Physics – Nuclear Physics
Scientific paper
Oct 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aps..dnp.hf002g&link_type=abstract
American Physical Society, Division of Nuclear Physics Annual Meeting, October 25-28, 2006, abstract #HF.002
Physics
Nuclear Physics
Scientific paper
To add 3-body forces when theory and data disagree is untenable when predictions are required. For the ``pion-less'' Effective Field Theory at momenta below the pion-mass, I provide a recipe to systematically estimate the typical size of 3-body forces in all partial waves and orders, including external currents [1]. It is based on the superficial degree of divergence of the 3-body diagrams which contain only two-body forces and the renormalisation-group argument that low-energy observables must be insensitive to details of short-distance dynamics. Na"ive dimensional analysis must be amended as the asymptotic solution to the leading-order problem depends for large off-shell momenta crucially on the partial wave and spin-combination considered. The typical strength of most 3-body forces turns out weaker than expected, demoting many to high orders. As application, the cross section of nd->tγ at thermal neutron energies bears no new 3-body force [2], besides those fixed by the triton binding energy and nd scattering length in the triton channel. It is calculated as [0.485(LO)+0.011(NLO)+0.007(NNLO) ];mb=[0.503±0.003];mb, converges and compares well with experiment, [0.509±0.015];mb. In contradistinction, potential models list a spread of [0.490.66];mb, depending on the 2-nucleon potential and inclusion of the δ(1232). [1] H.W. Grießhammer: Nucl. Phys. A760 (2005) 110 [2] H. Sadeghi, S. Bayegan and H.W. Grießhammer, in preparation.
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