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Scientists Smash Atoms to Smithereens, Revealing Hidden Nuclear Shapes







Scientists employ high-energy heavy ion collisions as a powerful tool to uncover intricate details of nuclear structure, offering insights with broad implications across various fields of physics.

Scientists have developed a novel technique using high-energy particle collisions at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy (DOE) Office of Science user facility for nuclear physics research located at DOE’s Brookhaven National Laboratory. Detailed in a newly published paper in Nature, this method complements lower-energy approaches for studying nuclear structure. It offers deeper insights into the shapes of atomic nuclei, enhancing our understanding of the building blocks of visible matter.

“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 principle axes that characterize a shape in between the ‘football’ and ‘tangerine,’” said Jiangyong Jia, a professor at Stony Brook University (SBU) who has a joint appointment at Brookhaven Lab and is one of the principal authors on the STAR Collaboration publication.

Deciphering nuclear shapes has relevance to a wide range of physics questions, including which atoms are most likely to split in nuclear fission, how heavy atomic elements form in collisions of neutron stars, and which nuclei could point the way to exotic particle decay discoveries. Leveraging improved knowledge of nuclear shapes will also deepen scientists’ understanding of the initial conditions of a particle soup that mimics the early universe, which is created in RHIC’s energetic particle smashups. The method can be applied to analyzing additional data from RHIC as well as data collected from nuclear collisions at Europe’s Large Hadron Collider (LHC). It will also have relevance to future explorations of nuclei at the Electron-Ion Collider, a nuclear physics facility in the design stage at Brookhaven Lab.

Ultimately, since 99.9% of the visible matter that people and all the stars and planets of the cosmos are made of resides in the nuclei at the center of atoms, understanding these nuclear building blocks is at the heart of understanding who we are.

“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,” Jia said. “Now that we’ve demonstrated a robust way to image nuclear structure, there will be many applications.”

From long exposure to freeze-frame snapshots

For decades, scientists used low-energy experiments to infer nuclear shapes  for example, by exciting the nuclei and observing photons, or particles of light, emitted as the nuclei decay back to the ground state. This method probes the overall spatial arrangement of the protons inside the nucleus, but only at a relatively long time scale.

“In low-energy experiments, it’s like taking a long-exposure picture,” said Chun Shen, a theorist at Wayne State University whose calculations were used in the new analysis.

Because the exposure time is long, the low-energy methods do not capture all the subtle variations in the arrangement of protons that can occur inside a nucleus at very fast timescales. And because most of these methods use electromagnetic interactions, they can’t directly “see” the uncharged neutrons in the nucleus.

“You only get an average of the whole system,” said Dean Lee, a low-energy theorist at the Facility for Rare Isotope Beams, a DOE Office of Science user facility at Michigan State University. Though Lee and Shen are not co-authors on the study, they and other theorists have contributed to developing this new nuclear imaging method.

Reconstructing shapes from debris

How exactly does STAR see that complexity if the nuclei get destroyed? By tracking how particles fly out  and how fast  from the most central, head-on nuclear smashups.

As the STAR scientists note in their Nature paper, “In an ironic twist, this effectively realizes [famous physicist] Richard Feynman’s analogy of the seemingly impossible task of ‘figuring out a pocket watch by smashing two together and observing the flying debris.’”

From years of experiments at RHIC, the scientists know that high energy nuclear collisions melt the protons and neutrons of the nuclei to set free their inner building blocks, quarks and gluons. The shape and expansion of each hot blob of this melted nuclear matter, known as a quark-gluon plasma (QGP), is determined by the shape of the colliding nuclei. The shape and size of each QGP blob directly affect pressure gradients generated in that blob of plasma, which in turn influence the collective flow and momentum of particles emitted as the QGP cools.

The STAR scientists reasoned they could “reverse engineer” this relationship to derive information about nuclear structure. They analyzed the flow and momentum of particles emerging from collisions and compared them with models of hydrodynamic expansion for different QGP shapes to arrive at the shapes of the originally colliding nuclei.

Website: International Research Awards on High Energy Physics and Computational Science.

#HighEnergyPhysics#ParticlePhysics#QuantumPhysics#AstroparticlePhysics#ColliderPhysics#HiggsBoson#LHC#QuantumFieldTheory#NeutrinoPhysics#PhysicsResearch#ComputationalScience#DataScience#ScientificComputing#NumericalMethods#HighPerformanceComputing#MachineLearningInScience#BigData#AlgorithmDevelopment#SimulationScience#ParallelComputing

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