CONSTRAIN NEUTRON STAR PROPERTIES WITH SpiRIT EXPERIMENT

The study of nuclear matter is an interdisciplinary endeavor that is relevant to both astrophysics and nuclear physics. Astrophysicists need to understand the properties of nuclear matter as some astrophysical objects are made of nuclear material. Nuclear physicists also need to understand the properties of nuclear matter as they are fundamental to the understanding of the existence of nuclei, their composition and the dynamics of nuclear collisions.Recent measurements of gravitational waves from binary neutron star mergers and precise neutron star radii from X-ray data of pulsars open a new channel for physicists to study nuclear matter. Such astronomical observations of neutron stars are sensitive to nuclear matter at high density that is usually inaccessible on earth. One of the ways physicists are able to reach such high density in laboratory is through heavy-ion collision. Transport model calculations that simulate nuclear collisions show that head-on collisions of heavy nuclei at high beam energy compress the overlapping region momentarily to densities comparable to that of the interior of neutron stars. To study neutron star where number of neutrons far exceeds that of protons, the dependence of nuclear properties on neutron-to-proton ratio (N/Z) needs to be understood. This dependence is quantified by the symmetry energy, which describes the difference in binding energy between pure neutron matter and matter with equal amount of protons and neutrons. The latter is also known as symmetric nuclear matter (SNM) which has been fairly well constrained. The amount of internal neutron star pressure that supports itself from gravitational collapse depends on the value of symmetry energy. Most of the existing heavy-ion collision data comes from collisions of stable isotopes. This limits the range of available N/Z in nuclear experiments. Extending results to a wider range of N/Z is one of the goals of SpiRIT experiment using projectiles provided by the cutting-edge Radioactive Isotope Beam Factory in RIKEN, Japan. SpiRIT time projection chamber (TPC) is constructed to measure charged pions spectra from the collision of neutron-rich system (132Sn + 124Sn), neutron-poor system (108Sn + 112Sn) and intermediate system (112Sn + 124Sn) at 270 MeV/u. By comparing fragmentation patterns for reactions with different number of neutrons, symmetry energy effects can be isolated. Some results from the analysis of pion spectra have been published and will be briefly reviewed in this work before we focus on light fragment observables that are also available from the TPC data. The data analysis software, with highlights on correction of some major detector aberrations, is discussed in details. Monte Carlo simulation of the SpiRIT TPC is then performed to understand the behavior of SpiRIT data and validate our data analysis procedure. Finally, Bayesian analysis is performed to compare transport model simulations with selected light fragment measurements using Markov-Chain Monte Carlo and Gaussian emulators. The observables are chosen to minimize systematic uncertainties from both the experiment and model. The posterior provides a comprehensive constraint on the symmetry energy parameters. Although previous analyses of pion spectra have already constrained the slope of symmetry energy at saturation density (L), its uncertainty can be reduced by 39% if pion results are combined with our new Bayesian posterior. The implications of symmetry energy constraint for neutron star will be discussed to demonstrate the importance of data from rare isotope heavy-ion collisions.