On Elementary and Composite Particles: the Case of Exotic Hadrons

By elementary hadron we mean a compact state made of quarks as opposite to a composite hadron, which is made up by other elementary hadrons. To investigate the structure of the exotic unstable hadrons one may either look for patterns in spectroscopy, as the quark model suggests, or resort to the physics of shallow bound states. In recent times, following an experimental study by the LHCb collaboration, a discussion has been raised on the role of the scattering effective range r_0 as a discriminating parameter between the elementary and composite hypotheses for the (3872). I will present a short introduction to the main aspects of this discussion.