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Advancements in Nuclear Physics: New Insights into Atomic Nuclei Shapes

Recent research has unveiled a groundbreaking technique utilizing high-energy particle collisions at the Relativistic Heavy Ion Collider (RHIC), operated by the U.S. Department of Energy at Brookhaven National Laboratory. This method enhances the understanding of atomic nuclei shapes, as discussed in a paper published in Nature. By complementing traditional low-energy approaches to nuclear structure determination, this innovation offers a richer perspective on the nuclei that constitute a significant part of observable matter.

According to Jiangyong Jia, a professor at Stony Brook University (SBU) and a key author of the STAR Collaboration paper, “In this new measurement, we not only quantify the overall shape of the nucleus — whether it’s elongated like a football or squashed down like a tangerine — but also the subtle triaxiality, the relative differences among its three principal axes that characterize a shape in between the ‘football’ and ‘tangerine.'” This nuanced understanding holds crucial implications for various fields of physics.

Determining the shapes of atomic nuclei could answer pivotal questions about nuclear fission processes, the formation of heavy elements in neutron star collisions, and obscure particle decay phenomena. Improved nuclear shape comprehension also enriches scientists’ insights into the primordial conditions of a particle soup that echoes the early universe, generated during RHIC’s intense particle collisions. Moreover, this approach can be applied to analyze additional data from RHIC and the Large Hadron Collider (LHC) in Europe, extending its relevance to future investigations at the Electron-Ion Collider, also being developed at Brookhaven Lab.

As emphasized by Jia, “The best way to demonstrate the robustness of nuclear physics knowledge gained at RHIC is to show that we can apply the technology and physics insights to other fields. Now that we’ve demonstrated a robust way to image nuclear structure, there will be many applications.”

From Long Exposure to High-Speed Imaging

For many years, researchers relied on low-energy experimentation to infer the shapes of atomic nuclei. This traditional method involved exciting nuclei and measuring the photons emitted as nuclei returned to their ground state. However, these techniques provided an overview of the spatial arrangement of protons within the nucleus but lacked the capability to detect rapid variations due to extended observation times.

“In low-energy experiments, it’s like taking a long-exposure picture,” explained Chun Shen, a theorist at Wayne State University, who contributed to the new analysis. Since these methods don’t operate on short time scales, they fail to capture intricate internal arrangements among protons and cannot directly observe uncharged neutrons.

Dean Lee, a low-energy theorist at the Facility for Rare Isotope Beams, noted, “You only get an average of the whole system.” Although Lee and Shen were not co-authors of the study, their work as theorists significantly aided the development of the new imaging method.

Conversely, the high-energy approach employed by RHIC facilitates the collection of multiple “freeze-frame” snapshots that provide detailed insights into both protons and neutrons, achieving a speed far surpassing that of traditional methods. “These snapshots come from different collision events,” remarked Jia, emphasizing that the nuclei are destroyed upon collision. Thus, by examining numerous images from various collisions, scientists can reconstruct the intricate 3D structure of the impacted nuclei.

Lee elaborated, “In each collision, you freeze time for a moment and look at where all the protons and neutrons are. The high-energy method captures a ton of information, capturing complexities overlooked by lower-energy methods.”

Reconstructing Nuclei Shapes from Collision Debris

The STAR detector’s ability to discern complexity arises from analyzing the trajectories and velocities of particles emerging from intense nuclear collisions. The STAR team’s insights connect to Richard Feynman’s analogy of determining a watch’s components by smashing two together and observing the fragments.

From extensive experiments at RHIC, scientists understand that high-energy collisions disrupt the protons and neutrons within nuclei, releasing their fundamental building blocks—quarks and gluons. The shape and behavior of the resulting quark-gluon plasma (QGP) is fundamentally influenced by the shape of the colliding nuclei. Variations in the QGP blobs’ shapes directly affect pressure gradients, modulating the collective flow and momentum of particles released during the QGP’s cooling phase.

The STAR team ingeniously derived a method to “reverse engineer” nuclear structures by correlating the flow patterns and momentum of the emerging particles with hydrodynamic models representing distinct QGP shapes, thus revealing the original states of the colliding nuclei.

Through comparing central collisions between gold nuclei, expected to be nearly spherical, and elongated uranium nuclei, the researchers anticipated differing flow patterns based on variations in nuclear shape. Observations supported this hypothesis, leading to a quantitative assessment of uranium nuclei shapes and a preliminary understanding of their three principal axes.

Overcoming Computational Challenges

Achieving accurate predictions from various hydrodynamic models involved complex computational tasks. Zhang, a former SBU postdoctoral fellow, dedicated more than a year to this work, utilizing the Open Science Grid to process around 20 million CPU hours—generating over ten million collision events for model fitting against experimental data.

Notable features in the STAR findings illuminated significant disparities between uranium and gold nuclei shapes, revealing unexpectedly complex characteristics in uranium nuclei that challenge previously held notions.

Broader Implications for Nuclear Physics

The introduction of this high-energy imaging technique aims to refine physicists’ knowledge of initial conditions in heavy ion collisions producing QGP at both RHIC and the LHC. Previous low-energy studies have established crucial links between nuclear structures and hydrodynamic flow patterns, facilitating the understanding that the QGP formed during these collisions behaves like a nearly perfect liquid. The new method provides an opportunity to verify these findings against low-energy techniques, promoting consistency and minimizing uncertainties in determining QGP properties.

This innovative approach is not limited to uranium but extends to other nuclei as well, particularly those with previously ambiguous structures. One avenue of research could involve isobar nuclei—pairs with an identical nucleon count yet dissimilar proportions. Such studies are vital in deriving model uncertainties linked to rare nuclear decay processes, such as neutrinoless double beta decay.

As Jia noted, this research has interdisciplinary implications, fostering connections across various subfields of nuclear physics. Increased dialogue and collaboration have arisen globally, demonstrated through numerous workshops and conferences addressing the intersections between high-energy and low-energy nuclear research.

This work received support from the DOE Office of Science, the U.S. National Science Foundation, and various international agencies, utilizing computational resources from the Scientific Data and Computing Center at Brookhaven Lab and the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory.

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