Comments on NNLO In the perturbative approach, for the total inclusive S.F.s and cross sections, once a comprehensive NLO calculation is in place, it is straightforward to include known NNLO corrections additively. However, one needs to realize that, unlike total inclusive F2,L, quantities such as ”F2c” is not well defined theoretically at NNLO and beyond. (It is not infra-red safe!) It is rather misleading to talk about a true “NNLO theory” of F2c (except within the 3-flv scheme, which has only a limited range of applicability). Extending global analysis to NNLO is certainly desirable, but not necessarily urgent for current applications (cf. excellent global fits), since experimental errors for most measured quantities, as well as other sources of uncertainties (such as parametrization, power-law corrections …), largely outweigh the NNLO corrections.

Comparison with MST and Thorne Papers On matters of principle, we seem to be in almost full agreement: both accept Collins’ general formalism, as well as the sensible SACOT and ACOTc prescriptions. Where do we depart? (In principle, differences must be of higher order in as.) We stay with the simple order-by-order approach; do not see any reason to depart from simplicity. RT and MST choose to differ, as described in RT’s talks. In practice, there can be discrepancies in the respective implementations. A careful comparison of results obtained with, in principle, similar inputs should be made, especially if apparent different conclusions are reached.

MST

Thorne hep-ph/0601245

GM global analysis and HERA I Charm Production data H1 NC e+p 96-97 F2c Zeus NC e+p 96-97 F2c H1 NC e+p 99-00 X Zeus NC e+p 98-00 F2c

Thorne hep-ph/0601245 NLO NNLO only?