Bring neutron stars to Earth

The artist’s impression shows two translucent eye sockets that represent tin cores, colliding and shattering in a shower of colorful cracks. Among these gaps, which represent protons, neutrons, and their clusters, is one pion, shown as another translucent sphere with two smaller spheres, representing quarks, inside. Credit: Erin O’Donnell / Facility for Rare Isotope Works

Imagine taking all the water in Lake Michigan – more than a quadrillion gallons – and squeezing it into a 4-gallon bucket, the kind you’d find at a hardware store.

A quick review of the numbers suggests that this must be impossible: that’s too much stuff and not enough space. But this extravagant density is a defining property of celestial objects called neutron stars. These stars are only about 15 miles wide, yet they contain more mass than our sun thanks to some extreme physics.

Led by researchers at Michigan State University, international collaboration has now copied the cosmic conditions of a neutron star on Earth to better explore that extreme science. The team shared their findings in the newspaper Letters on Physical Review.

For the experiment, the team selected tin to help create a dense, neutron-rich nuclear soup, helping it mimic the environment of neutron stars more closely. The team accelerated a beam from tin cores to nearly two-thirds of the speed of light at the RIKEN Nishina Japan Center for Accelerator Science. The research was funded by the Office of Nuclear Physics in the U.S. Department of Energy’s Office of Science, or DOE-SC, and the Department of Education, Culture, Sports, Science and Technology – Japan, or MEXT, Japan.

The researchers sent this beam through the thin tin target, or sheet, to shatter tin cores. The nuclei shatter and for just a moment – a trillion trillionth of a second – the wreckage exists as a super-dense region of nuclear buildings called protons and neutrons. Although this environment is transient, it lives long enough to create rare particles called pions (which is pronounced “pie-ons” – the “pi” comes from the Greek letter π).

By creating and detecting these pions, the team enables scientists to better answer long-standing questions about nuclear science and neutron stars. For example, this work may help scientists better characterize the internal pressure that prevents neutron stars from collapsing under their own gravity and becoming black holes.

“The experiment we conducted cannot be done anywhere other than inside neutron stars,” said Betty Tsang, a professor of nuclear science and a researcher at the National Superconducting Cyclotron Laboratory, or NSCL, at MSU.

Unfortunately scientists cannot set up a store within neutron stars. In addition to bulb temperatures and crushing gravitational forces, the nearest neutron star is about 400 light-years away.

Yet there is another place in the universe where scientists can observe matter packed to such an incredible density. This is in particle-accelerator laboratories, where scientists can smash the nuclei of atoms or nuclei, to squeeze large amounts of nuclear matter into very small volumes.

Of course, this is not a pie either.

“The experiment is very difficult,” Tsang said. “That’s why the team is so excited about this.” Tsang and William Lynch, a professor of nuclear physics in MSU’s Department of Physics and Astronomy, lead the Spartan contingent of researchers on the international team.

In order to realize their collective goals in this study, the collaborating institutes each played according to their strengths.

“That’s why we are accumulating collaborators,” Tsang said. “We’re solving problems by expanding the group and inviting people who really know what they’re doing.”

MSU, which is home to the U.S. graduate program in U.S. nuclear physics, has taken the lead in building the pion detector. The instrument, called the SπRIT Time Projection Chamber, was built with collaborators from the University of Texas A&M and RIKEN.

RIKEN’s particle accelerator offered the power and rare neutron-rich tin nuclei needed to create an environment reminiscent of a neutron star. Researchers from the Technical University, Darmstadt, in Germany contributed to the tin targets that were to meet required specifications. Students, staff and faculty from other institutions across Asia and Europe helped build the experiment and analyze data.

This experiment at RIKEN’s accelerator has helped push that understanding to new heights in terms of energy and density, but there are many more challenges.

When the Facility for Rare Isotope Works, or FRIB, operates in 2022, it also promises to be a center of international cooperation in nuclear science. And the facility will be uniquely equipped to continue to explore how nuclear systems behave at extreme energies and densities.

“When FRIB goes online, it will give us more choice of beams and let us make much more accurate measurements,” Tsang said. “And that will allow us to better understand the interiors of the neutron stars and discover things even more interesting, more surprising.”

Distant, rotating stars give clues to the forces that bind subatomic particles

Additional information:
J. Estee et al. Probing the Symmetric Energy by the Spectral Pion Relation, Letters on Physical Review (2021). DOI: 10.1103 / PhysRevLett.126.162701

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