This new phase will take place at CERN, the international high-
energy physics laboratory near Geneva, Switzerland.
The Department of Energy recently announced a grant of $2 million
to support NSD's Relativistic Nuclear Collisions Group in the
design and construction of detector components for the final
segment of the long-running heavy-ion studies program at CERN.
The new CERN run, due to begin in 1994, will involve the
acceleration of a beam of lead nuclei (atomic weight 208) to
energies of 180 billion electron volts (GeV) per nucleon.
LBL will provide 90,000 channels of electronics and read-out
systems for a time projection chamber (TPC) that is the heart of
"NA49" -- the largest of four or five experiments that will
utilize the new lead beam. The TPC will make use of integrated
chip technology pioneered at LBL by Stuart Kleinfelder and
Michael Wright of the Engineering Division's integrated circuit
group, and first used in the EOS TPC -- a time projection chamber
that went into operation at the Bevalac this spring. The EOS TPC was
designed and
built by a multidivisional LBL team of engineers, physicists, and
technicians, headed by Al Arthur of the Engineering Division.
This new chip technology is also being developed for use in the
next giant step in heavy-ion research, the Relativistic Heavy Ion
Collider (RHIC), due to go on line at Brookhaven National
Laboratory in 1997. LBL is playing a lead role in developing the
detector system for RHIC, known as STAR (Solenoidal Tracker at
RHIC).
At the Bevalac, the EOS TPC program -- lead by Hans-Georg Ritter
and Howard Wieman -- and the dilepton spectrometer (DLS)
program -- led by Lee Schroeder and Chuck Naudet -- are focusing
on what is known as the nuclear equation of state -- the response
of matter to changes in temperature and pressure. The equation of
state, or EOS, describes the changes that nuclear matter
undergoes when subjected to the most extreme conditions of
temperature and pressure, such as may exist in neutron stars,
supernovas, and during the big bang.
At the higher energies that will be available at CERN and RHIC,
scientists believe that these changes should eventually culminate
in a phase transition similar to what happens when water changes
to steam. This new form of matter -- never yet observed -- is an
undifferentiated soup of free quarks and gluons called the quark-
gluon plasma.
According to physicist Art Poskanzer, who is deputy spokesperson
for the NA49 collaboration and head of the LBL part of the
effort, the acceleration of lead beams at CERN will produce the
"highest density of nucleons (protons and neutrons) that will
ever be achieved on earth." Poskanzer explains this assertion by
pointing out that at even higher energies, like those that will
be available at RHIC, nucleons won't actually stop: they'll
simply pass through each other, leaving only a cloud of mesons to
be detected.
Studies of the nuclear equation of state began at the Bevalac in
the 1970s with experiments involving beams of neon at energies of
2 billion electron volts per nucleon. Later, when the Bevalac's
capabilities were boosted so that it could accelerate nuclei as
heavy as gold, the studies resulted in an important success --
the discovery of the phenomenon known as "collective flow."
Collective flow is thought to be a sign of the compression of
nuclear matter.
Following these early successes, the equation-of-state program
proceeded with two separate and complementary approaches. One
approach is to collide the heaviest possible nuclei -- those
containing hundreds of protons and neutrons rather than just a
few. This strategy increases the probability that many of the
nucleons will be involved in direct, head-on hits -- the ones
considered most likely to produce compressed nuclear matter.
At the Bevalac, the studies concentrated on ever more detailed
studies of collisions of the heaviest nuclei.
The second approach is to increase the velocity of the
accelerated particles, thus making the collisions that do occur
more energetic. In recent years, the goal has been to make these
energies more "relativistic" -- that is, closer to the speed of
light. In 1986, since no higher energy U.S. facility was
available, such detector collaborations as LBL's Plastic Ball,
streamer chamber, and others moved their base of operations to
CERN, where a major accelerator, the Super Proton Synchrotron
(SPS) has been made available for heavy-ion studies for about a
month out of each year.
The nuclei accelerated in these studies -- beginning with oxygen
in 1986 and moving to sulfur in 1988 -- were heavier than any
previously available at such high energies, though far lighter
than those that continued to be the focus at the Bevalac. The
final run of the sulfur beam at the SPS began on April 1 and has
recently been completed. In an effort led by Peter Jacobs and
Fred Bieser of NSD and Tom Noggle of the Engineering Division,
LBL provided 6000 channels of new "EOS-style" electronics for an
upgraded detector for this final sulfur run.
Now, the SPS injector at CERN will undergo major modifications to
enable it to accelerate beams of lead, permitting, for the first
time, experiments in which the energy is relativistic, and the
nuclei in the beam are at the high end of the periodic table.
The next major step in combining the two approaches will be in
the STAR detector at RHIC, in which the available energy
(achieved through colliding beams) will be 10 times higher, and
nuclei as heavy as gold will be available.
"The CERN experiment is on the direct path to STAR," says
Poskanzer, "not only because the integrated-chip TPC electronics
technology is the same, but also because the style of experiment
-- what we call event-by-event physics -- is a forecast of things
to come at RHIC." In previous heavy-ion experiments at these
energies, he explains, the researchers averaged out the
characteristics of many collisions. In this run, however (and in
RHIC in the future) it will be possible to measure most of the
particles emerging from the collisions, and to describe in detail
the outcome of each event.
Scientists in LBL's Nuclear Science Division are gearing up for
the latest phase in a comprehensive exploration of nuclear matter
that began over 15 years ago at the Bevalac.