The role of F_L(x,Q^2) in parton analyses
  We investigate the effect of the structure function F_L in global parton analyses of deep inelastic and related hard scattering data. We perform NLO and NNLO analyses which include the reduced cross section HERA data at high y, as well as earlier direct measurements of F_L. We find that the NNLO analysis gives a better description of F_L at low x than that performed at NLO. Nevertheless the data show evidence of the need of further contributions to F_L, which may be of higher-twist origin. We study such corrections both phenomenologically and theoretically via a renormalon approach. The higher-twist corrections extracted from a successful fit to the data are in general agreement with the theoretical expectations, but there is still room for alternative theoretical contributions, particularly at low x and Q^2. The importance of future measurements of F_L is emphasized. 
A.D. Martin, W.J. Stirling and R.S. Thorne