content_image/337718.jpg content_image/337719.jpg content_image/337717.jpg content_image/337715.jpg content_image/337716.jpg content_image/337714.jpg content_image/337713.jpg
The parameter U plays a fairly unimportant role, all the neutral current and low energy observables depend only on S and T [48]. In addition U is often predicted to be very small. In most of the models U should differ from zero by only a percent of T . 

In the case of an extra family with the doublet ( t ′ b ′ ) , the contribution to T and S param- eters are given by [34] 

with 

<img src='content_image/337718.jpg'>

<img src='content_image/337719.jpg'>

<img src='content_image/337717.jpg'>

## B. Vertex corrections to Z → b ¯ 

Including QCD and QED corrections, the Z → b ¯ decay width can be written as [51] 

where 

and δ ’s are various corrections which are discussed below. 

<img src='content_image/337715.jpg'>

<img src='content_image/337716.jpg'>

In the decay of the Z → b ¯ , the top quark mass enters in the loop correction to the vertex mediated by the W gauge boson. Due to spontaneous symmetry breaking effects the top mass can not be neglected in the calculation. In fact there is a top mass dependence that grows like m 2 t m 2 Z as in many other one-loop weak processes such as K − ¯ K , B − ¯ B (Δ F = 2 mixings), b → sℓ $^{+}$ℓ − etc. The additional contribution to the Zb ¯ b vertex, due to nonzero value of the top quark mass can be written as: 

δ q QED gives small final-state QED corrections that depend on the charge of final fermion, 

<img src='content_image/337714.jpg'>

<img src='content_image/337713.jpg'>

