Determination of the neutron skin of $^{208}$Pb from ultrarelativistic nuclear collisions

TL;DR

This paper introduces a novel experimental approach using ultrarelativistic heavy-ion collisions at the LHC to measure the neutron skin of $^{208}$Pb, providing results consistent with theoretical predictions and other experimental methods.

Contribution

It demonstrates a new method to determine neutron skin thickness in heavy nuclei through analysis of collision data and hydrodynamic modeling, expanding experimental techniques in nuclear physics.

Findings

Neutron skin of $^{208}$Pb measured as 0.217±0.058 fm.

Method yields results consistent with parity-violating electron scattering.

Establishes a new systematic approach for nuclear structure studies.

Abstract

Emergent bulk properties of matter governed by the strong nuclear force give rise to physical phenomena across vastly different scales, ranging from the shape of atomic nuclei to the masses and radii of neutron stars. They can be accessed on Earth by measuring the spatial extent of the outer skin made of neutrons that characterises the surface of heavy nuclei. The isotope $^{208}$Pb, owing to its simple structure and neutron excess, has been in this context the target of many dedicated efforts. Here, we determine the neutron skin from measurements of particle distributions and their collective flow in $^{208}$Pb+$^{208}$Pb collisions at ultrarelativistic energy performed at the Large Hadron Collider, which are sensitive to the overall size of the colliding $^{208}$Pb ions. By means of state-of-the-art global analysis tools within the hydrodynamic model of heavy-ion collisions, we infer a neutron skin $\Delta r_{np}=0.217\pm0.058$ fm, consistent with nuclear theory predictions, and competitive in accuracy with a recent determination from parity-violating asymmetries in polarised electron scattering. We establish thus a new experimental method to systematically measure neutron distributions in the ground state of atomic nuclei.