Measurement of the cross section of e + e − → ηπ + π − 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 − → ηρ 0 are studied for the first time. The Born cross sections are measured. No significant resonance structure is observed in the cross section lineshape.