Fluctuations of the quark densities in nuclei
Résumé
We study the static scalar susceptibility of the nuclear medium, i.e., the change of the quark condensate for a small modification of the quark mass. In the linear sigma model it is linked to the in-medium sigma propagator. At ordinary nuclear densities, when chiral symmetry is spontaneously broken, the scalar susceptibility is distinct from the pseudoscalar one, which is linked to the fluctuations of the quark pseudoscalar density. We show that the pseudoscalar one, which is large in the vacuum, owing to the smallness of the pion mass, follows the density evolution of the quark condensate and thus decreases. The scalar one instead increases due to the mixing with the softer modes of the nucleon-hole excitations. At normal nuclear matter density the two susceptibilities become much closer, a partial chiral symmetry restoration effect as they become equal when the full restoration is achieved.