NA49 experiment to continue at CERN

June 30, 1995

By Judith Goldhaber

A nucleus of lead is like a bag of marbles stuffed with 208 nucleons (protons and neutrons). Take two such bags, slam them together at high energies in a particle accelerator, and the chances are very high that shattering impacts will take place among the nucleons, with hundreds of fragments flying out of the collision.

In fact, scientists in LBNL's Nuclear Science Division -- who are heading off to Geneva this summer to do just that in the NA49 experiment at CERN -- estimate that every such collision between lead nuclei will produce about a thousand particles. The challenge is to find ways to identify as many of those particles as possible in order to reconstruct what was taking place at the moment of impact.

This fall's experimental program, beginning in November, will be the second year of running at CERN's Super Proton Synchrotron (SPS) accelerator for NA49. The experiment began taking data in November 1994, and future runs will continue for about six weeks each year for the next several years. In order to produce beams of lead nuclei at energies of 180 billion electron volts (GeV) per nucleon, the SPS was specially modified with a new ion source, a new linac, and an improved vacuum in the booster tank.

It's all part of Nuclear Science's continuing program -- underway since the early 1980s -- to produce and study nuclear matter under extreme conditions of temperature and pressure. A milestone along the way was the observation of the "collective flow" of compressed nuclear matter at LBNL's Bevalac in 1984. The ultimate goal of the CERN program is the observation of a phase transition from ordinary matter to a new form of matter called the quark-gluon plasma. Such matter -- an undifferentiated soup of free quarks and gluons -- probably played an important role in the first moments after the Big Bang, and may exist now in neutron stars and supernovas.

Physicist Peter Jacobs, who heads LBNL's part of the international team known as the NA49 collaboration, says that when protons and neutrons are tightly bound in a nucleus, they behave differently than they would if they were isolated. At high energies, the simultaneous collision of many protons and neutrons within a small volume can produce an especially hot and dense system in the center of mass of the collision. "Within this volume," says Jacobs, "we hope to see the phase transition to the quark-gluon plasma."

NA49 is a new experiment designed specifically to study the enormously complex results of the collisions of the heaviest nuclei. Until last year, the heaviest nucleus that could be accelerated at very high energy was sulfur (mass 32). Last year, the CERN SPS accelerated lead nuclei to 160 GeV per nucleon for the first time.

"What's new in NA49," says Jacobs, "is our ability to look simultaneously at many different aspects of each event by collecting almost all the particles coming out of the collision. Previous experiments have concentrated on one or a few of these signals. There have been hints of a phase transition but there are always ambiguities. With the ability to look at several signatures at once, we may be able to remove these ambiguities."

The heart of the NA49 experiment is an array of four time projection chambers, a type of particle detector invented at LBNL by physicist David Nygren in 1974. TPCs detect and identify particles by translating position (the position of particle tracks in space) into time (the time it takes the signal to drift through a given distance in a gas). TPCs have also figured in earlier studies of compressed nuclear matter at the Bevalac and at CERN, and will be used in the giant STAR detector being prepared for use at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab.

The NA49 collaboration includes about 100 scientists from 18 institutions in the United States and Europe. Among LBNL's major contributions to the effort is a system of integrated electronics similar to the one first developed for the "EOS" TPC at the Bevalac in 1992 (see Currents, Feb, 28, 1992). In this design, most of the electronic components are on the detector itself, drastically reducing the cost and complexity. For NA49, the LBNL team was responsible for the development and manufacture of electronics for the 182,000 TPC readout channels. This very large number of channels made it necessary to develop a number of new electronic components. According to Fred Bieser, chief engineer on the project, DOE's decision to entrust the NA49 electronics effort to LBNL was building on the Laboratory's known strength in the area of integrated electronics. "We are a world leader in this area," says Bieser.