SEMINAIRE DE L'IPN
New results from a number operator interpretation of the compositeness of bound and resonant states
A novel theoretical approach to the problem of the compositeness ($X$) of a resonance or bound state is developed on the basis of the expectation values of the number operators of the free particles in the continuum.
This formalism is specially suitable for effective field theories in which the bare elementary states are integrated out but that give rise to resonance and bound states when implemented in nonperturbative calculations.
Some examples from hadron physics will be given, e.g. concerning the sigma or rho resonances, which also illustrate the limitations of recent methods to evaluate X making use of only the on-shell T-matrix. A brief review on previous results to approach the calculation of X is presented.
José A. Oller