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Nucleon-pion-state contamination in nucleon form factors from the lattice
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Multi-particle states with additional pions are expected to be a non-negligible source of excited-state contamination in lattice simulations at the physical point. I will argue that chiral perturbation theory (ChPT) can be employed to calculate the contamination due to two-particle nucleon-pion states in nucleon observables. Results to leading order are presented for the nucleon vector and axial vector form factors.
Taking into account phenomenological results for the appearing low-energy coefficients (LECs) the impact of the nucleon-pion-states on the form factor results can be estimated.
Comparison with lattice data works in most cases reasonably well even though the source-sink separations in lattice correlation functions are currently too small to apply ChPT with confidence. Source-sink separations of about 2 fm and larger seem to be necessary to
compute the form factors with few-percent precision.