Excited state systematics in extracting nucleon electromagnetic form factors from the lattice

Nucleon form factors are central observables of hadronic physics and provide details of the nucleon's distribution of charge and magnetisation. Currently, lattice simulations fall short of the accuracy achieved by experiment. Furthermore, the simulations of the nucleon electromagnetic form factor fail to reproduce experimental results [1, 2]; thus it is important to ensure that systematic effects are under control in lattice simulations. We present recent results for the nucleon electromagnetic (EM) form factors using lattice QCD, including the determination of the charge radii. The standard approach is to extract the form factors via a plateau fit to the lattice data using a 'large-enough' time separation between the operators at the source and sink. To check that this removes excited state contaminations to an acceptable level, we employ two further extraction methods: a fit that explicitly accounts for the contamination; and the use of a summed operator insertion, which suppresses the contamination. A comparison of the methods allows for the study of systematic effects related to excited state contributions entering in the Q² dependence of the form factors. This work (also presented in [3]) provides an update on results previously presented in [4] and follows the methodology used in a recent study of the nucleon's axial form factor [5]. Similar methods have been used in [6]. Our simulations use non-perturbatively O(a) improved Wilson fermions in N_f = 2 QCD, measured on the CLS ensembles. Further details of the lattice ensembles used may also be found in [3].