Electromagnetic Nucleon-to-Delta Transition in Chiral Effective-Field Theory

We perform a relativistic chiral effective-field theory calculation of the pion electroproduction off the nucleon ($e^- N \to e^- N \pi$) in the Delta(1232)-resonance region. After fixing the three low-energy constants, corresponding to the magnetic (M1), electric (E2), and Coulomb (C2) $\gamma N\Delta$ couplings, our calculation provides a prediction for the momentum-transfer and pion-mass dependence of the $\gamma N\Delta$ form factors. The prediction for the pion-mass dependence resolves the discrepancy between the recent lattice QCD results and the experimental value for the "C2/M1 ratio" at low $Q^2$.