Electroweak Precision Observables within a Fourth Generation Model with General Flavour Structure

TL;DR

This paper calculates how a hypothetical fourth generation of fermions affects electroweak precision observables, considering general flavor mixing, and finds that these observables can constrain new mixing angles effectively.

Contribution

It provides a comprehensive one-loop calculation of EWPOs with a general fourth-generation flavor structure, including non-oblique corrections, without approximations.

Findings

EWPOs are sensitive to two fourth-generation mixing angles.

Constraints on these angles are competitive with flavor physics.

Non-oblique corrections are several permille and significant for global fits.

Abstract

We calculate the contributions to electroweak precision observables (EWPOs) due to a fourth generation of fermions with the most general (quark-)flavour structure (but assuming Dirac neutrinos and a trivial flavour structure in the lepton sector). The new-physics contributions to the EWPOs are calculated at one-loop order using automated tools (FeynArts/FormCalc). No further approximations are made in our calculation. We discuss the size of non-oblique contributions arising from Z--quark--anti-quark vertex corrections and the dependence of the EWPOs on all CKM mixing angles involving the fourth generation. We find that the electroweak precision observables are sensitive to two of the fourth-generation mixing angles and that the corresponding constraints on these angles are competitive with those obtained from flavour physics. For non-trivial 4x4 flavour structures, the non-oblique contributions lead to relative corrections of several permille and should be included in a global fit.