Feature

Squeezing the Atomic Nucleus

Dr David Krofcheck

How hard is it to squeeze an atomic nucleus? Recent experiments, using relativistic beams of gold nuclei impacting on a gold foil target, have provided new insights into the behaviour of highly compressed nuclear matter.

During a collision between two gold nuclei, the protons and neutrons in the region of the impact are subjected to extreme pressure, density, and temperature. By varying the collision energy, it is possible to explore different pressures and densities. Nuclear matter appears to "soften" as the normal protons and neutrons inside the gold nuclei are compressed to several times their normal density.

But, what really happened in the highly compressed nuclear matter, and why is it important to know?

A collaboration of 54 physicists from 14 universities and laboratories, including Auckland University, has been asking such questions. The "E895" collaboration is studying fixed-target, gold-on-gold collisions at beam energies of 2, 4, 6 and 8 GeV per nucleon. The Alternating Gradient Synchrotron at Brookhaven National Laboratory generated these relativistic beams of gold nuclei.

The gold beams strike a target foil located in a magnetic dipole field. A detector, called a Time Projection Chamber, provides three-dimensional curved track information on the outgoing charged particle fragments resulting from the high-energy collisions. The chamber also samples atomic ionisation of the argon-CH₄ gas mixture produced by the charged fragments as they pass through. Tracking and ionisation measurements allow the resulting particles to be identified and their momentum measured. "Lightweight" pions and kaons through to "heavyweight" helium and lithium nuclei were the primary fragments found.

Perspective view of a 4GeV per nucleon gold beam colliding with a gold foil target. The resulting fragments are shown passing through a Time Projection Chamber.

For each gold-gold collision, the E895 researchers used the particle identification and momentum data to reconstruct the nuclear reaction plane. The average magnitude of the protons' momentum components parallel to a reaction plane, and the azimuthal distribution of the protons about the plane, were measured.

Changes in proton momenta and azimuthal distributions with beam energy implied the following scenario: in the region between 2-4 GeV per nucleon, there is a substantial decrease in the nuclear pressure gradient with respect to the matter density. This represents the phase space location of the "softest point" of nuclear matter; the matter density at which it is easy to squeeze atomic nuclei!

Nuclear reaction models based upon "normal" nuclear matter have difficulty in simulating the protons' "behaviour". One provocative suggestion is that the "soft" nuclear matter has been so tightly squeezed that a phase transition into a collection of free quarks and gluons (the constituents of protons and neutrons) has occurred.

This bizarre state of matter is known as the quark-gluon plasma. It may have existed in the early universe just microseconds after the Big Bang, and possibly exists inside neutron stars. Further studies are planned for mid-2000, using collisions between counter-rotating gold beams at Brookhaven's Relativistic Heavy Ion Collider (RHIC), to probe even higher energy density nuclear matter.