A combination of all ZEUS and H1 inclusive data for zero polarisation was published and subject to a detailed QCD analysis [@HERAPDF20], yielding the parton distribution function (PDF) set HERAPDF2.0 and its variants. For the analysis presented here, the ZEUS HERAII data taken at the centre-of-mass energy of 318GeV were used separated into sets with positive and negative polarisation as published by the ZEUS collaboration [@ZEUS2NCe; @ZEUS2NCp; @ZEUS2CCe; @ZEUS2CCp]. All other data sets were used as originally published by H1 [@H1allhQ2; @H1FL1; @H1FL2; @Adloff:1999ah; @Adloff:2000qj; @Adloff:2003uh; @Collaboration:2009kv; @Collaboration:2009bp] and ZEUS [@ZEUSFL; @Breitweg:1998dz; @zeuscc97; @Chekanov:2001qu; @Chekanov:2002zs; @Chekanov:2002ej; @Chekanov:2003vw; @Chekanov:2003yv] for unpolarised beams.

Standard Model formalism {#sec-pred}
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Inclusive deep inelastic $ep$ scattering can be described in terms of the kinematic variables $Q^2$, $x_{\rm{Bj}}$ and $y$. The negative four-momentum-transfer squared, $Q^2$, is defined as $Q^2 = -q^2 = -(k-k')^2$, where $k$ and $k'$ are the four-momenta of the incoming and the scattered electron, respectively. The Bjorken scaling variable, $x_{\rm{Bj}}$, is defined as $x_{\rm{Bj}}=Q^2/2P \cdot q$, where $P$ is the four-momentum of the incoming proton. In the quark-parton model (QPM) the kinematic variable $x_{\rm{Bj}}$ is equal to the fractional momentum of the struck quark, $x$. The fraction of the electron energy transferred to 