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.
outward streaming particles provide all the tomographic 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.
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.
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)