Köln 2004 – scientific programme

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HK: Physik der Hadronen und Kerne

HK 13: Poster Session: Theory

HK 13.5: Poster

Tuesday, March 9, 2004, 13:30–15:30, Foyer

Spectral functions of isoscalar scalar and isovector electromagnetic form factors of the nucleon at two-loop order — •Norbert Kaiser — Physik Department T39, Technische Universität München, D-85747 Garching, Germany

We calculate the imaginary parts of the isoscalar scalar and isovector electromagnetic form factors of the nucleon up to two-loop order in chiral perturbation theory. Particular attention is paid on the correct behavior of Im σ_N(t) and Im G_E,M^V(t) at the two-pion threshold t₀=4 m_π² in connection with the non-relativistic 1/M-expansion. We recover the well-known strong enhancement near threshold originating from the nearby anomalous singularity at t_c = 4m_π²−m_π⁴/M² = 3.98 m_π². In the case of the scalar spectral function Im σ_N(t) one finds a significant improvement in comparison to the lowest order one-loop result. Higher order ππ-rescattering effects are however still necessary to close a remaining 20%-gap to the empirical scalar spectral function. The isovector electric and magnetic spectral functions Im G_E,M^V(t) get additionally enhanced near threshold by the two-pion-loop contributions. After supplementing their two-loop results by a phenomenological ρ-meson exchange term one can reproduce the empirical isovector electric and magnetic spectral functions fairly well.
N. Kaiser, Phys. Rev. C68, 025202 (2003).
Work supported in part by BMBF, GSI and DFG.