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 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.

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)