Fusion of nuclear physics experiments and astronomical observations to advance equation of state research

For most stars, neutron stars and black holes are their final resting places. When a supergiant star runs out of fuel, it expands and then quickly collapses in on itself. This action created a neutron stellar object denser than the Sun, crammed into a space 13 to 18 miles wide. In such a highly condensed stellar environment, most electrons combine with protons to form neutrons, creating a dense ball of matter composed primarily of neutrons. Researchers are trying to understand the forces that control this process by creating dense matter by colliding neutron-rich nuclei in the laboratory and making detailed measurements.

A research team led by William Lynch and Betty Tsang at the Facility for Rare Isotope Beams (FRIB) focused on understanding neutrons in dense environments. Lynch, Tsang and their collaborators used 20 years of experimental data from accelerator facilities and neutron star observations to understand how particles interact in nuclear matter at various densities and pressures. The team wanted to determine how the ratio of neutrons to protons affects the nuclear forces in the system. The team recently worked on natural astronomy.

“In nuclear physics we are often limited to studying small systems, but we know exactly which particles are in our nuclear systems. Stars provide us with incredible opportunities because they are crucial to how nuclear physics plays out. Large systems that act on each other, but we don’t know exactly what particles are inside them,” said Lynch, a professor of nuclear physics at FRIB and the Department of Physics and Astronomy at Michigan State University (MSU).

“They are interesting because the density varies so much in such a large system. Nuclear forces play a dominant role in them, but we know very little about this role.”

When a star 20-30 times the mass of the Sun runs out of fuel, it cools, collapses and explodes as a supernova. After this explosion, only the deepest material inside the star condenses to form a neutron star. The neutron star has no fuel to burn, and over time it radiates its remaining heat into the surrounding space.

Scientists expect that the material in the outer core of a cold neutron star will be roughly similar to the material in the nucleus of an atom, but there are three differences: the neutron star is larger, the interior is denser, and most of the nuclei are neutrons. Deep within a neutron star’s core, the composition of the neutron star’s material remains a mystery.

“If experiments can provide more guidance about the forces acting within them, we can better predict their internal composition and phase transitions,” Lynch said. “Neutron stars provide a great research opportunity to combine these disciplines.” “

Accelerator facilities like FRIB help physicists study how subatomic particles interact under the exotic conditions more common in neutron stars. When researchers compare these experiments to neutron star observations, they can calculate the equation of state (EOS) for particles interacting in a low-temperature, dense environment.

EOS describes matter under specific conditions and how its properties change with density. Solving EOS in a variety of environments helps researchers understand the effects of the strong nuclear force inside dense objects such as neutron stars in the universe. It could also help us understand more about what happens to neutron stars as they cool.

“This is the first time we have brought together such a rich set of experimental data to explain the equation of state under these conditions, which is important,” said Tsang, professor of nuclear science at FRIB. “Previous efforts used theory to explain the low density and low energy of nuclear matter. end. We want to use all the data from our previous experience with the accelerator to obtain a comprehensive equation of state.”

Researchers seeking EOS typically calculate it at higher temperatures or lower densities. They then draw conclusions about the system under a wider range of conditions. In recent years, however, physicists have come to realize that the EOS obtained from experiments is only relevant to a specific density range.

Therefore, the team needed to bring together data from various accelerator experiments that use different measurements of collision nuclei to replace these assumptions with data. “In this work, we asked two questions,” Lynch said. “For a given measurement, what density does the measurement detect? After that, we ask what the measurement tells us about the equation of state at that density.”

In the recent paper, the team combined experiments it conducted at accelerator facilities in the United States and Japan. It brings together data from 12 different experimental constraints and three neutron star observations. The researchers focused on determining the equation of state for nuclear matter, ranging from half to three times the saturation density of the nucleus (the density of all stable nuclei). By producing this comprehensive EOS, the team provides the larger nuclear physics and astrophysics community with a new benchmark for more accurately modeling the interactions of nuclear matter.

The team improved medium-density measurements that cannot be provided by neutron star observations) through experiments at the GSI Helmholtz Center for Heavy Ion Research in Germany, the RIKEN Nishina Accelerator Science Center in Japan, and the National Superconducting Cyclotron Laboratory (the predecessor of FRIB). To achieve the key measurements discussed in this paper, their experiments helped fund advances in data collection technologies for active targets and time projection chambers that are being used in many other experiments around the world.