“We want to know what holds the world together at its inmost folds”
For more than ten years now, the Large Hadron Collider (LHC) at the CERN nuclear research centre near Geneva has been providing data from particle collisions at high energy levels, allowing conclusions to be drawn about the structure of atomic nuclei. In the process, protons and atomic nuclei of lead are accelerated to something approaching the speed of light. Physicist Prof. Michael Klasen from the Institute of Theoretical Physics at the University of Münster was involved in a review paper summarising the current state of knowledge in this field. In this interview with Christina Hoppenbrock, he uses this to provide insights into the progress being made in nuclear research.
Atomic nuclei can’t be studied under the microscope, but physicists still know a lot about what particles these nuclei consist of and what the properties of these particles are …
The physicist Ernest Rutherford and his colleagues discovered the atomic nucleus by the scattering of alpha particles. Today we also investigate atomic nuclei by means of scatter experiments, but at higher energies. As early as 1949, Hans Jensen, who supervised my PhD, and Maria Goeppert-Mayer deduced a shell structure of the protons and neutrons in the nucleus, similar to that of the electrons in the atomic shell, and for that they were awarded the Nobel Prize for Physics.
For more than ten years now, in the most powerful particle accelerator in the world – the Large Hadron Collider at the CERN Nuclear Research Centre in Geneva – light and heavy atomic nuclei have been colliding with one another with unprecedented levels of energy. What has physics gained from these experiments?
What information do the experimental data provide on these elementary particles?
The distribution of quarks and gluons in the proton has been well-known since experiments carried out at the German Electron Synchrotron in Hamburg 20 years ago. However, this distribution changes in heavy nuclei. Research undertaken in the past ten years at the LHC have given us so much information that we now have a good idea of the motion of the protons and neutrons – and also of the binding effects in the nucleus. This means, for example, that we can now investigate several things: from what energy level gluons begin to melt; what role quarks play in this; how they are released in the quark-gluon plasma; and whether this is also actually possible in the case of lighter nuclei.
We’re talking about physical experiments. What role does theoretical physics play?
Theoretical work puts the large number of measurements from different experiments into a systematic framework, reveals contradictions, initiates further investigations, and leads ultimately to a new picture of the atomic nucleus. This is only possible if the processes can be computed using mainframes with a high degree of precision and then subjected to comprehensive statistical analyses. We in Münster have made a decisive contribution to that over the past few years.
A hitherto unique electron-ion accelerator is to be built at the Brookhaven National Laboratory in Upton, New York. What insights do you hope for from this accelerator, as well as from other accelerators?
Nuclear particles can be tracked more precisely in experiments with electron beams, which means that from 2030 we should even have a three-dimensional image of the atomic nucleus and be able to study the scattering in correlated quarks and gluons. That would work even better with accelerators with even higher energies, such as are planned at CERN. Then the fusion of quarks and gluons could be directly demonstrated, enabling their phase transition into the plasma state to be tracked.
This article is from the university newspaper wissen|leben No. 8, 13 December 2023.
Original publications
P. Duwentäster, T. Jezo, M. Klasen, K. Kovarik et al., Phys.Rev.D 105 (2022); DOI: 10.1103/PhysRevD.105.114043
M. Klasen, H. Paukkunen, DOI: 10.1146/annurev-nucl-102122-022747