Direct $F_L$ measurement at high $Q^2$ at HERA

A measurement of the longitudinal structure function $F_{L}(x,Q^{2})$ derived from inclusive deep inelastic neutral current $ep$ scattering cross section measurements at high negative four-momentum transfer squared, $35 \leq Q^2 \leq 800~$GeV$^2$, is presented. The data were taken in 2007 with the H1 detector at HERA at a positron beam energy of $27.5$~GeV and proton beam energies of $920, 575$ and $460$~GeV. The measurements of the NC cross sections from this analysis, combined with the reported earlier NC cross section measurements for the same data periods at medium $Q^{2}$ ($12 \leq Q^2 \leq 90~$GeV$^2$), allowed to measure $F_{L}(x,Q^{2})$ in the range $12 \leq Q^2 \leq 800~$GeV$^2$ and Bjorken-$x$ $0.00028\leq x \leq 0.0353$. The measured longitudinal structure function is found to be consistent with NLO/NNLO QCD predictions