Scientists at the Relativistic Heavy Ion Collider (RHIC) have achieved a groundbreaking milestone by observing the directed flow of hypernuclei, which are rare nuclei containing at least one hyperon, in particle collisions. Hyperons, characterized by the presence of a “strange” quark, are believed to be abundant in neutron stars, the densest objects in the universe. By simulating these extreme conditions in the laboratory, researchers aim to unravel the interactions between hyperons and nucleons.
The study conducted at RHIC revealed the directed flow of hypernuclei during particle collisions, offering unprecedented insights into the abundant yet elusive matter found in neutron stars. By closely examining simulated conditions, scientists gained valuable knowledge about interactions crucial for comprehending the intricate structures of neutron stars. These observations, which mirror the flow patterns of regular nuclei, have the potential to refine theoretical models associated with neutron stars.
Physicists at the Relativistic Heavy Ion Collider (RHIC) have recently published their first-ever observation of the directed flow of hypernuclei. These short-lived, scarce nuclei possess at least one “hyperon” alongside ordinary protons and neutrons. Hyperons contain a “strange” quark, replacing one of the up or down quarks found in ordinary nucleons. Such intriguing matter is believed to be abundant in the cores of neutron stars, which rank among the densest and most exotic entities in the universe. While embarking on a journey to neutron stars for studying this exotic matter remains a science fiction concept, particle collisions offer scientists a means to gain insight into these celestial objects from the confines of Earth’s laboratories.
“The conditions inside a neutron star may still be far beyond our current laboratory capabilities, but at this stage, it represents the closest approximation we can achieve,” remarked Xin Dong, a physicist from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (LBNL) and a participant in the study. “By comparing our laboratory data with existing theories, we can attempt to infer what transpires within a neutron star.”
Neutron stars materialize when massive stars collapse at the end of their lifecycle. By studying the collective flow of hypernuclei in high-energy heavy ion collisions, scientists can gain valuable insights into hyperon-nucleon interactions within the nuclear medium and unravel the inner composition of neutron stars.
To conduct their research, scientists employed the STAR detector at RHIC, a nuclear physics research facility operated by the U.S. Department of Energy’s Office of Science at Brookhaven National Laboratory. The detector facilitated the study of flow patterns exhibited by the debris resulting from the collisions of gold nuclei. These patterns are generated by the immense pressure gradients unleashed during the collisions. By comparing the flow of hypernuclei to that of similar ordinary nuclei composed solely of nucleons, researchers aimed to gain insights into the interactions between hyperons and nucleons.
“In our familiar world, nucleon-nucleon interactions form the basis of ordinary atomic nuclei. However, when we delve into the realm of neutron stars, interactions between hyperons and nucleons—about which we still have limited knowledge—become highly relevant to understanding their structure,” explained Yapeng Zhang, another member of the STAR team from the Institute of Modern Physics of the Chinese Academy of Sciences, who spearheaded the data analysis alongside his student Chenlu Hu. Analyzing the flow of hypernuclei allows scientists to uncover the hyperon-nucleon interactions responsible for the formation of these extraordinary particles.
The newly published data in the journal Physical Review Letters provides quantitative information that theorists can utilize to refine their descriptions of hyperon-nucleon interactions, which underpin the creation of hypernuclei and shape the macroscopic structures of neutron stars.
Zhang stated, “There are currently no definitive calculations that establish the nature of these hyperon-nucleon interactions. This measurement has the potential to constrain theories and serve as a variable input for further calculations.”
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Unveiling the Flow Patterns
Previous experiments have demonstrated that the flow patterns of regular nuclei generally follow a mass-scaling relationship. In other words, the greater the number of protons and neutrons in a nucleus, the more pronounced the collective flow in a specific direction. This suggests that these nuclei inherit their flow characteristics from the interactions between their constituent protons and neutrons, governed by the strong nuclear force.
The results obtained by the STAR team in this study indicate that hypernuclei conform to the same mass-scaling pattern. Consequently, hypernuclei are most likely formed via the same mechanism.
“In the coalescence mechanism, nuclei (including hypernuclei) form based on the strength of interactions between their individual components,” explained Dong. “This mechanism provides insights into the interactions between nucleons in regular nuclei and nucleons and hyperons in hypernuclei.”
Furthermore, observing similar flow patterns and mass scaling relationships in both normal nuclei and hypernuclei implies that the interactions between nucleons and hyperons are remarkably similar.
The flow patterns also offer valuable information about the matter produced during particle collisions, including its temperature, density, and other properties.
Zhang elaborated, “The pressure gradient generated in the collision induces asymmetry in the direction of outgoing particles. Therefore, the observed flow reflects the manner in which the pressure gradient is established within the nuclear matter.”
“The measured flow of hypernuclei could open up new avenues for studying hyperon-nucleon interactions under finite pressure and at high baryon density,” added Zhang.
To gain a deeper understanding of the properties of the matter involved, scientists will utilize additional measurements of how hypernuclei interact with the surrounding medium.
The Advantages of Low Energy
This research achievement was made possible by the versatility of RHIC, which can operate across an extensive range of collision energies. The measurements were carried out during Phase I of the RHIC Beam Energy Scan, a systematic investigation of gold-gold collisions spanning from 200 GeV per colliding particle pair down to 3 GeV.
To reach the lowest energy levels, RHIC operated in “fixed-target” mode. In this configuration, a beam of gold ions circled the 2.4-mile-circumference RHIC collider and collided with a gold foil placed inside the STAR detector. This low energy setting grants scientists access to the highest baryon density, a parameter closely linked to the pressure generated in the collisions.
“At this lowest collision energy, where the matter created in the collision is extremely dense, the production of nuclei and hypernuclei is more abundant compared to higher collision energies,” explained Yue-Hang Leung, a postdoctoral fellow from the University of Heidelberg, Germany. “The low-energy collisions are the only ones that generate a sufficient number of these particles, providing the necessary statistical data for our analysis. No one has ever conducted such research before.”
Implications for Neutron Stars
The fact that hypernuclei seem to form through coalescence, just like ordinary nuclei, suggests that they, like their regular counterparts, are created during the later stages of the collision process.
“At this late stage, the density of hyperon-nucleon interaction that we observe is not particularly high,” stated Dong. “Therefore, these experiments may not directly simulate the environment of a neutron star.”
Nevertheless, he emphasized, “This data is fresh. We need input from our theoretical colleagues, who must consider this new data on hyperon-nucleon interactions when constructing new neutron star models. The collective efforts of both experimentalists and theorists are necessary to fully understand this data and establish the relevant connections.”
Reference: “Observation of Directed Flow of Hypernuclei 3ΛH and 4ΛH in √sNN=3 GeV Au+Au Collisions at RHIC” by B. E. Aboona et al. (STAR Collaboration), 24 May 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.212301
This research received funding from the DOE Office of Science (NP), the U.S. National Science Foundation, and various international organizations and agencies listed in the scientific paper. The STAR team utilized computing resources at the Scientific Data and Computing Center at Brookhaven Lab, the National Energy Research Scientific Computing Center (NERSC) at DOE’s Lawrence Berkeley National Laboratory, and the Open Science Grid consortium.
Frequently Asked Questions (FAQs) about hypernuclei
What did the scientists observe at the Relativistic Heavy Ion Collider?
The scientists at the Relativistic Heavy Ion Collider (RHIC) observed the directed flow of hypernuclei, which are rare nuclei containing at least one hyperon, in particle collisions.
Why are hyperons important in this study?
Hyperons, which contain a “strange” quark, are believed to be abundant in neutron stars, one of the universe’s densest objects. By studying hypernuclei, researchers aim to understand the interactions between hyperons and nucleons and gain insights into neutron star structures.
How did the scientists conduct their study?
The scientists used the STAR detector at RHIC to study the flow patterns of debris emitted from collisions of gold nuclei. They compared the flow of hypernuclei to that of similar ordinary nuclei made of nucleons to gain insights into the interactions between hyperons and nucleons.
What are the implications of the observed flow patterns?
The flow patterns observed in both normal nuclei and hypernuclei indicate that the interactions between nucleons and hyperons are remarkably similar. These observations provide quantitative information for refining theoretical models of hyperon-nucleon interactions and contribute to understanding the formation of hypernuclei and the structures of neutron stars.
How does this research relate to neutron stars?
Although the laboratory conditions cannot fully replicate those inside a neutron star, studying hypernuclei flow allows scientists to infer and gain valuable insights into the hyperon-nucleon interactions and properties relevant to neutron stars. The data obtained from this study can inform the development of new neutron star models that incorporate hyperon-nucleon interactions.
