Ricerca dello stato eta_b a partire da transizioni radiative della risonanza Upsilon(2S)

In this thesi we describe the search for the ground state of bottomonium, $\eta_b(1S)$ in $\Upsilon(2S)$ decays, using data taken on the peak of this resonance in Experiments 67 and 71 (2S). These analyses also profit from data taken in the proximity of $\Upsilon(1S)$ resonance (exp.65), and 60 MeV below the $\Upsilon(4S)$ peak (Exp.47 to 67), to study the properties of the continuum. The M1 radiative transition $\Upsilon(2S)\rightarrow\gamma\eta_b(1S)$ is identified as a peak in the inclusive photon spectrum. The data sample integrated by Belle is currently the world largest. This thesis provides the first measurement of the $\eta_b$ width together with the first observation of the rare transition $\chi_{b0} \rightarrow \gamma \Upsilon(1S)$