Neutron stars as laboratories for fundamental physics: from the QCD phase diagram to neutron dark decays

The advent of multimessenger astronomy marks an exciting era for novel probes of the Standard Model and beyond. I first discuss how astrophysical observations of neutron stars could shed light on the zero-temperature QCD phase diagram across broad density scales. Model-independent bounds on the correlation between nuclear and astrophysical observables are presented, from which the constraining power of future gravitational wave observations is forecasted. The implications of perturbative QCD are also carefully examined. While perturbative QCD calculations may favor lower sound speed inside neutron stars, they cannot impact astrophysical observables. A robust, model-insensitive constraint on the minimum sound speed beyond neutron star densities is derived. I show that the existence of massive pulsars informs both the maximum and the minimum sound speed across the entire QCD phase diagram. The second part of my talk focuses on neutron star constraints on neutron dark decays. Motivated by the neutron lifetime puzzle, it is hypothesized that neutrons may decay into dark states containing either a fermion carrying unit baryon number B=1 (dark neutrons) or three fermions each carrying B=1/3 (dark quarks). While the observation of massive pulsars requires strong self-repulsion among dark neutrons, up to 6 flavors of non-interacting dark quarks can be compatible with astrophysics. Dark quarks lighter than about 313 MeV form halos surrounding normal neutron stars, and any such new states lighter than about 270 MeV are ruled out by the existence of low-mass neutron stars. Possible gravitational-wave observables of dark quark halos are discussed.