of
unconfined quarks and gluons?  This is what RHIC set out to show.

Let's look at what happened.  In the RHIC accelerator itself two beams of gold
ions, atoms stripped of all their electrons, are clashed at several interaction
zones around the ring-shaped facility.  Every nucleus is a bundle of 197
protons and neutrons, each of which shoots along with an energy of up to 100
GeV.  Therefore, when the two gold projectiles meet in a head-on "central
collision" event, the total collision energy is 40 TeV (40 trillion electron
volts).  Of this, typically 25 TeV serves as a stock of surplus energy---call
it a fireball---out of which new particles can be created.   Indeed in many
gold-gold smashups as many as 10,000 new particles are born of that fireball. 
Hubble-quality pictures of this blast of particles
(http://www.bnl.gov/RHIC/full_en_images.htm), shows the aftermath of the
fireball, but not the fireball itself.

The outward streaming particles provide all the forensic evidence for
determining the properties of the fireball.  To harvest this debris, the RHIC
detectors must be agile and very fast. The recreation of the frenzied quark era
is ephemeral, lasting only a few times 10^-24 seconds. The size of the fireball
is about 5 femtometers, its density about 100 times that of an ordinary
nucleus, and its temperature about 2 trillion degrees Kelvin or (in energy
units) 175 MeV.  RHIC was built to create that fireball.  But was it the
much-anticipated quark-gluon plasma?  The data unexpectedly showed that the
fireball looked nothing like a gas.
For one thing, potent jets of mesons and protons expected to be squirting out
of the fireball, were being suppressed.

Now, for the first time since starting nuclear collisions at RHIC in the year
2000 and with plenty of data in hand, all four detector groups operating at the
lab have converged on a consensus opinion.
They believe that the fireball is a liquid of strongly interacting quarks and
gluons rather than a gas of weakly interacting quarks and gluons.  The RHIC
findings were reported at this week's April meeting of the American Physical
Society (APS) in Tampa, Florida in a talk delivered by Gary Westfall (Michigan
State) and at a press conference attended by several RHIC scientists.

Brookhaven physicist Samuel Aronson said that having established the
quark-gluon-liquid nature of the pre-protonic universe, RHIC expected to plumb
the liquid's properties, such as its heat capacity and its reaction to shock
waves.  The liquid is dense but seems to flow with very little viscosity.  It
flows so freely that it approximates an ideal, or perfect, fluid, the kind
governed by the standard laws of hydrodynamics.  At least in its flow
properties the quark liquid is therefore a classical liquid and should not be
confused with a superfluid, whose flow properties (including zero viscosity)
are dictated by quantum mechanics.

One of the reasons for RHIC's previous hesitancy in delivering a definitive
pronouncement was concern over the issue of whether the observed nuclear liquid
was composed of truly deconfined quarks and gluons or of quarks confined within
hadrons, or maybe even a mixture of quarks and hadrons.  According to William
Zajc (Columbia Univ. and spokesperson for the PHENIX detector group at RHIC),
the patterns of particles flying out of the fireball, including preliminary
data on heavier, charm-quark-containing particles such as D mesons, support the
quark liquid picture.

To summarize, the main stories here are (1) that based on the evidence of the
RHIC data, the universe in the microsecond era would seem to consist of a novel
liquid of quarks and gluons; (2) that RHIC has reproduced small fragments of
this early phase of the universe for detailed study; and (3) that these results
are vouched for by all four RHIC groups.  If there had been delays in making an
announcement of the results or if the exact nomenclature for the novel nuclear
matter had been left unsettled, the RHIC physicists at the press conference
seemed more interested in pursuing their new kind of experimental science---a
sort of fluid-dynamical cosmology.

(All four groups are also concurrently publishing  "white paper" summaries of
their work in the journal Nuclear Physics A.  Preprints are available as
follows:
BRAHMS, http://arxiv.org/abs/nucl-ex/0410020 ;
PHENIX, http://arxiv.org/abs/nucl-ex/0410003 ;
PHOBOS, http://arxiv.org/abs/nucl-ex/0410022 ; and
STAR, http://arxiv.org/abs/nucl-ex/0501009)

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