At two-thirds of the normal nuclear matter density, the neutron matter pressure (P) is directly related to the neutron-skin thickness (delta-r_np) of 208Pb.
The experimental values of delta-r_np (including values obtained in Heavy Ion Collision experiments) cluster around 0.2 fm corresponding to P ~ 1.5x1027 bar. (1 bar ~ 1 atmosphere).
The data agree better with Chiral Effective Field Theory calculations that include 3-nucleon (3n) forces (shaded region). Calculations that include only nnforce (lower two lines) predict much lower pressure and neutron skin thickness of 208Pb.
References: Horowitz & Piekarewicz, PRL 86 (2001) 5647. Typel & Brown, PRL 85 (2000) 5296. Tsang et al, PRL 102 (2009) 122701. Hebeleret al., PRL 105 (2010) 161102.