Window on the Universe: Equation of State of Dense Matter

Michigan State University, East Lansing MI

Investigators

Abstract

The groundbreaking LIGO-Virgo discovery of the GW170817 gravitational wave signals from a neutron star collision marked a major milestone in Astronomy. For the first time, scientists not only detected gravitational waves, but also observed light in the form of gamma rays, visible light, and radio waves from the same event. This remarkable achievement—made possible by advanced U.S. scientific investments—launched a new era of “multi-messenger” astronomy, wherein different signals from an event, like the GW170817 event, are combined to provide a better understanding of that event. Data from the GW170817 event also confirmed that these stellar collisions are sites where heavy elements are produced via nucleosynthetic processes. To explore questions on these processes under laboratory controlled conditions, the PI’s, their graduate student and collaborators will examine additional “messages” from nuclear collisions that compress matter to high densities similar to that attained in the GW170817 event. Then they will use these data to test different hypotheses about the relevant forces inside neutron stars and how they depend on density and on the relative numbers of neutrons and protons—key to understanding both the smallest of atomic nuclei and the densest of stars. The PI’s, their graduate student and interested undergraduate students focused on answering these questions with data from an experiment performed in 2024 by the SPiRIT collaboration at the RIBF accelerator facility in Japan. This award focuses on analyzing and publishing data from that experiment. This research provides students and postdoctoral researchers with hands-on experience in building detectors, analyzing and simulating complex data, and in the application of machine learning and statistical modeling. Such training facilitates their development into future leaders in science, technology, and national security. The PI’s, their students, post-doctoral research associates and international collaborators performed a major experiment in 2024 at the RIKEN RIBF research center in Japan. This experiment combined the SRIT time projection chamber, built by scientists from Michigan State University’s Facility for Rare Isotope Beams (FRIB) and the Cyclotron Lab at Texas A&M, with the SAMURAI spectrometer and readout electronics supplied by scientists at the RIBF facility in Japan. The 2024 experiment exploits the sensitivity of charged pion production to the difference between the forces on neutrons and the forces on protons within the dense neutron-rich matter compressed by colliding two heavy nuclei. Previous results by this collaboration—recently published in Nature Astronomy—illustrates how such experiments can constrain assumptions about the forces on neutrons and protons in the dense neutron-rich matter that is central to both nuclear collisions and neutron stars. This new experiment probes these forces at higher energies and densities and provides meaningful tests of theoretical reaction models of dense matter. The PI’s, and their group members and collaborators are focused on the analyses and interpretation of this new data on the production of charged pions. At the higher energies of the new experiment, theory also predicts that pion production should have the virtue of being less sensitive to non-resonant pion production and to theoretical uncertainties in the mechanisms of non-resonant pion production. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.