10. specify the allowed processes in the event generation

chapter nine

10.1 an example to start with: Z boson plus two partons

MG may be used to produce processes with also partons (quarks or gluons) in the final state
One example of such a generation is a Z boson accompanied by two partons (where the definition of j can be found in chapter 3):

generate p p > z j j
output zjj_default

10.1.1 the anatomy of the process

The outcome of the process generation will contain diagrams featuring strong and the electroweak coupling vertices, indicated as QCD and QED respectively in the MG drawings
To find the images of the calculated diagrams:

cd zjj_default/
find . -name "*.ps"

MG will generate the processes with the minimal number of QED vertices needed. In the following example diagram, there are:
- one QED vertex for the production of the Z boson
- two QCD vertices for the production of the two final state quarks

10.2 the electro-weak production of a Z boson with two partons

The same process may be produced with a purely electroweak process:
This may be obtained by forcing the number of QCD vertices in the process:

generate p p > z j j QCD=0
output zjj_ewk

nota bene the request QCD=N allows for up to N QCD vertices in the event (the same holds for the QED vertices)
nota bene the request QED==k allows for exactly k QED vertices in the event (the same holds for the QCD vertices)

10.3 exercises

what is the value of the cross section in the two cases considered?
what does it happen if both the QED and QCD numbers are fixed?
repeat the same exercise for the generation of pairs of top quarks at the LHC

chapter eleven