# Minimum bias runs¶

With the introduction of the new underlying event model from Herwig++ 2.3 onwards, we are able to simulate minimum bias events. However, the default setup is tailored for simulating underlying event activity in hard scattering events. We therefore have to change the settings in a few places. First, the specific minimum bias matrix element, MEMinBias, has to be selected

cd /Herwig/MatrixElements
insert SimpleQCD:MatrixElements[0] MEMinBias


Then, the cuts have to be altered to reflect the fact that we are using virtually any trigger selection.

##################################################
# Cuts on the hard process. MUST be ZERO for MinBias
##################################################
cd /Herwig/Cuts
set JetKtCut:MinKT 0.0*GeV
set QCDCuts:MHatMin 0.0*GeV

# Need this cut only for min bias
set QCDCuts:X1Min 0.055
set QCDCuts:X2Min 0.055


alternatively one can also set it using MinBiasCuts:

set LHCGenerator:EventHandler:Cuts /Herwig/Cuts/MinBiasCuts


The remaining settings that need to be changed are the ones in the underlying event model, i.e. in the MPIHandler. The parameter that tells the model which hard process is identical to QCD jet production has to be set to zero as our primary hard process is QCD-like,

set /Herwig/UnderlyingEvent/MPIHandler:IdenticalToUE 0


The settings so far enable the new underlying event model with both perturbative and non-perturbative contributions. For completeness we report the switches that are available to turn the soft model off. In this case the simulation is identical to any version 2.x before 2.3.

set /Herwig/UnderlyingEvent/MPIHandler:softInt Yes


The modification that calculates the overlap function of soft scatters from the elastic t-slope data can be controlled by

set /Herwig/UnderlyingEvent/MPIHandler:twoComp Yes


An example using these setting can be found in LHC-MB.in.