At HERA heavy quarks may contribute up to 30 % of the structure function F2. The potential of including heavy-quark data in the ZEUS PDF fits is explored, using D ∗ double differential cross-sections as well as the inclusive quantities F c¯c 2, F c¯c 2. The introduction of heavy quarks requires an extension of the DGLAP formalism. The effect of using different heavy flavour number schemes, and different approaches to the running of αs, are compared. Parton Density Function (PDF) determinations are usually global fits [2, 3, 4, 5], which use inclusive cross-section data and structure function measurements from deep inelastic lepton hadron scattering (DIS) data as well as some other exclusive cross-ections. The kinematics of lepton hadron scattering is described in terms of the variables Q 2, the invariant mass of the exchanged vector boson, Bjorken x, the fraction of the momentum of the incoming nucleon taken by the struck quark (in the quark-parton model), and y which measures the energy transfer between the lepton and hadron systems. The differential cross-section for the neutral current (NC) process is given in terms of the structure functions by d 2 σ(e ± p) dxdQ
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