LBL Components Drive Heavy Ion Experiment at CERN

June 19, 1992

By Judith Goldhaber


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.

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.