Illuminating the nucleon spin

In this short note the QED extension of the nucleon spin sum rule is considered. To this end, the leading-order (LO) QED evolution kernels for the quark/gluon helicity and orbital-angular-momentum (OAM) distributions are calculated, as well as their lepton and photon analogue distributions introduced for the first time. The LO evolution kernels of the latter are also calculated, both in QCD and QED. Putting all together, the nucleon spin sum rule remains scale-invariant in QCD$\times$QED, as expected, which represents a check of the newly obtained results. This theoretical development will allow in the future the quantification of the contributions of lepton and photon distributions to the nucleon spin, and a more precise control over the uncertainties.

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