Using a sample of 10 billion $J/{ \psi}$ events collected with the BESIII detector, the decays $\eta' \to \pi^{ +}\pi^{ -}\pi^{ +}\pi^{ -}$, $\eta' \to \pi^{ +}\pi^{ -}\pi^{ 0}\pi^{ 0}$ and $\eta' \to 4 \pi^{ 0}$ are studied via the process $J/{ \psi}\to\gamma\eta'$. The branching fractions of $\eta' \to \pi^{ +}\pi^{ -}\pi^{ +}\pi^{ -}$ and $\eta' \to \pi^{ +}\pi^{ -}\pi^{ 0}$ $\pi^{ 0}$ are measured to be $( 8.56 \pm 0.25({ \rm stat.}) \pm 0.23({ \rm syst.}) ) \times { 10^{ - 5}}$ and $(2.12 \pm 0.12({ \rm stat.}) \pm 0.10({ \rm syst.})) \times { 10^{ - 4}}$, respectively, which are consistent with previous measurements but with improved precision. No significant $\eta' \to 4 \pi^{ 0}$ signal is observed, and the upper limit on the branching fraction of this decay is determined to be less than $1.24 \times { 10^{ -5}}$ at the $90\%$ confidence level. In addition, an amplitude analysis of $\eta' \to \pi^{ +}\pi^{ -}\pi^{ +}\pi^{ -}$ is performed to extract the doubly virtual isovector form factor $\alpha$ for the first time. The measured value of $\alpha=1.22 \pm 0.33({ \rm stat.}) \pm 0.04({ \rm syst.})$, is in agreement with the prediction of the VMD model.
Categories: hep-ex