Electron structure function at LEP energies

The hadronic part of the Electron Structure Function (ESF) has been measured for the first time, using e +e − data collected by the DELPHI experiment at LEP, at electron scattering energy from √ s = 91.18 − 209.5 GeV. The data analysis is simpler than that of the measurement of the photon structure function. The ESF data are compared to predictions of phenomenological models based on the photon structure function. The GRVLO and SAS parameterisations do not seem to reproduce the data at high virtualities of the probing photons. It is shown that the quasi-real photon virtuality contribution is significant. The presented data can serve as a cross-check of the photon structure function analyses and help in refining existing parametrisations.