Size measurement of dynamically generated resonances with finite boxes

The structure of dynamically generated states is studied from a viewpoint of the finite volume effect. We establish the relation between the spatial size of a stable bound state and the finite volume mass shift. In a single-channel scattering model, this relation is shown to be valid for a bound state dominated by the two-body molecule component. We generalize this method to the case of a quasi-bound state with finite width in coupled-channel scattering. We define the real-valued size of the resonance in a given closed channel using the response to the finite volume effect on the channel. Applying this method to physical resonances we find that Lambda(1405) and f_{0}(980) are dominated by the Kbar N and K Kbar scattering states, respectively, and that the distance between Kbar N (K Kbar) inside Lambda(1405) [f_{0}(980)] is 1.7-1.9 fm (2.6-3.0 fm). The root mean squared radii of Lambda(1405) and f_0(980) are also estimated from the mean distance between constituents.