Calculating Z0 Decay Matrix Element: Finding the Error

[marlon1]

I want to calculate the matrix element of Z0 decay into a fermion, antofermion pair given the propagator, vertex element and fermi's golden rule.

But somewthing is going wrong.

First the propagator has a term 1/Mz^2 so
the MatrixElement^2 will go like 1/Mz^4.

but the vertex factor has a term Mz^2 so the matrix element will finally go
1/Mz^2

But according to the answer the matrix element must go like Mz^2 so I'm wrong a factor Mz^4

take a look here;

https://www.physicsforums.com/attachment.php?attachmentid=8956&d=1169839397"

 

[mjsd]

why do you have a propagtor when it is a tree-level decay... look at your Feymann diagram and you should see no propagtor.. unless I have misunderstood your interaction.