One- and two-phonon wobbling excitations in triaxial 165Lu

High-spin states in 165Lu have been investigated by in-beam γ-ray coincidence spectroscopy using the EUROBALL spectrometer array. Two new excited rotational bands have been discovered with features similar to a previously known triaxial superdeformed band in that nucleus. Comparison of the decay pattern of these bands, in particular the unusually large E2 transition strength from the first excited to the yrast superdeformed band, to theoretical calculations shows that they belong to a family of wobbling excitations with phonon numbers nw=0, 1 and 2. These results, together with evidence for nuclear wobbling in the neighbouring isotopes 163Lu and 167Lu, firmly establish this mode of excitation in the A=165 mass region. The observation of wobbling is a unique signature of stable nuclear triaxiality.