2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/182652The $ψ'\to J/ψπ^+π^-$ decay process provides a new way to extract the $ππ$ $S$ wave phase shifts up to $0.59GeV$. In this paper we derive the formulae for extracting the $ππ$ $S$ wave phase shifts from the invariant mass spectrum of $ππ$ in the $ψ'\to J/ψπ^+π^-$ decay.7 pages, 1 figure. A misprint in Eq.(13) correctedHigh Energy Physics - PhenomenologyHigh Energy Physics - Experimenttext