Observation of a State $X\left(2600\right)$ in the ${\pi }^{+}{\pi }^{-}\eta \prime $ System in the Process $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}\eta \prime $

Based on $\left(10087±44\right)×{10}^{6}\text{}\text{}J/\psi $ events collected with the BESIII detector, the process $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ is studied using two largest decay channels of the ${\eta }^{\prime }$ meson, ${\eta }^{\prime }\to \gamma {\pi }^{+}{\pi }^{-}$ and ${\eta }^{\prime }\to \eta {\pi }^{+}{\pi }^{-}$, $\eta \to \gamma \gamma $. A new resonance, which we denote as the $X\left(2600\right)$, is observed with a statistical significance larger than $20\sigma $ in the ${\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ invariant mass spectrum, and it has a connection to a structure around ${1.5\text{}\text{}\mathrm{GeV}/c}^{2}$ in the ${\pi }^{+}{\pi }^{-}$ invariant mass spectrum. A simultaneous fit on the ${\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ and ${\pi }^{+}{\pi }^{-}$ invariant mass spectra with the two ${\eta }^{\prime }$ decay modes indicates that the mass and width of the $X\left(2600\right)$ state are $2618.3±2.{0}_{-1.4}^{+16.3}\text{}\text{}\mathrm{MeV}/{c}^{2}$ and $195±{5}_{-17}^{+26}\text{}\text{}\mathrm{MeV}$, where the first uncertainties are statistical, and the second systematic.