New Approach to Determine Neutron Distribution in Atomic Nuclei

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In a lead nucleus (left) the distribution of neutrons is more diffuse compared t
In a lead nucleus (left) the distribution of neutrons is more diffuse compared to the distribution of the protons. When such nuclei collide, the spatial extent of the neutron distribution affects the matter produced (middle), leaving measurable imprints in the distributions of detected particles (right). | Govert Nijs (CERN)

Physicists from Heidelberg, the Netherlands and Switzerland, and the USA establish new value for the neutron skin of lead-208

Measurement data from collisions of heavy ions can provide insight into the neutron distribution in atomic nuclei, according to an international research team led by Dr Giuliano Giacalone from the Institute for Theoretical Physics of Heidelberg University. The researchers used data from particle collisions in the Large Hadron Collider (LHC) at the European research centre CERN in Geneva (Switzerland) to determine the neutron skin of lead-208 isotopes. According to Dr Giacalone, this new experimental research approach makes it possible to systematically determine the distribution of neutrons in atomic nuclei.

Protons and neutrons, the building blocks of atomic nuclei, interact through the strong interaction, also known as the nuclear force. It is one of the four fundamental forces in the Standard Model of particle physics and is at the origin of a wealth of phenomena, from the shape of nuclei to the properties of exotic matter that makes up neutron stars. Calculating these interactions from fundamental theory is a very challenging task, as Dr Giacalone explains. While scientists have a good idea of how the protons in nuclei are distributed, it is far more difficult to determine the distribution of neutrons. Information can be gained from the thickness of the neutron skin, a shell of excess neutrons that collect on the surface of especially heavy atomic nuclei like lead-208.

"By determining the extent of the neutron skin, we hope to obtain a better understanding of neutron-rich matter and the physical forces acting on it," explains Dr Govert Nijs of the Massachusetts Institute of Technology (USA), one of the cooperation partners in the international research team. Lead-208 is especially well-suited for such experimental searches because it is rich in neutrons, has a simple spherical structure, and is readily available in nature. Earlier experiments have already yielded a specific value for the extent of this shell of excess neutrons, yet an independent determination coming from a particle collider experiment did not exist. Using the data sets from the collisions of lead ions, the researchers were able to achieve such a determination and yield a new comparison value in processes based on the strong interactions of quarks and gluons at high energies.

In the LHC, the world’s largest particle accelerator, lead-208 nuclei are brought into collision to reproduce what is known as the quark-gluon plasma, a primordial state of matter in which quarks and gluons, two types of elementary particles, move freely as they did moments after the Big Bang. The neutron skin of lead-208 not only influences the collision rate of particles but also the number, the average momenta, and the angular distribution of the new particles produced by the collision. Using Bayesian inference - a method to determine theoretical model parameters for a given set of experimental observations - the physicists were able to draw conclusions about the total radius of the nucleus as well as the radii of the proton and neutron distributions. The result of their difference produces a value for the neutron skin of lead-208 of 0.2170.058 femtometres.

"Our results are based solely on existing data sets, such that they can be improved in the future and challenge more seriously other experimental findings," states Dr Wilke van Schee of Utrecht University (Netherlands) and CERN. Future measurements of observables that offer a closer view of the neutron distributions could help improve the precision of the neutron skin determination from the LHC data. The researchers not only hope to gain a better understanding of neutron-rich matter but also to obtain, indirectly, insight into the behaviour of the strong interaction.

The research in Heidelberg was done under the auspices of Collaborative Research Centre 1225, "Isolated quantum systems and universality under extreme conditions" (ISOQUANT). The research results were published in the journal "Physical Review Letters".

G. Giacalone, G. Nijs, W. van der Schee: Determination of the Neutron Skin of 208Pb from Ultrarelativistic Nuclear Collisions. Physical Review Letters 131, 202302 (15 November 2023).