# Hard subprocess¶

## Set center-of-mass energy of hard subprocess¶

The center-of-mass energy of the hard subprocess can be set by the following command:

cd /Herwig/Generators
set [NameOfGenerator]:EventHandler:LuminosityFunction:Energy [Center-of-Mass energy in GeV]
cd /


## The gauge boson in my Drell-Yan process has zero pT¶

We no longer apply the initial-state recoil to the intermediate particles in the hard process in order to enable the implementation of gauge boson pair production at NLO and other future developments. This information was only ever provided as a debugging record with HepMC code 11, and users were strongly advised not to rely on it. If you look at the stable decay products of the gauge boson in the final state and get the momentum of the intermediate boson by summing them up, the momentum of the gauge boson is correct and has the right recoils applied.

## Can I simulate more than one process?¶

For processes simulated with Matchbox you can have as many Process commands as you need, while for the internal matrix
elements you can insert as many matrix element lines as you like in the input files. By default, cut settings apply to all of them together.

For each event, a given process is chosen in proportion to its contribution to the overall cross-section. If the cross-sections of the processes differ widely, the weaker processes will not contribute unless you generate enough events!

## Which cuts are being applied?¶

To determine the current cut settings for a given run, you need to enable the first debugging level using the -d1 command line flag, e.g.

Herwig run LHC.run -N10 -d1


The cut settings will then be listed near the top of the .log file.