Determining the Sign of the Z-Penguin Amplitude

TL;DR

This paper uses precision electroweak measurements to constrain the sign and magnitude of the Z-penguin amplitude in minimal-flavor-violation models, deriving bounds on rare decay branching ratios.

Contribution

It determines the sign of the Z-penguin amplitude consistent with experimental data and derives bounds on rare decay processes within constrained minimal-flavor-violation.

Findings

The sign of the Z-penguin amplitude matches the Standard Model.

Bounds on non-standard contributions to the Inami-Lim function C.

Limits on branching ratios of rare meson decays.

Abstract

We point out that the precision measurements of the pseudo observables R_b^0, A_b, and A_FB^0,b performed at LEP and SLC suggest that in models with minimal-flavor-violation the sign of the Z-penguin amplitude is identical to the one present in the standard model. We determine the allowed range for the non-standard contribution to the Inami-Lim function C and show by analyzing possible scenarios with positive and negative interference of standard model and new physics contributions, that the derived bound holds in each given case. Finally, we derive lower and upper limits for the branching ratios of K^+ -> pi^+ nu nubar, K_L -> pi^0 nu nubar, K_L -> mu^+ mu^-, B -> X_d,s nu nubar, and B_d,s -> mu^+ mu^- within constrained minimal-flavor-violation making use of the wealth of available data collected at the Z-pole.