Evidence for the Cusp Effect in $\eta \prime $ Decays into $\eta {\pi }^{0}{\pi }^{0}$

Using a sample of $4.3×{10}^{5}$ ${\eta }^{\prime }\to \eta {\pi }^{0}{\pi }^{0}$ events selected from the ten billion $J/\psi $ event dataset collected with the BESIII detector, we study the decay ${\eta }^{\prime }\to \eta {\pi }^{0}{\pi }^{0}$ within the framework of nonrelativistic effective field theory. Evidence for a structure at ${\pi }^{+}{\pi }^{-}$ mass threshold is observed in the invariant mass spectrum of ${\pi }^{0}{\pi }^{0}$ with a statistical significance of around $3.5\sigma $, which is consistent with the cusp effect as predicted by the nonrelativistic effective field theory. After introducing the amplitude for describing the cusp effect, the $\pi \pi $ scattering length combination ${a}_{0}-{a}_{2}$ is determined to be $0.226±0.06{0}_{\mathrm{stat}}±0.01{3}_{\mathrm{syst}}$, which is in good agreement with theoretical calculation of $0.2644±0.0051$.