Excited states and precision results for nucleon charges and form factors
May 16, 2022
The exponentially falling signal-to-noise ratio in all nucleon correlation functions, and the presence of towers of multihadron excited states with relatively small mass gaps makes extraction of matrix elements of various operators within the ground state nucleon challenging. Theoretically, the allowed positive parity states with the smallest mass gaps are the N(p)π(−p), N(0)π(0)π(0), N(p)π(0), N(0)π(p), . . ., states. A priori, the contribution of these states arises at one loop in chiral perturbation theory (χPT), however, in many cases the contributions are enhanced. In this talk, I will review four such cases: the correlation functions from which the axial form factors, electric and magnetic form factors, the Θ-term contribution to neutron electric dipole moment (nEDM), and the pion-nucleon sigma term are extracted. Including appropriate multihadron states in the analysis can lead to significantly different results compared to standard analyses with the mass gaps taken from fits to 2-point functions. The χPT case for Nπ states is the most clear in the axial/pseudoscalar form factors which need to satisfy the PCAC relation between them. Our analyses, supported by χPT, suggests similarly large effects in the calculations of the Θ-term and the pion-nucleon sigma term that have significant phenomenological implications.
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