The magnetic dipole, the electric quadrupole and the Coulomb quadrupole amplitudes for the transition \gamma N\to \Delta are calculated in quenched lattice QCD at \beta=6.0 with Wilson fermions. Using a new method combining an optimal combination of interpolating fields for the $\Delta$ and an overconstrained analysis, we obtain statistically accurate results for the dipole form factor and for the ratios of the electric and Coulomb quadrupole amplitudes to the magnetic dipole amplitude, R_{ EM} and R_{ SM}, up to momentum transfer squared 1.5 GeV^2. We show for the first time using lattice QCD that both R_{ EM} and R_{ SM} are non-zero and negative, in qualitative agreement with experiment and indicating the presence of deformation in the N- Delta system.
Categories: hep-lat nucl-ex nucl-th