Confinement through a relativistic generalization of the linear   interaction
  Generalizing a covariant framework previously developed, it is shown that confinement insures that meson $\to q+\bar{q}$ decay amplitudes vanish when both quarks are on-shell. Regularization of singularities in a covariant linear potential associated with nonzero energy transfers (i.e. $q^2=0, q^{\mu}\neq0$) is improved. 
Cetin Savkli, Franz Gross