Merging the communities of heavy ions and neutron stars

10/28/2020 Jessica Raley for ICASU

Written by Jessica Raley for ICASU

Group photo from a virtual workshop entitled "From heavy-ion collisions to neutron stars."
Group photo from a virtual workshop entitled "From heavy-ion collisions to neutron stars."

In a new paper, a team of physicists from the University of Illinois, Kent State, and the University of Houston articulate a central conclusion from their recent virtual workshop: mapping the complete quantum chromodynamics (QCD) phase diagram will require an interdisciplinary collaboration of experts in lattice QCD, nuclear physics, relativistic hydrodynamics, gravitational waves, computational astrophysics, computer science, and statistical theory.

In August, Illinois Physics professors Jorge Noronha, Jacquelyn Noronha-Hostler, and Nicolás Yunes teamed up with Professor Veronica Dexheimer from Kent State and Professor Claudia Ratti from the University of Houston to host a workshop entitled, “From heavy-ion collisions to neutron stars.” According to the workshop website, the purpose was to bring together “the communities of heavy-ion and neutron star physics to discuss ways of mapping out the high-energy equation of state across the entire QCD phase diagram.” The organizers’ summary of the workshop is now available on the arXiv.

At low densities and high temperatures, lattice QCD prescribes the equation of state. However, the equation of state remains unknown at higher densities and lower temperatures, such as in the core of a neutron star. 

Phase diagram of QCD showing an estimate of the region that can be studied using heavy-ion collisions vs. neutron stars and their mergers. The X axis is temperature and the Y axis is baryon chemical potential.
Phase diagram of QCD showing an estimate of the region that can be studied using heavy-ion collisions vs. neutron stars and their mergers. The X axis is temperature. The  Y axis is baryon chemical potential.

Data from the Laser Interferometer Gravitational Wave Observatory (LIGO) provide a great deal of information about what happens when two neutron stars collide, and this information may reveal clues about the equation of state at such high densities.

Professor Yunes, who directs the Illinois Center for Advanced Studies of the Universe (ICASU), says, “The densities and pressures are so high in the core of neutron stars that we currently lack an understanding of the true phases of matter in this regime. Matter could transform into phases that resemble pasta, or even into a soup of quarks, known as a ‘quark-gluon plasma.” 

The quark-gluon plasma can be produced experimentally in heavy-ion collisions here on Earth, where we can study its properties at high temperatures over a range of densities. However, scientists do not know whether quark-gluon plasmas also exist at much lower temperatures and higher densities, such as at the core of a neutron star. LIGO may hold the key to answering this question.

According to Professor Ratti, at this workshop, “The equation of state of strongly interacting matter was identified as one of the most urgent observables for both heavy-ion physics and astrophysics."

Professor Dexheimer adds, “Understanding extremely dense and extremely hot matter requires contributions from different areas of research, including nuclear physics, high energy physics, and gravitation. This implies bringing together experts from institutions across the country and beyond.”

When ICASU was created earlier this year, one of the primary goals was to promote interdisciplinary collaboration. Associate Director, Professor Noronha, says “Nuclear physics, high energy physics, and gravitation are 3 of the 4 main pillars of ICASU, so this workshop was a perfect fit. It is representative of exactly the kind of collaboration we imagined when we created the center.”

One issue that the workshop participants tackled was the difficulty of communicating across disciplines. Professor Noronha-Hostler says, “We created this workshop to establish a common language between research communities, initiate cross-disciplinary collaborations, and identify the most crucial questions that the scientific community will need to address to map out the QCD phase diagram in the years to come.”

“Communication between different disciplines within physics can be challenging, because each community uses highly specialized language,” notes Professor Yunes. “Even the word ‘model’ may mean completely different things to a relativistic hydrodynamics specialist and a gravitational wave data analyst. The first step toward mapping the whole QCD phase diagram is to develop a common language.”

The August 2020 workshop was funded by the National Science Foundation (NSF), and the organizers have already secured additional funds to host another workshop next summer, one they hope participants will be able to attend in person at the Urbana campus. 

Professor Ratti notes that this year’s meeting was “a kickoff to identify common goals between different communities and to explore the interests of these communities in working together.” Given the success of this year's workshop, she says, the organizers plan to host a series of meetings with the goal of “finding solutions to many open questions in the field of strongly interacting matter under extreme conditions.”

“It was fantastic to see the excitement and engagement from the community! We had around 100 participants and would have likely had many more if we had advertised on a larger scale, something we hope to do next time around,” Professor Noronha-Hostler concludes. 


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This story was published October 28, 2020.