Scattering off unstable states

Unstable states that live long enough may appear as in(out)going particles in scattering experiments. Yet, the standard QFT approach strictly applies only to fully stable asymptotic states. This is evident when scattering involving unstable particles develops a $t$-channel singularity at specific angles. We employ a finite-time formalism leading to analytic results without the singularity (even in the infinite-time limit), thus solving the problem at a fundamental level. In turn, the approach also justifies treating long-lived particles, like weakly decaying pions, as stable during strong interactions.