Determination of the longitudinal structure function at HERA from radiative events

In deep-inelastic electron proton scattering, the emission of photons collinear with the incident electrons leads to a reduction of the effective beam energy. It is argued that this effect can be used to measure the longitudinal structure function at HERA. For an integrated luminosity of 200 pb-1, the statistical accuracy on R is in the range of ΔR≅0.05-0.25 for 6x10-4 ≤ x ≤ 2x10-2 and 15 GeV2 ≤ Q2 ≤ 120 GeV2. Systematic effects from photon identification and from the background of elastic ep scattering in the luminosity monitor, as well as corrections from bin size effects and from cutting out low pT events are discussed. (orig.)