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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