Multi-hadron-state contamination in nucleon observables from chiral perturbation theory

Multi-particle states with additional pions are expected to be a non-negligible source of the excited-state contamination in lattice simulations at the physical point. It is shown that baryon chiral perturbation theory (ChPT) can be employed to calculate the contamination due to two-particle nucleon-pion states in various nucleon observables. Results to leading order are presented for the nucleon axial, tensor and scalar charge and three Mellin moments of parton distribution functions: the average quark momentum fraction, the helicity and the transversity moment. Taking into account experimental and phenomenological results for the charges and moments the impact of the nucleon-pion-states on lattice estimates for these observables can be estimated. The nucleon-pion-state contribution leads to an overestimation of all charges and moments obtained with the plateau method. The overestimation is at the 5-10% level for source-sink separations of about 2 fm. Existing lattice data is not in conflict with the ChPT predictions, but the comparison suggests that significantly larger source-sink separations are needed to compute the charges and moments with few-percent precision.Comment: 17 pages, 8 figures. Talk given at the 35th International Symposium on Lattice Field Theory, 18 - 24 June 2017, Granada, Spai

Three-particle NππN\pi\pi state contribution to the nucleon two-point function in lattice QCD

The three-particle $N\pi\pi$-state contribution to the QCD two-point function of standard nucleon interpolating fields is computed to leading order in chiral perturbation theory. Using the experimental values for two low-energy coefficients the impact of this contribution on lattice QCD calculations of the nucleon mass is estimated. The impact is found to be at the per mille level at most and negligible in practice.Comment: 16 pages, 5 figures. V2: Error in eq. (A2) corrected, figures 3 to 5 updated, conclusions unchanged. Discussion of FV corrections expanded. Typos corrected, references added. Matches published version in Phys. Rev.