$B(E2)$ anomalies in the yrast band of $^{170}\mathrm{Os}$

International audienceBackground: The neutron-deficient osmium isotopic chain provides a great laboratory for the study of shape evolution, with the transition from the soft triaxial rotor in Os168 to the well-deformed prolate rotor in Os180, while shape coexistence appears around N=96 in Os172. Therefore, the study of the Os isotopic chain should provide a better understanding of shape changes in nuclei and a detailed scrutiny of nuclear structure calculations. In this paper, the lifetimes of the low-lying yrast states of Os170 have been measured for the first time to investigate the shape evolution with neutron number. Purpose: Lifetimes of excited states in the ground-state band of Os170 are measured to investigate the shape evolution with neutron number in osmium isotopes and compare with state-of-the-art calculations. Methods: The states of interest were populated via the fusion-evaporation reaction Nd142(S32,4n) at a bombarding energy of 170 MeV at the ALTO facility from IPN (Orsay, France). Lifetimes of the 21+ and 41+ states in Os170 were measured with the recoil-distance Doppler-shift method using the Orsay universal plunger system. Results: Lifetimes of the two first excited states in Os170 were measured for the first time. A very small B(E2;41+→21+)/B(E2;21+→01+)=0.38(11) was found, which is very uncharacteristic for collective nuclei. These results were compared to state-of-the-art beyond-mean-field calculations. Conclusions: Although theoretical results give satisfactory results for the energy of the first few excited states in Os170 and the B(E2;21+→01+) they fail to reproduce the very small B(E2;41+→21+), which remains a puzzle