Recent astronomical measurements of neutron star mergers have reinvigorated questions aboutthe nature of dense matter in the universe. The densities reached in these stars range from normalnuclear densities ρo to upwards of 4ρo or even 9ρo in some models, where ρo = 1.7 × 1014 g cm−3 ;representing extremely dense, neutrons rich matter. The symmetry energy describes the differencein binding energy between pure neutron matter and symmetric matter with equal number of neutronsand protons. It is the density dependence of the symmetry energy that is related to the outwardpressure, which supports the star, preventing it from collapsing under its own gravitation force. Foryears, theory and experiment have combined to progressively constrain the Equation of State (EoS)of symmetric matter, and it was only in the last decade that the density dependence of pure neutronmatter, the symmetry energy, had been constrained at normal nuclear densities and lower, ρ < ρo .Still large uncertainties, and discrepancies on preliminary constraints, remain at densities around2ρo which are critical importance to the understanding the properties of neutron stars.Besides observing neutrons stars directly, the laboratory provides the only way to study densenuclear matter at different densities and proton-neutron asymmetries. It has been shown thatcharged pions are produced from nucleon-nucleon collisions through the short-lived intermediate ∆resonances, in the early, dense parts of Heavy Ion Collisions (HICs). Yet, existing pion experimentaldata tailored to answering questions about the symmetry energy was lacking and suffers fromunknown systematic errors. This prompted the construction of the Samurai Pion-Reconstructionand Ion Tracker Time Projection Chamber (SπRIT TPC) which was constructed to measure chargedpions produced in neutron-rich HICs with a high efficiency. This Dissertation encompasses the firstpion spectra from the most neutron-rich (132Sn + 124Sn), and neutron-poor (108Sn + 112Sn) systemsmeasured at 270 AMeV taken at the RIBF facility at RIKEN. The total pion ratio was measured towithin 4% and the pion spectra are measured with a high degree of accuracy, marking the first timepion spectra – at energies relevant to the symmetry energy – have been published. Also, we willcompare the results with the most recent transport models which can simulate pion production. Wewill highlight some of the successes and hope to motivate a discussion in the theoretical communityon how to better reproduce the pion phenomena in neutron-rich HICs, which is important for notonly constraining the density dependence of the symmetry energy, but also for understanding the∆ and pion roles in neutron stars.
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