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We analyze the origin of the structure observed in the reaction $J/\psi \to\gamma \eta'\pi^+\pi^-$ for $\eta'\pi^+\pi^-$ invariant masses close to theantiproton-proton ($\bar pp$) threshold, commonly associated with the $X(1835)$resonance. Specifically, we explore the effect of a possible contribution fromthe two-step process $J/\psi \to \gamma \bar NN \to \gamma \eta'\pi^+\pi^-$.The calculation is performed in distorted-wave Born approximation which allowsan appropriate inclusion of the $\bar NN$ interaction in the transitionamplitude. The $\bar NN$ amplitude itself is generated from a correspondingpotential recently derived within chiral effective field theory. We are able toreproduce the measured spectra for the reactions $J/\psi \to \gamma \bar pp$and $J/\psi \to \gamma \eta'\pi^+\pi^-$ for invariant masses around the $\barpp$ threshold. The structure seen in the $\eta'\pi^+\pi^-$ spectrum emerges asa threshold effect due to the opening of the $\bar pp$ channel.

$J/\psi \to \gamma\eta'\pi^+\pi^-$ and the structure observed around the $\bar pp$ threshold