3He impurity excitation spectrum in liquid 4He
  We evaluate microscopically the excitation spectrum of the 3He impurity in liquid 4He at T=0 and compare it with the experimental curve at the equilibrium density. The adopted correlated basis perturbative scheme includes up to two independent phonons, intermediate correlated states and the correlation operator is built up with two- and three-body correlation functions. The experimental spectrum is well described by the theory along all the available momentum range. A marked deviation from the simple Landau-Pomeranchuck, quadratic behavior is found and the momentum dependent effective mass of the impurity increases of about 50% at q around 1.7 A^-1 respect to its q=0 value. No signature of roton-like structures is found. 
A.Fabrocini(INFN and Dept.of Physics, Pisa) and A.Polls(Dep.d'Estr.i   Const.de la Materia, Barcelona)