Incompressibility of neutron-rich matter

The saturation properties of neutron-rich matter are investigated in a relativistic mean-field formalism using two accurately calibrated models: NL3 and FSUGold. The saturation properties-density, binding energy per nucleon, and incompressibility coefficient-are calculated as a function of the neutron-proton asymmetry alpha equivalent to(N-Z)/A to all orders in alpha. Good agreement (at the 10% level or better) is found between these numerical calculations and analytic expansions that are given in terms of a handful of bulk parameters determined at saturation density. Using insights developed from the analytic approach and a general expression for the incompressibility coefficient of infinite neutron-rich matter, i.e., K-0(alpha)=K-0+K-tau alpha(2)+..., we construct a hybrid model with values for K-0 and K-tau as suggested by recent experimental findings. Whereas the hybrid model provides a better description of the measured distribution of isoscalar monopole strength in the Sn isotopes relative to both NL3 and FSUGold, it significantly underestimates the distribution of strength in Pb-208. Thus, we conclude that the incompressibility coefficient of neutron-rich matter remains an important open problem.