Determination of the compositeness of resonances from decays: the case of the B⁰_sto J/psi f₁(1285)

We develop a method to measure the amount of compositeness of a resonance, mostly made as a bound state of two hadrons, by simultaneously measuring the rate of production of the resonance and the mass distribution of the two hadrons close to threshold. By using different methods of analysis we conclude that the method allows one to extract the value of 1-Z with about $0.1$ of uncertainty. The method is applied to the case of the $\bar B^0_s \to J/\psi f_1(1285)$ decay, by looking at the resonance production and the mass distribution of $K \bar K^*$.