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On the role of isospin violation in the pion-nucleon σ-term

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arxiv 2305.07045 v2 pith:VDQS3UX3 submitted 2023-05-11 hep-ph hep-latnucl-th

On the role of isospin violation in the pion-nucleon σ-term

classification hep-ph hep-latnucl-th
keywords sigmapion-nucleonconventionscorrespondingdeterminationsextractionincludedisospin
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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In recent years, a persistent tension between phenomenological and lattice QCD determinations of the pion-nucleon $\sigma$-term $\sigma_{\pi N}$ has developed. In particular, lattice-QCD calculations have matured to the point that isospin-violating effects need to be included. Here, we point out that the standard conventions adopted in both fields are incompatible, with the data-driven extraction based on the charged-pion mass, but lattice-QCD conventions relying on the mass of the neutral pion to define the isospin limit. The corresponding correction amounts to $\Delta \sigma_{\pi N}=3.1(5)\,\text{MeV}$ when evaluated in chiral perturbation theory with low-energy constants determined from a Roy-Steiner analysis of pion-nucleon scattering as well as $\sigma_{\pi N}$ itself. It reduces the tension with lattice QCD, and should be included in the comparison to phenomenological determinations. We also update the extraction from pionic atoms accounting for the latest measurement of the width of pionic hydrogen, $\sigma_{\pi N}=59.0(3.5)\,\text{MeV}$, and provide the corresponding set of scalar couplings of the nucleon.

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