Contributions due to the longitudinal virtual photon in the semi-inclusive ep collision at HERA

The importance of contributions due to the longitudinally polarised virtual photon and the longitudinal-transverse interference term, in the unpolarised ep collisions is discussed. The numerical calculations for the Compton process, e p --> e gamma X, at the HERA collider were performed in the Born approximation. The various distributions in the ep centre-of-mass and Breit frames are presented. These cross sections are dominated by the transversely polarised intermediate photon, even for large values of its virtuality.