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  • Open Access

Scattering off unstable states

Francesco Giacosa*

Vanamali Shastry†

  • *Contact author: francesco.giacosa@gmail.com
  • †Contact author: vanamalishastry@gmail.com
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Phys. Rev. D 113, 116002 – Published 1 June, 2026

DOI: https://doi.org/10.1103/24fy-h8fp

Abstract

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 phenomenological level. In turn, the approach also justifies treating long-lived particles, like weakly decaying pions, as stable during strong interactions.

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Phys. Rev. D 113, 116002– Published 1 June, 2026
  • Received 12 July 2025
  • Accepted 8 May 2026
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