Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
2001-09-25
Phys.Rev. D65 (2002) 085001
Physics
High Energy Physics
High Energy Physics - Phenomenology
20 pages (Latex); minor changes included, accepted for publication in Phys. Rev. D
Scientific paper
10.1103/PhysRevD.65.085001
Fundamental properties of unstable particles, including mass, width, and partial widths, are examined on the basis of the Nielsen identities (NI) that describe the gauge dependence of Green functions. In particular, we prove that the pole residues and associated definitions of branching ratios and partial widths are gauge independent to all orders. A simpler, previously discussed definition of branching ratios and partial widths is found to be gauge independent through next-to-next-to-leading order. It is then explained how it may be modified in order to extend the gauge independence to all orders. We also show that the physical scattering amplitude is the most general combination of self-energy, vertex, and box contributions that is gauge independent for arbitrary s, discuss the analytical properties of the NI functions, and exhibit explicitly their one-loop expressions in the Z-gamma sector of the Standard Model.
Grassi Pietro Antonio
Kniehl Bernd A.
Sirlin Alberto
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