Threshold of pion condensation

Abstract

The threshold density for pion condensation in neutron matter is determined with a model including nuclear interactions and the $\Delta$ resonance in $\pi$-nucleon scattering. Depending on the magnitude of $N-\Delta$ correlations, pion condensation can have a threshold as low as 0.1 neutrons/${\mathrm{fm}}^3$ or not occur at all. Applying the model to nuclear matter, we find that in the absence of $N-\Delta$ correlations physical pions have a very large effective mass. With $N-\Delta$ correlations, these effects are diminished, but pion condensation could still occur at twice normal nuclear matter density.