v 1 1 0 Fe b 19 98 STRANGE PHASES IN NEUTRON STARS

The equation of state and the properties of neutron stars are studied for a phase transition to a charged kaon condensate. We study the mixed phase by using Gibbs condition with comparison to the hitherto applied Maxwell construction. Implications for kaon condensation and for the mass-radius relation of condensed neutron stars are examined. The high density behavior of the equation of state of nuclear matter is still quite unknown and has led to speculations about the appearance of new phases. At sufficiently high density, there should be a phase transition to a quark plasma. In the following, we will discuss the onset of kaon condensation in neutron matter which might happen at lower density. Kaon condensation in connection with β-stable matter has already been studied in [1]. It received considerable attention after the work of [2] which included strong in-medium effects for the kaons. Once the effective kaon energy hits the electrochemical potential, neutrons can convert to protons and K −. Using chiral perturbation theory, this happens at (3 − 4)ρ 0 [3]. As the kaon is a boson and condenses, the equation of state is considerably softened and the maximum mass of a neutron star is lowered to 1.5M ⊙ [4]. This is turn provides a scenario for low mass black holes as proposed by Bethe and Brown [5]. As a side remark, it was found before the kaon condensation scenario was introduced that the less spectacular appearance of hyperons results also in a lower maximum mass of neutron stars [1]. The essential ingredient for kaon condensation is the lowering of the effective mass of the kaon in the medium. One knows from kaon-nucleon scattering that the s-wave K + N scattering is repulsive. The low density theorem then states that the optical potential in the nuclear medium is then also repulsive and about +30 MeV at normal nuclear density. Surprisingly, also the scattering for the antiparticle, the K − , shows repulsion which is due to the appearance of the Λ(1405) resonance just below threshold [6]. A recent analysis of K − atoms suggest that the optical potential of the K − can be as deeply attractive as –200 MeV at normal nuclear density [7]. A coupled channel calculation by