Note: This press release has been adapted from an original release by Brookhaven National Laboratory. Read the original release.
Physicists studying collisions of gold ions at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy Office of Science user facility for nuclear physics research at DOE’s Brookhaven National Laboratory, are embarking on a journey through the phases of nuclear matter – the stuff that makes up the nuclei of all the visible matter in our universe.
A new analysis of collisions conducted at different energies shows tantalizing signs of a critical point – a change in the way that quarks and gluons, the building blocks of protons and neutrons, transform from one phase to another. Physicists from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) contributed to the findings, published March 5 by RHIC’s STAR Collaboration in the journal Physical Review Letters. The findings will help physicists map out details of these nuclear phase changes to better understand the evolution of the universe and the conditions in the cores of neutron stars.
“If we are able to discover this critical point, then our map of nuclear phases – the nuclear phase diagram – may find a place in the textbooks, alongside that of water,” said Bedanga Mohanty of India’s National Institute of Science and Research, one of hundreds of physicists collaborating on research at RHIC using the sophisticated STAR detector. Mohanty is a former Berkeley Lab postdoctoral researcher.
As Mohanty noted, studying nuclear phases is somewhat like learning about the solid, liquid, and gaseous forms of water, and mapping out how the transitions take place depending on conditions like temperature and pressure. But with nuclear matter, you can’t just set a pot on the stove and watch it boil. You need powerful particle accelerators like RHIC to turn up the heat.
RHIC’s highest collision energies “melt” ordinary nuclear matter (atomic nuclei made of protons and neutrons) to create an exotic phase called a quark-gluon plasma (QGP). Scientists believe the entire universe existed as QGP a fraction of a second after the Big Bang – before it cooled and the quarks bound together (glued by gluons) to form protons, neutrons, and eventually, atomic nuclei. But the tiny drops of QGP created at RHIC measure a mere 10-13 centimeters across (that’s 0.0000000000001 centimeter.) and they last for only 10-23 seconds! That makes it incredibly challenging to map out the melting and freezing of the matter that makes up our world.
“Strictly speaking if we don’t identify either the phase boundary or the critical point, we really can’t put this [QGP phase] into the textbooks and say that we have a new state of matter,” said Nu Xu, a STAR physicist at Berkeley Lab.
Tracking phase transitions
To track the transitions, STAR physicists took advantage of the incredible versatility of RHIC to collide gold ions (the nuclei of gold atoms) across a wide range of energies.
“RHIC is the only facility that can do this, providing beams from 200 billion electron volts (GeV) all the way down to 3 GeV. Nobody can dream of such an excellent machine,” Xu said.
The changes in energy turn the collision temperature up and down and also vary a quantity known as net baryon density that is somewhat analogous to pressure. Looking at data collected during the first phase of RHIC’s “beam energy scan” from 2010 to 2017, STAR physicists tracked particles streaming out at each collision energy.
“The idea for the beam energy scan was conceived at Berkeley Lab in 2004,” noted Hans-Georg Ritter, a former head of the Relativistic Nuclear Collisions Program in Berkeley Lab’s Nuclear Science Division.
The STAR physicists then performed a detailed statistical analysis of the net number of protons produced. A number of theorists had predicted that this quantity would show large event-by-event fluctuations as the critical point is approached.
The reason for the expected fluctuations comes from a theoretical understanding of the force that governs quarks and gluons. That theory, known as quantum chromodynamics, suggests that the transition from normal nuclear matter (“hadronic” protons and neutrons) to QGP can take place in two different ways: at high energies and at lower energies.
“Quantum chromodynamics, the theory of the strong interaction – one of the fundamental forces of nature – covers physics ranging from the early universe to compact stars,” said Volker Koch, a theorist at Berkeley Lab.
“At vanishing baryon density, the transition from quark-gluon plasma to hadronic particles is a smooth crossover,” meaning it is slow and gradual. This type of transition is known as a phase transition – like water evaporating into a gas or melting into a liquid. When it reaches a certain baryon density, the transition is expected to be of “first order,” meaning it is abrupt and rapid. The point where the smooth transition turns into an abrupt one is referred to as a critical point and is characterized by large fluctuations.
“Realizing this QCD critical point is an important landmark,” Koch said. If it is discovered, it would establish – for the first time – a phase transition that involves the fundamental particles of the Standard Model: quarks and gluons.” The Standard Model of particle physics is like a rule book for the physics of the universe.
At high energies and temperatures, where protons and anti-protons are produced in pairs and the net baryon density is close to zero, physicists have evidence of a smooth crossover between the phases. It’s as if protons gradually melt to form QGP, like butter gradually melting on a counter on a warm day.
But at lower energies, they expect what’s called a first-order phase transition – an abrupt change like water boiling at a set temperature as individual molecules escape the pot to become steam. Nuclear theorists predict that in the QGP-to-hadronic-matter phase transition, net proton production should vary dramatically as collisions approach this switchover point.
“At high energy, there is only one phase. The system is more or less invariant, normal,” Xu said. “But when we change from high energy to low energy, you also increase the net baryon density, and the structure of matter may change as you are going through the phase transition area.
The oscillations they see in these higher orders, particularly the skew (or kurtosis), are reminiscent of another famous phase change observed when transparent liquid carbon dioxide suddenly becomes cloudy when heated, the scientists say. This “critical opalescence” comes from dramatic fluctuations in the density of the CO2 – variations in how tightly packed the molecules are.
“In our data, the oscillations signify that something interesting is happening, like the opalescence,” Mohanty said.
Yet despite the tantalizing hints, the STAR scientists acknowledge that the range of uncertainty in their measurements is still large. The team hopes to narrow that uncertainty to nail their critical point discovery by analyzing a second set of measurements made from many more collisions during phase II of RHIC’s beam energy scan, from 2019 through 2021.
The entire STAR collaboration was involved in the analysis, Xu noted, with a particular group of physicists – including Xiaofeng Luo, a former PhD student at Berkeley Lab who is from China; Yu Zhang, a PhD student at the Lab; Ashish Pandav, a PhD student from India; and Toshihiro Nonaka, from Japan. The researchers met weekly with the U.S. scientists – over many time zones and virtual networks – to discuss and refine the results.
The work is also a true collaboration of the experimentalists with nuclear theorists around the world and the accelerator physicists at RHIC. The latter group, in Brookhaven Lab’s Collider-Accelerator Department, devised ways to run RHIC far below its design energy while also maximizing collision rates to enable the collection of the necessary data at low collision energies.
“We are exploring uncharted territory,” Xu said. “This has never been done before. We made lots of efforts to control the environment and make corrections, and we are eagerly awaiting the next round of higher statistical data,” he said.
This study was supported by the DOE Office of Science (NP), the U.S. National Science Foundation, and a wide range of international funding agencies listed in the paper. RHIC operations are funded by the DOE Office of Science. Data analysis was performed using computing resources at the RHIC and ATLAS Computing Facility (RACF) at Brookhaven Lab, the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory, and via the Open Science Grid consortium.
NERSC is a DOE Office of Science user facility.
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