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HK: Hadronen und Kerne

HK 37: Hadronic Probes IV, Interaction of Pions

HK 37.2: Gruppenbericht

Dienstag, 17. März 1998, 17:30–18:00, C

Isospin violation in the pion–nucleon system — •Ulf-G. Meißner and Sven Steininger — FZ Jülich, IKP (Th)

Isospin symmetry violation is one of the most fundamental topics in nuclear physics. Its understanding enables one to get a handle on the light quark masses. For doing that, one needs a consistent framework to disentangle the isospin violation due to the quark mass difference m_d−m_u from the one of electromagnetic origin. We construct the complete effective chiral pion–nucleon Lagrangian in the presence of virtual photons to one loop [1]. This effective field theory approach allows one to systematically study quark mass and electromagnetic effects in reactions like pion photo/electroproduction, elastic pion–nucleon scattering or π N → π π N, which have been or will be measured in the respective threshold regions to high precision. As an application, we consider strong and electromagnetic isospin violation for scattering of neutral pions off nucleons, improving a calculation of Weinberg performed some 20 years ago [2]. We show that for the scattering lengths these isospin violating terms are of the same size as the purely hadronic ones. The overall effect of isospin violation in the π⁰N scattering lengths is of the order of 25%. We also analyze isospin–violating effects for the πN σ–term. These can be as large as 10% for the absolute value but are negligible for the shift to the Cheng–Dashen point.

[1] Ulf-G. Meißner and S. Steininger, [hep-ph/9709453], Phys.Lett.B, in print.

[2] S. Weinberg, Trans. N.Y. Acad. of Sci. 38 (1977) 185.