New data from RHIC could help understand hyperon-nucleon interactions

Scientists at the Relativistic Heavy Ion Collider (RHIC) have made a significant discovery regarding the directed flow of hypernuclei. These rare and short-lived nuclei contain “hyperons,” which are particles consisting of at least one “strange” quark instead of the usual up or down quarks found in ordinary nucleons (such as protons and neutrons). Hyperons are believed to be prevalent in the dense cores of neutron stars, which are extremely dense and exotic astronomical objects. While physically visiting neutron stars for study remains in the realm of science fiction, particle collisions can offer scientists valuable insights into these celestial bodies within the controlled environment of a laboratory.

Xin Dong, a physicist from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (LBNL) who participated in the study, stated that while laboratory conditions are still far from replicating those found in a neutron star, they provide the closest approximation currently available. By comparing data obtained from these laboratory experiments with existing theories, researchers can attempt to infer the behavior of matter in neutron stars.

The study utilized the STAR detector at RHIC, which is a facility for nuclear physics research located at Brookhaven National Laboratory and funded by the DOE Office of Science. The scientists focused on analyzing the flow patterns of debris resulting from collisions between gold nuclei. These patterns arise due to the immense pressure gradients generated during the collisions. By comparing the flow of hypernuclei with that of ordinary nuclei composed solely of nucleons, scientists aimed to gain a deeper understanding of the interactions between hyperons and nucleons.

Yapeng Zhang, a member of STAR from the Institute of Modern Physics of the Chinese Academy of Sciences, who spearheaded the data analysis alongside his student Chenlu Hu, emphasized the importance of hyperon-nucleon interactions in understanding the structure of neutron stars. While nucleon-nucleon interactions form ordinary atomic nuclei in our familiar world, the dynamics change significantly within the extreme environment of a neutron star. Tracking the flow of hypernuclei allows scientists to gather insights into the hyperon-nucleon interactions that lead to the formation of these exotic particles.

The findings, recently published in Physical Review Letters, furnish quantitative information that theorists can employ to refine their models of hyperon-nucleon interactions, which play a vital role in shaping both hypernuclei and the overall structure of neutron stars.

Zhang stated that currently, there are no well-established calculations regarding hyperon-nucleon interactions. The measurement obtained from this study could potentially constrain theories and provide a variable input for these calculations, advancing our understanding of these intriguing particles and their interactions.

Go with the flow

Previous experiments have demonstrated that the flow patterns of regular nuclei exhibit a scaling behavior with mass. This means that as the number of protons and neutrons in a nucleus increases, so does its collective flow in a specific direction. This scaling behavior suggests that the flow of regular nuclei arises from the interactions between their constituent protons and neutrons, which are governed by the strong nuclear force.

The recent findings from the STAR experiment indicate that hypernuclei also follow this mass-scaling pattern. This implies that hypernuclei likely form through the same mechanism as regular nuclei.

Xin Dong explained that the formation of nuclei and hypernuclei depends on the strength of interactions between their individual components. The coalescence mechanism provides insights into the interactions between nucleons (in nuclei) and nucleons/hyperons (in hypernuclei).

The similarity in flow patterns and mass scaling between regular nuclei and hypernuclei suggests that the interactions between nucleons and hyperons in hypernuclei are very similar to nucleon-nucleon interactions.

The observed flow patterns also provide information about the properties of the matter produced in particle collisions, such as its temperature, density, and other characteristics.

Yapeng Zhang noted that the pressure gradient generated during collisions induces asymmetry in the direction of outgoing particles. Therefore, the observed flow reflects how the pressure gradient is established within the nuclear matter.

The measurement of hypernuclei flow opens up new possibilities for studying hyperon-nucleon interactions under high baryon density and finite pressure conditions.

In future studies, scientists aim to gather additional measurements regarding how hypernuclei interact with the surrounding medium, which will provide further insights into its properties.

The benefits of low energy

The success of this research heavily relied on the versatility of RHIC, which can operate across a wide range of collision energies. The measurements were conducted during Phase I of the RHIC Beam Energy Scan, which involved studying gold-gold collisions spanning from 200 GeV per colliding particle pair down to 3 GeV.

To achieve the lowest energy level, RHIC operated in “fixed-target” mode, where one beam of gold ions circled around the 2.4-mile-circumference collider and collided with a gold foil placed inside the STAR detector. This low energy setting allowed scientists to explore the highest baryon density, a measurement related to the pressure generated during the collisions.

Yue-Hang Leung, a postdoctoral fellow from the University of Heidelberg, Germany, explained that at the lowest collision energy, where the created matter is extremely dense, nuclei and hypernuclei are produced more abundantly compared to higher collision energies. The low-energy collisions provided the necessary statistical data for the analysis, making it a unique accomplishment.

In terms of the relevance to neutron stars, the formation of hypernuclei through coalescence, similar to ordinary nuclei, suggests that both types of nuclei are created during the later stages of the collision system’s evolution.

Xin Dong acknowledged that the density of hyperon-nucleon interaction observed at this late stage may not directly simulate the conditions in a neutron star. However, the obtained data is fresh and requires input from theoretical experts. Incorporating this new data on hyperon-nucleon interactions into neutron star models is crucial. Dong emphasized the need for collaboration between experimentalists and theorists to understand the data and establish connections between the findings and neutron star research.

Source: Brookhaven National Laboratory

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