Longitudinal wobbling in 133La

Excited states of La-133 have been investigated to search for the wobbling excitation mode in the low-spin regime. Wobbling bands with n omega=0and 1 are identified along with the interconnecting Delta I=1, E2 transitions, which are regarded as one of the characteristic features of wobbling motion. An increase in wobbling frequency with spin implies longitudinal wobbling for La-133, in contrast with the case of transverse wobbling observed in Pr-135. This is the first observation of a longitudinal wobbling band in nuclei. The experimental observations are accounted for by calculations using the quasiparticle-triaxial-rotor (QTR) model, which attribute the appearance of longitudinal wobbling to the early alignment of a pi proton pair.