The Compton Amplitude, lattice QCD and the Feynman-Hellmann approach

New powerful method of computations in the lattice QCD is presented

The problem of computations in lattice QCD is discussed. The LQCD is a source of first-principle results for hadronic matrix elements. It is important to compute the Compton amplitude needed for computation of the structure functions. The Compton amplitude is a 4-point correlation function and hence it is difficult to compute using standard approaches of the Lattice QCD. To circumvent this problem the authors apply a Feynman–Hellmann approach which they developed in their previous publications. This approach has many advantages: it avoids operator mixing, renormalisation is simple, it allows investigation of power corrections to the leading behaviour of the OPE.
The approach is described in details with numerical result for the valence PDF as example of its application. The authors additionally discuss other possible applications of the approach developed, in particular, spin dependent structure functions and electromagnetic corrections to the proton – neutron mass splitting.
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