>From: physnews@aip.org
>Reply-To: physnews@aip.org
>To: ljk4@MSN.COM
>Subject: Physics News Update 728
>Date: Wed, 20 Apr 2005 10:42:37 -0400
>
>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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