Measuring the Thickness of the Neutron Skin with Ultra-Relativistic Heavy Ion Collisions

Researchers determined the neutron skin of lead-208 from experimental data collected in lead-lead collisions at the CERN Large Hadron Collider.

Image courtesy of Govert Nijs and CERN
At the CERN Large Hadron Collider, lead nuclei (left) are collided to form a quark-gluon plasma (middle), which eventually decays into particles (right).

The Science

Heavy nuclei such as lead-208 are characterized by an outer shell of mostly neutrons, called the neutron skin. When these nuclei are collided at speeds close to the speed of light, the quarks and the gluons that make up the protons and the neutrons melt into a state of matter called the quark-gluon plasma. The shape and the size of the quark-gluon plasma these collisions produce are affected by the thickness of the neutron skin. This makes it possible to infer this thickness from experimental observations.

The Impact

In an atomic nucleus, the size of the neutron skin is sensitive to the strong interaction among protons and neutrons. The same interaction governs the behavior of the matter that composes neutron stars. These are among the most mysterious objects observed in the sky, with a mass roughly one to two times that of the Sun but with a radius of only about 6 miles. Therefore, determinations of the neutron skin on Earth permit us to deepen our knowledge of these extreme celestial objects.


The thickness of the neutron skin of a heavy nucleus affects the shape and the size of the quark-gluon plasma produced in high-energy collisions of two nuclei. This in turn affects the distributions of particles emitted from these collisions, measured by particle detectors at the Large Hadron Collider. Researchers have developed sophisticated theoretical models to describe these collisions, including advanced tools that allow scientists to tune the model parameters to describe a vast set of experimental data points.

In this analysis, researchers used the Trajectum hydrodynamic model, which was tuned to 670 experimental data points measured at the Large Hadron Collider. One of the parameters of this model is the size of the neutron skin. By comparing the model parameters to the experimental results, the researchers found the value for the neutron skin of lead-208 that best describes the data. This supplies the first determination of the neutron skin for lead nuclei using collider data.


Giuliano Giacalone
Heidelberg University 

Govert Nijs
Massachusetts Institute of Technology & CERN

Wilke van der Schee
Utrecht University & CERN


One of the researchers was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy (the Heidelberg STRUCTURES Excellence Cluster). Another researcher was  supported by the Department of Energy Office of Science, Office of Nuclear Physics.


Giacalone, G., Nijs, G., and van der Schee, W. Determination of the neutron skin of 208Pb from ultrarelativistic nuclear collisions, Physical Review Letters 131 202302 (2023). [DOI: 10.1103/PhysRevLett.131.202302]

Related Links

The open source Trajectum relativistic hydrodynamics code was employed in this analysis.

Highlight Categories

Program: NP

Performer: University