Sigma model effective action for strong localization
  Bose gas in a random external field is considered. The sigma model like effective action both for weak and strong random fields compared with the interaction between particles is derived by averaging over the random field and integration over the quantum fluctuations in the framework of the Keldysh-Schwinger technique for irreversible processes. Using this effective action the properties of the system in strongly fluctuating random field are analyzed. 
A.Babichenko and V.Babichenko