Smashing heavy nuclei reveals proton size

highlight article | September 21, 2022

Theoretical study uses the accuracy of new heavy-ion collision data to predict how gluons are distributed within protons and neutrons

DOE/US Department of Energy

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The science

Atomic nuclei are made up of protons and neutrons, collectively called nucleons. Nucleons, in turn, consist of quarks and gluons. Understanding how these inner building blocks are distributed within nuclei can reveal how large protons and neutrons appear when examined at high energies. This work used comparisons between model calculations and new precision data from collisions of heavy ions (which contain many protons and neutrons) to access the distribution of gluons and predict the size of the proton.

The impact

Identifying and accurately measuring factors that are sensitive to nucleon size will help physicists describe quark-gluon plasma (QGP) more accurately. This is a hot, dense form of nuclear matter, formed when individual protons and neutrons “melt” in heavy-ion collisions, mimicking conditions in the early Universe. This knowledge can remove significant uncertainties about the initial state of the manufactured QGP. Knowing more about the initial state of QGP provides input to the modeling that scientists use to derive the viscosity and other properties of QGP. The results also complement measurements of proton size based on the distribution of quarks within the proton.


It might be hard to imagine that the debris from violent heavy-ion collisions – which break down the boundaries of protons and neutrons and create thousands of new particles – could be used to gain detailed insights into the properties of nucleons. Recent advances in experimental methods together with improved theoretical modeling have made this possible. Based on a state-of-the-art model for the colliding nuclei and the hydrodynamic evolution of the quark-gluon plasma generated in the collision, this work shows that certain observables are highly sensitive to the sizes of the protons and neutrons inside the collision nuclei.

Comparing the model with data from experiments also shows that the gluon distribution within protons and neutrons is rather lumpy – not as smooth and spherical as modeled using naive assumptions. Current and future measurements using collisions of different nuclei at the Relativistic Heavy Ion Collider (RHIC), a Department of Energy (DOE) user facility at Brookhaven National Laboratory, and at the Large Hadron Collider (LHC) at CERN, together with a sophisticated theoretical program, is presented give a more detailed insight into the distribution of gluons inside protons and neutrons, inside and outside heavy nuclei and how it behaves with changing collision energies. This fundamentally important information will be studied with even greater precision in the Electron-Ion Collider to be built at Brookhaven.


This research was funded by the Deutsche Forschungsgemeinschaft and the DOE Office of Science, Office of Nuclear Physics. The research used computing resources from the Open Science Grid supported by the National Science Foundation.

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