Lifetime of Single-Particle Excitations in a Dilute Bose-Einstein   Condensate at Zero Temperature
  We study the lifetime of single-particle excitations in a dilute homogeneous Bose-Einstein condensate at zero temperature based on a self-consistent perturbation expansion of satisfying Goldstone's theorem and conservation laws simultaneously.It is shown that every excitation for each momentum ${\bf p}$ should have a finite lifetime proportional to the inverse $a^{-1}$ of the $s$-wave scattering length $a$, instead of $a^{-2}$ for the normal state, due to a new class of Feynman diagrams for the self-energy that emerges upon condensation. We calculate the lifetime as a function of $|{\bf p}|$ approximately. 
Kazumasa Tsutsui and Takafumi Kita
cond-mat.quant-gas cond-mat.supr-con