Charm production in deep inelastic scattering from threshold to high Q(2)

Charm final states in deep inelastic scattering constitute similar to 25% o f the inclusive cross-section at small x as measured at HERA. These data ca n reveal important information on the charm and gluon structure of the nucl eon if they are interpreted in a consistent perturbative QCD framework whic h is valid over the entire energy range from threshold to the high energy l imit. We describe in detail how this can be carried out order-by-order in P QCD in the generalized (MS) over bar formalism of Collins (generally known as the ACOT approach), and demonstrate the inherent smooth transition from the 3-flavor to the 4-flavor scheme in a complete order alpha (s) calculati on, using a Monte Carlo implementation of this formalism. This calculation is accurate to the same order as the conventional NLO F-2 calculation in th e limit Q/m(c) much greater than 1. It includes the resummed large logarith m contributions of the 3-flavor scheme (generally known in this context as the fixed-flavor-number or FFN scheme) to all orders of alpha (s) 1n(m(c)(2 )/Q(2)). For the inclusive structure function, comparison with recent HERA data and the existing FFN calculation reveals that the relatively simple or der-alpha (s) (NLO) 4-flavor (m(c) not equal = 0) calculation can, in pract ice, be extended to rather low energy scales, yielding good agreement with data over the full measured Q(2) range. The Monte Carlo implementation also allows the calculation of differential distributions with relevant kinemat ic cuts. Comparisons with available HERA data show qualitative agreement; h owever, they also indicate the need to extend the calculation to the next o rder to obtain better description of the differential distributions.