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 Xray 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 heavyion collision. Transport model calculations that simulate nuclear collisions show that headon 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 neutrontoproton 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 heavyion 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 cuttingedge Radioactive Isotope Beam Factory in RIKEN, Japan. SpiRIT time projection chamber (TPC) is constructed to measure charged pions spectra from the collision of neutronrich system (132Sn + 124Sn), neutronpoor 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 MarkovChain 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 heavyion collisions.
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Electronic Theses & Dissertations
 Copyright Status
 Attribution 4.0 International
 Material Type

Theses
 Authors

Tsang, Chun Yuen
 Thesis Advisors

Tsang, Betty M.B
 Committee Members

Lynch, William
Danielewicz, Pawel
Brown, Edward
DeYoung, Tyce
 Date
 2022
 Subjects

Nuclear physics
 Program of Study

Physics  Doctor of Philosophy
 Degree Level

Doctoral
 Language

English
 Pages
 184 pages
 Permalink
 https://doi.org/doi:10.25335/r3jph814