Factorization and Non-Factorization of In-Medium Four-Quark Condensates
  It is well-established for the vacuum case that in the limit of a large number of colors N_c the four-quark condensates factorize into products of the two-quark condensate. It is shown that in the combined large-N_c and linear-density approximation four-quark condensates do not factorize in a medium of pions (finite temperature system) but do factorize in a medium of nucleons (nuclear system). 
Stefan Leupold (Giessen Univ.)