When Tiny, Energetic Worlds Collide

A new analysis provides a clearer picture of the universe by considering the yield of fast particles in grazing versus head-on nuclear collisions.

Image courtesy of Oak Ridge National Laboratory
When two nuclei of lead collide, the number of fast particles the collision produces depends on whether the nuclei graze each other, so-called peripheral collisions (colored data points), or have more head-on or central collisions (faint grey lines).

The Science

Scientists use collisions of heavy ions moving near the speed of light to recreate and investigate the quark-gluon plasma (QGP). The QGP is a state of matter at extremely high temperature and density that prevailed in the early universe. In this state, nuclei melt into constituent quarks and gluons, which subsequently flow like a fluid over large distances. By measuring the attenuation (loss of energy) of fast particles that are travelling through this medium, nuclear physicists extract fundamental information about the QGP and gain new knowledge about conditions that existed shortly after the Big Bang.

The Impact

The energy loss of a fast quark or gluon as it traverses a volume of QGP was a key piece of evidence in the discovery of the QGP. Scientists conducted this original research at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York. In early analyses of nucleus-nucleus collisions in QGP, nuclear scientists noted that the energy loss was high in central (head-on) and lower in peripheral (grazing) impacts. The scientists then studied how to best account for different energy losses in various smaller-scale collisions. These include collisions of two protons or of a proton and a nucleus of a heavy atom like lead. A new analysis solved the mystery of energy disparities by weighting the factors that contribute to (apparent) energy losses in diverse collision types.


Fast particles quickly lose energy when they interact with QGP matter, a phenomenon called “jet quenching.” Their attenuation can be quantified by relating the measured number of particles produced in nuclear collisions to that from elementary proton-proton collisions, assuming the former results from many “billiard-ball”-type collisions of the latter. Physicists call the difference in the number of particles produced in different collisions the nuclear modification factor, or “RAA.” Peripheral nuclear collisions should produce no attenuation (R~1), while jet quenching would result in strong attenuation (up to R~0.2) for more head-on, central, collisions. However, before the recent study, scientists observed a value of R of about 0.8 (not the predicted result closer to R~1) in peripheral collisions. This surprising result was first obtained in the early 2000s at RHIC, a DOE Office of Science user facility for the study of nuclear physics. Researchers ascribed the result to modest jet quenching. In this recent study, scientists used a particle production model inspired from proton–proton collisions without jet quenching and performed rigorous measurements of even the most peripheral collisions. For the first time they unambiguously demonstrated the necessity of accounting for geometry and the way collisions were selected for the determination of R. In this way, researchers could connect the dots between peripheral collisions among heavy ions and collisions with other systems involving protons. This corroboration of theory and experiment provides a clearer window to the early universe and will improve analyses of data from ongoing experiments at the Large Hadron Collider and RHIC, as well as at the RHIC’s future sPHENIX experiment.


Constantin Loizides (ORNL), loizidesca@ornl.gov


This material is based upon work supported by the Department of Energy Office of Science, Office of Nuclear Physics. The research also used the ALICE experiment at the CERN Large Hadron Collider, which is partially supported by the Department of Energy Office of Science


Acharya, S. et al. (ALICE Collaboration), “Analysis of the apparent nuclear modification in peripheral Pb-Pb collisions,” Phys. Letters B793 (2019). [DOI: 10.1016/j.physletb.2019.04.047]

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