Measurement of the cross section of $e^{+}e^{-}\to\eta\pi^{+}\pi^{-}$ at center-of-mass energies from 3.872 GeV to 4.700 GeV

Using data samples with an integrated luminosity of 19 fb−1 at twenty-eight center-of-mass energies from 3.872 GeV to 4.700 GeV collected with the BESIII detector at the BEPCII electron–positron collider, the process e+e− → ηπ+π− and the intermediate process e+e− → ηρ0 are studied for the first time. The Born cross sections are measured. No significant resonance structure is observed in the cross section lineshape.