Collisions between gold nuclei have revealed unexpected behavior in matter compressed to extreme densities.
The experiment takes place at the RHIC collider in the United States. Gold nuclei are accelerated there and then projected at one another. For a fraction of a second, the collision creates matter far denser than that found in an ordinary atomic nucleus.
Collision between gold ions recorded by the STAR detector at RHIC. The lines represent the trajectories of thousands of particles produced in the collision.
Credit: Brookhaven National Laboratory / Wikimedia Commons — CC BY 2.0.
The researchers want to understand how this matter changes state depending on its temperature and density. To do so, they vary the energy of the collisions.
The STAR detector then records the thousands of particles produced. Their motion preserves information about the very brief state that existed just after the collision. The researchers focused in particular on the small variations in their lateral motion from one collision to another.
This is where the anomaly appears. In the most head-on collisions, these variations drop sharply between 3 and 5.2 GeV. They then remain very small up to 7.7 GeV, before rising again at higher energies. The signal clearly departs from a regular trend.
Momentum correlations measured by STAR during gold-gold collisions as a function of energy. A dip appears at the lowest energies studied.
Credit: STAR Collaboration/Brookhaven National Laboratory.
Why does this dip interest physicists so much? Nuclear matter can exist in different states, depending on its temperature and density. The researchers suspect that there is a “critical point” where the way it transitions from one state to another changes profoundly.
Near such a point, matter could absorb a great deal of energy without its temperature changing significantly. From one collision to another, temperature differences should therefore decrease. The dip measured by STAR corresponds precisely to the type of behavior expected in this situation.
However, this result is not yet sufficient to announce the discovery of a critical point. A collision model lacking this phenomenon does not correctly reproduce the observed signal. But other mechanisms could produce a comparable anomaly, and several explanations remain possible.