Pinpointing a Crucial Juncture in Matter Transformation: When the Universe Alters Its Course

A team of physicists at the Relativistic Heavy Ion Collider (RHIC) are scrutinizing phase changes in nuclear matter resulting from collisions of gold ions to pinpoint a key turning point in such metamorphoses. By recreating and closely examining the transition of quark-gluon plasma—a state of matter prevalent after the Big Bang— they are proposing that variations in the formation of lightweight atomic nuclei could signify this turning point. Certain anomalies in the data hint towards potential fluctuations, although additional research is necessary for conclusive proof.

The evaluation of lightweight atomic nuclei derived from gold ion collisions provides valuable understanding into phase changes in primordial matter.

Physicists scrutinizing data from gold ion collisions at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Brookhaven National Laboratory, are in search of evidence that definitively establishes a critical point in the transformation of nuclear matter from one phase to another.

The latest insights from the RHIC’s STAR Collaboration, published in Physical Review Letters, suggest that predictions on the number of lightweight atomic nuclei produced from collisions could be key in mapping nuclear phase changes. Confirming a critical point—a juncture where nuclear matter alters its transformation phase—is instrumental to answering fundamental questions about the composition of our universe.

The central element of the STAR detector at Brookhaven’s Relativistic Heavy Ion Collider is the Time Projection Chamber, which monitors and identifies particles derived from ion collisions. Credit: Brookhaven National Laboratory

“The nuclear phase diagram can be visualized as a bridge linking the past— including the Big Bang and the early universe—to the matter we observe today and even neutron stars,” explained Xiaofeng Luo, a member of the RHIC’s STAR Collaboration from Central China Normal University (CCNU), who spearheaded a group of students in this study. “It holds significant scientific value and furthers our understanding of our origin.”

In Search of the Critical Point

The collisions at the RHIC replicate a hot, dense state of matter that only existed briefly after the Big Bang roughly 14 billion years ago. This matter, known as quark-gluon plasma (QGP), is a mixture of “free” quarks and gluons— the fundamental constituents of the protons and neutrons that comprise atomic nuclei. By colliding heavy ions at varying energies, RHIC physicists can explore how these collisions create this primordial soup and how it transitions back into ordinary nuclear matter.

To identify signs of a critical point—where the transition from QGP to ordinary matter shifts from a smooth crossover (similar to butter slowly melting on a warm day) to a sudden change (akin to water abruptly boiling)—the scientists look for fluctuations in the measures derived from the collisions.

The process of charting nuclear phase changes is akin to studying how water transitions under varying temperature and pressure conditions (or net baryon density for nuclear matter). RHIC’s collisions “melt” protons and neutrons to create quark-gluon plasma (QGP). STAR physicists are experimenting with collisions at different energies, adjusting the “knobs” of temperature and baryon density, to search for signs of a “critical point.” Credit: Brookhaven National Laboratory

A previous study found compelling signs of the kind of fluctuations scientists would expect around the critical point by examining the number of net protons produced at different collision energies. Protons, each consisting of three quarks, form as the QGP cools and can act as proxies for the overall baryon density (baryons are all particles composed of three quarks, including neutrons).

Scientists predict that as the baryon density of matter increases, protons and neutrons are more likely to coalesce to form lightweight atomic nuclei when the QGP “freezes out.” Therefore, in this study, they tried to monitor the yield of a type of lightweight atomic nucleus known as a triton—consisting of one proton and two neutrons. Detecting fluctuation patterns in triton production might help pinpoint the critical point.

As in the previous study, the data were gathered by the Solenoidal Tracker at RHIC, a particle detector known as STAR, during the first phase of the Beam Energy Scan (BES-I). This program recorded images of collisions at various energies and temperatures from 2010 to 2017, documenting changes in the numbers and types of particles being emitted. This new analysis expands upon a paper that Brookhaven physicist Zhangbu Xu and colleagues published in 2017, predicting that the yield ratio of light nuclei such as tritons should be linked to the critical point.

Monitoring fluctuations in the yield ratio of lightweight nuclei such as deuterons and tritons from collisions within the STAR detector should be responsive to a critical point. The data (red points) generally align with predictions (shaded areas), but two anomalous points could be indicators of the kind of fluctuations scientists anticipate around the critical point. Credit: STAR Collaboration

“The creation of these light nuclei requires a certain baryon density,” said Dingwei Zhang, a member of the RHIC’s STAR Collaboration and PhD student at CCNU. “If the system nears the critical point, the baryon density fluctuates significantly. Through this analysis, we aimed to observe these fluctuations, hence pinpointing the critical point.”

The data from most of the collision energies analyzed corresponded to theoretical models of how new nuclei would form as protons and neutrons coalesce. However, at two points—from collisions at 19.6 billion election volts (GeV) and 27 GeV—the data diverged from the baseline predicted by the model, potentially indicating the sought-after fluctuations.

The points offer a combined significance that still falls short of the threshold necessary to claim a physics discovery.

“We hoped this analysis would be sensitive to the critical point,” Luo said. “We are very pleased to see these outliers, and it’s definitely promising. Ultimately, if the critical point exists in the energy range we covered, all these observables should give a consistent signal.”

The team is eager to analyze a wealth of additional collision data. In 2021, the STAR collaboration successfully concluded the second phase of the Beam Energy Scan (BES II), which captured images of gold collisions at various RHIC energies, including the lowest energy of 3 GeV.

“We hope that the BES II data will enhance our sensitivity to a critical point signal,” Luo said. “With higher statistics, we may be able to reach the level of significance required to claim a discovery. And that would be monumental.”

Reference: “Beam Energy Dependence of Triton Production and Yield Ratio (Nt×Np/N2d) in Au+Au Collisions at RHIC” by M. I. Abdulhamid et al. (STAR Collaboration), 16 May 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.202301

The research was sponsored by the DOE Office of Science (NP), the U.S. National Science Foundation, and a variety of international organizations and agencies as acknowledged in the scientific paper.

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• RHIC – Relativistic Heavy Ion Collider
• Quark-Gluon Plasma – CERN
• Physical Review Letters – Journal
• Beam Energy Scan at RHIC
• DOE Office of Science – Nuclear Physics
• U.S. National Science Foundation