IMPORTANCE OF A MEASUREMENT OF F_L (X, Q²) AT HERA

I investigate what a direct measurement of the longitudinal structure function F_L (x, Q²) could teach us about the structure of the proton and the best way in which to use perturbative QCD for structure functions. I assume HERA running at a lowered beam energy for approximately 4-5 months and examine how well the measurement could distinguish between different theoretical approaches. I conclude that such a measurement would provide useful information on how to calculate structure functions and parton distributions at small x.