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Holographic Estimate of Electromagnetic Mass

Using the gauge/gravity duality, we calculate the electromagnetic contributions to hadron masses, where mass generates dynamically by strong QCD interactions. Based on the Sakai-Sugimoto model of holographic QCD we find that the electromagnetic mass of proton is $0.48~{\rm MeV}$ larger than that of neutron, which is in agreement with recent lattice results. Similarly for pions we obtain $m_{π^{\pm}}-m_{π^0}=1.8~{\rm MeV}$, roughly half of the experimental value. The electromagnetic mass of pions is found to be independent of $N_c$ and 't Hooft coupling and its scale is set only by the Kaluza-Klein scale of the model, $M_{\rm KK}=949~{\rm MeV}$.