‘Fire-streaks’ are created in collisions of atomic nuclei

IMAGE: Fragments of extremely hot matter, produced in the collision of heavy atomic nuclei at the SPS accelerator at the European CERN centre, move away from each other at high velocities,…
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Credit: Source: IFJ PAN, Iwona Sputowska

At very high energies, the collision of massive atomic nuclei in an accelerator generates hundreds or even thousands of particles that undergo numerous interactions. At the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow, Poland it has been shown that the course of this complex process can be represented by a surprisingly simple model: extremely hot matter moves away from the impact point, stretching along the original flight path in streaks, and the further the streak is from the plane of the collision, the greater its velocity.

When two massive atomic nuclei collide at high energies, the most exotic form of matter is formed: the quark-gluon plasma behaving like a perfect fluid. The theoretical considerations of physicists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow, Poland show that after impact the plasma forms into streaks along the direction of impact, moving faster the further away it moves from the collision axis. The model, its predictions and the effects of their confrontation with hitherto experimental data are presented in the journal Physical Review C.

Collisions of atomic nuclei occur extremely rapidly and at distances of merely hundreds of femtometres (i.e. hundreds of millionths of one billionth of a metre). The physical conditions are exceptionally sophisticated and direct observation of the phenomenon is not currently possible. In such situations, science copes by constructing theoretical models and confronting their predictions with the data collected in experiments. In the case of these collisions, however, a huge disadvantage is that the resulting conglomerate of particles is the quark-gluon plasma. Interactions between quarks and gluons are dominated by forces that are so strong and complex that modern physics is not capable of describing them precisely.

“Our group decided to focus on the electromagnetic phenomena occurring during the collision because they are much easier to express in the language of mathematics. As a result, our model proved to be simple enough for us to employ the principles of energy and momentum conservation without too much trouble. Later on, we found that despite the adopted simplifications the model predictions remain at least 90% consistent with experimental data”, says Dr. Andrzej Rybicki (IFJ PAN).

Massive atomic nuclei accelerated to high velocities, observed in the laboratory, are flattened in the direction of motion as a result of the effects of the theory of relativity. When two such proton-neutron ‘pancakes’ fly towards each other, the collision is generally not central: only some of the protons and neutrons of one nucleus reach the other,…

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