  Many neutron star (NS) properties, such as the proton fraction within a NS,
reflect the symmetry energy contributions to the Equation of State that
dominate when neutron and proton densities differ strongly. To constrain these
contributions at supra-saturation densities, we measure the spectra of charged
pions produced by colliding rare isotope tin (Sn) beams with isotopically
enriched Sn targets. Using ratios of the charged pion spectra measured at high
transverse momenta, we deduce the slope of the symmetry energy to be $42 < L <
117$ MeV. This value is slightly lower but consistent with the $L$ values
deduced from a recent measurement of the neutron skin thickness of $^{208}$Pb.
