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What are the phases of matter in environments where the strong interaction is dominant? This has been a longstanding goal of nuclear physics. In my talk, I will focus on the dense and cool part of the phase diagram. These conditions are found in neutron stars, which we can use as laboratories to understand more about the strong interaction. I will focus on the development of new phenomenological models for neutron star physics. These models are typically fit to isospin-symmetric nuclear matter saturation properties. However, neutron stars in chemical equilibrium have a large fraction of neutrons to baryons ~90%. We use pure neutron matter predictions from chiral effective field theory to calibrate four relativistic mean-field theories for neutron star physics while also satisfying standard nuclear saturation properties. We test these nuclear models against astrophysical constraints, including the recent measurement of a 0.77 solar mass compact object inside supernova remnant HESS 1731-347. We find that a model with only nucleonic degrees of freedom cannot support this observation and other more precise astrophysical constraints at the 68% credibility level. We instead find that a weak first-order phase transition to quark matter around twice nuclear saturation density can support this and all other stellar-structure constraints.
Nobuyuki Matsumoto