Flavour Mixing, Gauge Invariance and Wave-function Renormalisation

Abstract

We clarify some aspects of the LSZ formalism and wave function renormalisation for unstable particles in the presence of electroweak interactions when mixing and CP violation are considered. We also analyse the renormalisation of the CKM mixing matrix which is closely related to wave function renormalisation. We critically review earlier attempts to define a set of "on-shell" wave function renormalisation constants. With the aid of an extensive use of the Nielsen identities complemented by explicit calculations we corroborate that the counter term for the CKM mixing matrix must be explicitly gauge independent and demonstrate that the commonly used prescription for the wave function renormalisation constants leads to gauge parameter dependent amplitudes, even if the CKM counter term is gauge invariant as required. We show that a proper LSZ-compliant prescription leads to gauge independent amplitudes. The resulting wave function renormalisation constants necessarily possess absorptive parts, but we verify that they comply with the expected requirements concerning CP and CPT. The results obtained using this prescription are different (even at the level of the modulus squared of the amplitude) from the ones neglecting the absorptive parts in the case of top decay. The difference is numerically relevant.

arXiv comment19 pages, plain latex, one ps figure

References in corpus (14)

• CP Violation in Top Physics
• The Nielsen Identities of the SM and the definition of mass
• Gauge dependence of the on-shell renormalized mixing matrices
• Fermion Mixing Renormalization and Gauge Invariance
• Width and Partial Widths of Unstable Particles in the Light of the Nielsen Identities
• Width and Partial Widths of Unstable Particles
• Gauge-Independent W-Boson Partial Decay Widths
• Renormalization of the Cabibbo-Kobayashi-Maskawa Matrix
• On-Mass-Shell Renormalization of Fermion Mixing Matrices
• CP Violation and Family Mixing in the Effective Electroweak Lagrangian
• The Renormalization Group Evolution of the CKM Matrix
• Gauge parameter dependence in gauge theories (revised: subsection 2.3)
• Precision Tests of the Standard Model
• Recent developments in the study of unstable particle processes

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