OMS scheme of UV renormalization in the presence of unstable fundamental particles

A generalization of the on-mass-shell scheme of UV renormalization (the OMS scheme) to the case of presence of unstable fundamental particles is proposed. Its basic ingredients are as follows: (i) the renormalized mass coincides with a real part of the position of the complex pole of the corresponding propagator, (ii) the imaginary part of the on-shell self-energy coincides with the imaginary part of the complex pole position. The latter property implies the gauge-invariance of the imaginary part of the on-shell self-energy in the OMS scheme and its direct connection with the width of the unstable particle. Starting with the three-loops this connection becomes nontrivial.