Relativistic corrections for two- and three-body flux tube model

We generalize the relativistic flux tube model for arbitrary two- or three-body systems. The spin-independent and spin-dependent contributions of the flux tube to the total Hamiltonian are computed in perturbation theory. In particular, we show that the spin-dependent part exhibits a universal spin-orbit form: It does not depend on the nature of the confined particles. The general equations we present, being well defined for light particles, can thus be applied to usual as well as exotic hadrons such as hybrid mesons and glueballs.