Sam Wormley
2005-04-20 15:32:16 UTC
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)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.